How to Modify SAT Solvers to Produce Resolution Refutations for Unsatisfiable Instances?

Question: In recent SAT competitions, there is a Certified UNSAT track. The problem instances are all unsatisfiable and the solvers are asked to produce certificates for unsatisfiability. One way is to produce a resolution refutation for the set of clauses in a problem instance. How can a SAT solver, say MiniSAT, be modified to do this? Can this be done for every solver? I searched the Internet and found little information. A reference is good for me.
Comment: Here is an example: http://www.mat.unb.br/ayala/MFingerSAT.pdf. CDCL solvers correspond to general resolution, whereas without clause learning you get only treelike resolution. These concepts are explained in lecture notes on the area, which can be found online.
Answer: A clause-learning SAT solver can be described approximately as follows. There is a stash of learned clauses, which starts empty. We run a recursive procedure, which at each point in time, first checks whether the current assignment (initially empty) contradicts an axiom or something in the stash. If so, there is no need to explore this branch. Otherwise, it picks a variable, and tries both assignment recursively. When a recursive call corresponding to a partial assignment $\alpha$ returns, we learn the clause $\lnot \alpha$. At any given point in time we are allowed to forget clauses. (It's important for the proof that each clause is learned, even if we forget it later on).
We now take the transcript of the algorithm and consider the clauses in the order in which they are learned. We prove that whenever a clause $c$ is learned, it can be derived using the rules of resolution from existing clauses or from axioms; in particular, at the very end we derive contradiction.
We allow the weakening rule in the refutation we construct; we can always get rid of weakening later on without increasing the refutation length (exercise).
There are three cases to consider. If the clause $c$ was learned because the corresponding assignment contradicted an axiom $c'$ or a clause in the stash $c'$, then $c$ is a weakening of $c'$. The interesting case is when $c = \lnot \alpha$ was learned since both assignment to some variable $x$ failed to produce a satisfying assignment. These assignments correspond to the learned clauses $c \lor x$ and $c \lor \lnot x$, from which $c$ follows by resolution.
Note that if we never use clauses in the stash then the resulting refutation is tree-like (every clause is used once), assuming we state each axiom whenever it's needed (rather than once and for all). Also, in any case the size (number of lines) of the refutation is the same as the number of recursive calls.
Answer: The simplest method of modifying a typical DPLL-based CDCL solver is to have it output every clause it learned while deciding a formula is unsatisfiable.  Such a list of clauses constitutes a RUP proof, which can be used to either construct a traditional resolution-based proof of unsatisfiability or show unsatisfiability directly using a somewhat different verification scheme.  See "Verification of Proofs of Unsatisfiability for CNF Formulas" and "Verifying RUP Proofs of Propositional Unsatisfiability" for the details.
Answer: If you know something about CDCL conflict clause generation, you will know that they are produced by resolution per se. For example, on page 137 of Handbook of Satisfiability, clause learning is done by the following procedure:
$$\omega_L^{d,i}=\begin{cases}\alpha(\kappa)&{\rm if\ }i=0\\
\omega_L^{d,i-1}\odot\alpha(l)&{\rm if\ }i\ne0\land\xi(\omega_L^{d,i-1},l,d)=1\\
\omega_L^{d,i-1}&{\rm if\ }i\ne0\land\forall_l\xi(\omega_L^{d,i-1},l,d)=0
\end{cases}$$
Here, $\alpha(v)$ is the antecedent of $v$. The first line specifies the base case. The recursion will ensue following the second line. The third line signals a fixed point and the process halts. Note in the second line, $\odot$ signifies resolution.
So just print the generation process and you will get a resolution refutation of the unsatisfiable formula.
Answer: Not every solver can be thus modified, and people are searching for new solvers that cannot produce resolution refutations. (Resolution is slow and we want fast solvers.)
But CDCL solvers can p-simulate general resolution. That means they can produce resolution refutations that are only polynomially larger than general resolution. The method is written in Pipatsrisawat and Darwiche's On the power of clause-learning SAT solvers as resolution engines.
But that is not a good answer, because we want to show SAT solvers are slow, instead of how efficient they are. One way is to show general resolution p-simulates CDCL solvers. But people are too happy proving how good SAT solvers are to consider the other direction.
I don't know how to answer my own question. That's why I seem to turn this to a discussion forum. But that's why I'm asking in the first place.
Those who know why "CDCL solvers correspond to general resolution" please stand up and explain why general resolution is as good as CDCL SAT solvers!
Comment: The paper is just proving something else. What you want to show is that the transcript of a CDCL refutation can be converted to a resolution refutation. There won't be any polynomial blow-up in this direction.
Comment: Should it be that solvers p-simulate general resolution, or that general resolution p-simulates solvers? I guess it's the latter because we want to lowerbound the runtime of solvers.
Comment: I think this paper is proving wrong things.
Comment: The direction that general resolution is as good as CDCL SAT solvers is not there. Who can provide such a reference? Thanks a lot.
Comment: Welcome to CS.SE.  This is a question-and-answer site, not a discussion forum.  Please don't use answers to request clarification or ask new questions.  If you have a follow-up question, please post it separately.  Also, please make your answer self-contained: instead of referring to the comments, include the information in your answer.  Comments exist only to help people improve their post and can be deleted at any time.  I will delete the question part of your answer; please edit it to include a full citation to the paper you're referring to.


How do valence shell configurations affect attraction and repulsion forces between atoms?

Question: It seems clear that the electrons of one atom are attracted to the protons of another and that this is the source of attractive forces between atoms. However, it is not clear why an atom with a full valence shell such as a noble gas or covalently bonded atom is incapable of bonding or being attracted to other atoms. Are the electrons not attracted to protons in the same way? Is it because the repulsion between the electrons wins out over the electron-proton attractive forces. Furthermore, why do valence shell arrangements with few electrons bring about attractive forces?
Comment: Possible duplicate of [Why are noble gases stable](http://chemistry.stackexchange.com/questions/7520/why-are-noble-gases-stable)
Comment: ``it is not clear why an atom with a full valence shell such as a noble gas or covalently bonded atom is incapable of bonding or being attracted to other atoms`` - It is, in some cases.
Answer: Firstly, why should covalently bonded atoms be incapable of bonding? Bond changes are the very basis of quite a huge amount of chemical reactions. Oxygen occurs in the $\ce{O2}$ form, which is covalently bonded, but readily breaks its O–O bond in order to form other bonds.
Now, to your question. Why are elements in group 18 (generally: think of the xenon fluorides) inert? Why is sodium so willing to give up its electron, while neon doesn't form any compounds at all, when they only differ by one proton in the nucleus?
The answer lies in the orbital energies. You can find a table listing the outer shell orbital energies page 16 of Appendix B of Miessler and Tarr's Inorganic Chemistry (5th ed.). Let us compare the outer orbital energies of Ne and Na. Neon has two 2s electrons with an energy of $-48.47\ \mathrm{eV}$ and four 2p electrons with an energy of $-21.59\ \mathrm{eV}$, while the 3s electron in the sodium atom lies at $-5.14\ \mathrm{eV}$. The difference is overwhelming. When a ionic compound is formed, an electron is removed from the less electronegative atom and added to the more electronegative atom, forming a cation and an anion. Now, the Na electron is much easier to remove than the Ne electron – you just need $-5.14\ \mathrm{eV}$ to do the task, while eliminating the Ne electron is much more difficult. The electron is then placed into an atom such as fluorine, whose outer 2p subshell lies at $-18.65\ \mathrm{eV}$: you needed energy to remove the Na electron, but energy is released when you place it in the F atom (since it is put into a low energy orbital). Placing an electron from F to Na wouldn't release energy because it is much more stable in the fluorine atom, due to their orbital energies (if you want to simplify things my anthropomorphizing the electrons, you could say they're "happier" in the fluorine atom). Removing an electron from the neon valence shell is very difficult because it has very low energy; adding one doesn't bring much because since the second shell is full, you would have to add it to the third shell – its energy is similar to the Na 3s shell, so it's not convenient.
I have to note here that while the first ionization energy corresponds more or less to the orbital energy (Na: $495.8\ \mathrm{kJ/mol}$ which corresponds to $5.139\ \mathrm{eV/atom}$), the electron affinity doesn't (F: $-3.399\ \mathrm{eV/atom}$), because of electron-electron repulsion in the fluorine p subshell. That's why an isolated $\ce{Na+ - F-}$ compound cannot exist: the additional energy released when forming the lattice is needed to form a stable entity.
Having said this, we can move on. You may ask: Yes, but why are the F electrons more stable than the Na ones, when Na has more protons? The reason is that the outer Na electrons are in the third shell, and thus they have higher energies. In a hydrogen-like atom (an atom with only one electron, which is clearly an oversimplification, since it doesn't involve screening) the orbitals have the following energies:
$$E \approx -13.6 \left(\frac{Z}{n}\right)^2 \mathrm{~eV}$$
You can see that raising the $Z$ (atomic number) value from 9 (F) to 11 (Na) isn't enough for lowering the energy, since $n$ (the principal quantum number, i.e. the shell) changes from 2 to 3.
So, this is why sodium forms positive ions, fluorine forms negative ions, and neon doesn't form any ions at all (chemically). Nota bene: as I said, the formula above doesn't take screening into account – when going down a group, it (usually: see d-block contraction) prevails over the growing nucleus. That's why Ar is less inert than Ne, even though the formula above would suggest the contrary.
But what about covalent compounds? According to VB theory, bonding occurs through orbital overlap. Lower energy orbitals mean smaller overlaps, which is inconvenient if you want a stable bond: Carbon forms bonds while neon doesn't because its orbitals are less stable (see table) and can overlap with other orbitals. Its orbitals are less stable because neon has more protons with the same principal quantum number – see the equation above to understand the concept.
In short: noble gases, especially those positioned higher in the periodic table, don't bond because their outer orbitals have the lowest energies. Ultimately, this is the result of having the largest number of protons possible for a given outer shell in the ground state.
Answer: If you consider Hydrogen bonding to itself, the bond is stabilized both by the electric attraction between proton and electron. As well the 1s orbital of both atoms merge into one bonding orbital, this single orbial is more energetically stable because electrons like to be paired off (spin 1/2 and -1/2) in orbitals.
The electron configurations of the noble gas feature valence shells with completely filled orbital pairs. According to the Pauli Exclusion Principle (PEP), no two electrons may inhabit the same quantum state in the same system. If all available electronic states for the atom are filled then there is no stabilizing phenomena for either the loss or gain of a new electron, and the formation of bonds is not favored energetically.
An analogy commonly used for the PEP is that there is an additional repulsive "force" between elections beyond the electric repulsion, it is precisely this "force" that prevents the noble gasses from bonding. Though it's not so much a force as it's simply the inherent fermionic behavior of the electrons at play. However, exotic noble gas compounds do exist, but are not terribly stable.
Comment: Spin pairing has a net energy cost associated with it, not an stability gain. The lower energy of bonding orbitals has nothing to do with spin pairing.
Comment: I don't understand *"If all available electronic states for the atom are filled then there is no stabilizing phenomena for either the loss or gain of a new electron"* - there is no such thing as running out of electronic states
Comment: There are noble gas compounds that are almost stable as a rock.


What are the benefits of the different weapons in survival mode?

Question: I have started playing MW3 recently and have completed 13 rounds in Resistance map. I have been using the Assault Riffle ACR 6.8. What kind of weapons I should use for survival mode where I can get more bang for my buck?
Comment: This depends heavily on your play style.  The only possible objective interpretation I see is damage per round, which you can see yourself from the weapon stats.  I don't think this is a good question.
Comment: Besides damage, there are other things like reload time, accuracy and sprint speed. As agent86 pointed out, LMGs are less useful in certain situations like running away from juggernauts. As I have mentioned in my question, I just started off playing MW3. Bear with me please.
Answer: When I play survival, I tend to "live off the land" and take weapons off of dead enemy soldiers instead of buying weapons at the shop.  In Resistance in particular, it's pretty easy to set up shop in the center house, like I explained here:
What is best sniper position on Resistance in Spec Ops: Survival mode?
Remember that you can also carry two primary weapons in Survival - so if enemies are using AK-47's and ammo for that is plentiful on the ground, you can use that gun alongside another AR that you've purchased or found in an earlier round.  Between rounds you can go back and just buy ammo, or pick up upgrades for these weapons if you prefer them.
Having 2 ARs means that if you empty a clip and there's still a lot of guys to shoot, you can switch to your off-hand AR and continue firing faster than you could've reloaded.
I don't tend to like LMGs or SMGs, the LMGs take too long to reload, and the SMGs do pitiful damage against enemies in the later rounds.  You might find them useful, depending on your strategy and preferences, however.
Not buying weapons means that you have more cash left over for other things, such as sentry guns, squads of AI helpers, and air support to deal with Juggernauts and other difficult situations.
Here is the weapon list and their rankings:

Larger size
Comment: A solid answer, but I disagree with your comments about LMGs. At waves 30+ an LMG is almost a necessity.
Answer: I have gotten to LvL 39-40 on Bakarra which is the hardest diffuclty by using the best combination that I have seen, along with other people I know that have gotten past lvl 65 using this combo. Use the L86 LMG, which reloads pretty damn fast for an lmg and allows you to have the ammo you need for the higher levels along with the power to chop to the helicopters if you manage to use your predators or whatever you use. Secondly purchase the sleight of hand perk if you're lvl 50 and a grip and a red dot sight instead of a holographic sight which will also increase your visuals for the game.
The second weapon you will need to save up for is the AA-12, it does run out of ammo fast but will take out a juggernaut in 2 clips (sleight of hand allows you to shoot off 12 rounds in about 2-3 seconds if that) up to about lvl 40 since that is as far as me and my buddys have gone to without dying for one stupid reason or another or running out of ammo due to poor planning. Buy a grip for this shotgun as it will increase your accurarcy when shooting the juggs. Use flashbangs and if you have a partner alternate.
Good tips to use through this is do not purchase the explosive turrents they will kill you more than help. Use C4, about 7-8 will kill a single jugg up to lvl 28 if I'm not mistaken so thats one less you have to deal with and 2 less if your partner kills with a predator and none if you then shoot your predator off and kill one. Also remember using shield squads doesnt help you kill people but it does give you time to prep after the round starts or during a road when you need to reload or get out of the Red vision stage from taking too much damage. I would advise that you and your partner set up claymores up the steps in Bakarra or in resistance (bakarra the 2 story next to the grenade, turret, armor pelican case.). This will allow you to see the jugg and blow up your C4 and your partner to shoot a predator off without being attacked by those damned C4 dogs.
Comment: Totally agree on the L86 and C4. Your AA-12 experience is interesting, I'll try that out.
Answer: i have been trying to figure out the best guns for MW3 survival and from what i have learned the best gun to use is the MP7 and the MK46. MP7 can take down a regular juggernaut with less than one mag if you aim for the head. shooting anywhere else would take 2-4 mags. The MK46 can take down an armored juggernaut with only two mags. I got to wave 67 in seatown with a this set-up. Sleight of hand would also be very helpful.
Answer: I would have to say l86 lmg and any gun that the enemy uses. If they are using ak-47s pick one up as it saves time any money.
Perks:

Slight of Hand
Self Revive

Equipment:

C4 (10)
Claymores (10)
Auto or grenade sentries depending on the location
Flashbangs

Air support:

Riot shield team
Predator missile
Answer: Here's what I like to do:
Kill the first man and then pick up his weapon. Then finish off the rest with your pistol, picking up all of the ammo as it drops.  By this time you should have a half sock(?) gun.  Next, switch out your pistol when a new gun gets dropped, instead of buying a new weapon. Continue doing this throughout the playthrough, switching out older guns for newer guns, and by about round 25 you should have enough to buy 2 LMGs.
Answer: What me and my partner do is buy everything as soon as we have enough money to get it. That means at wave 6 we are buying our first lmg and by wave 12 we have sleight of hand. The reason we do this is because with a lot of ammo it is easier to concentrate on getting rampages and kill streaks and also by buying all the expensive things first by the time we reach level 38 we have about 40-50k a piece (get rich or die tryin) and that allows for us to properly prepare for the harder rounds with predators, rioters, and air strikes without breaking the bank.
Answer: I agree, on the earlier levels, just use whatever weapons the enemies are dropping, allowing you to build up a bit of a buffer of cash that you will need later on.
Enemies get progressively more hitpoints as the waves go on, so by around Wave 15+ you will need to carry at least one LMG to cut down all the troops they throw at you.
By Wave 35+, it's virtually very, very difficult to kill all the enemies without you running out of ammo, unless you carry two LMGs.
By Wave 50, I'm fairly certain it's impossible to proceed without two LMGs.
Comment: Largely depends on the map and weapons your partner uses. I've made 51 on Seatown with L86 and FAD. Mostly because my partner didn't use a FAD so all the dropped ammo was available to me. Generally I use L86 and MG36, but it's about keeping steady points for C4, claymores etc.
Answer: The buying everything when you have the money is easiest for helicopter and Juggernauts. When using LMGs, you don't have to move; you can throw frags, sit and shoot at his head or your partner can go to the back of him. Or you and your partner can take turns shooting his middle armor plate easily taking a Juggernaut out. You can get easy practice by doing underground - a helicopter comes at wave 6, then a Juggernaut at wave 10, 2 at wave 15,and 3 at wave 20 and so on.


Why would God have to tell Joshua three different times to "be strong and of good courage"?

Question: In Joshua 1 we have:

Verse 6: be strong and very courage
Verse 7: only be strong and very courage
Verse 9: have I not commanded you? Be strong and of good courage

Why did God give this command to Joshua three times?
Answer: The triple command to "be strong of good courage" is actually two commends as will be seen shortly.  The first two instances to "be strong and of good courage" are because of the dual task given to Joshua by God, namely:

V6: for (= because) you shall give these people the inheritance of the land that I swore to their fathers I would give them.  That, is, Joshua will be the leader of the people.
V7: Be careful to observe all the law that My servant Moses commanded you.  Keeping the covenant law was not for wimps but required great courage and strength.
V9: Finally, the third instance is actually an encouragement based on the source of Joshua's strength and courage precisely because, "the LORD your God is with you wherever you go."

Thus, Joshua was given a superhuman dual task of leading the Israelites into the promised land AND providing an exemplar of covenant keeping law keeper.  These were beyond the capabilities of any human but could be accomplished with divine assistance.
Benson notes the following:

Joshua 1:6. Be strong and of a good courage — Joshua, though a person of great courage and resolution, whereof he had given
sufficient proof, yet needed these exhortations, partly because his
work was great, and difficult, and long, and in a great measure new;
partly because he had a very mean opinion of himself, especially if
compared with Moses: and remembering how perverse and ungovernable
that people were, even under Moses, he might very well suspect the
burden of ruling them would be too heavy for his shoulders.
Comment: Thank you for this wonderful insight to the scriptures
Answer: http://numerology.center/biblical_numbers_number_3.php

Number 3 is one of the most important numbers through Bible, it is a number of harmony, of God's presence and of completeness. Most of the significant events in Bible were accompanied by this number or by number 7. It is connected to the Holy Trinity, to three patriarchs before the flood – Abel, Enoch and Noah, and three patriarchs after the flood – Abraham, Isaac, and Jacob. New Testament has 27 books which is 33 (333), and it is largely connected with Jesus.
He prayed three times before being arrested, he was put at the cross at the third hour of the day and died at the ninth hour which is 3pm. Number 3 is the number of eternal life, as he was resurrected after three days of being dead. Three people saw the Holy Transfiguration of Jesus, they were" Peter, John and James.

Peter denied the Lord 3 times in
Matthew 26:75

Then Peter remembered the word Jesus had spoken: "Before the rooster crows, you will disown me three times.

Jesus in turn asked Peter 3 times.
John 21:17

The third time he said to him, "Simon son of John, do you love me?" Peter was hurt because Jesus asked him the third time, "Do you love me?" He said, "Lord, you know all things; you know that I love you." Jesus said, "Feed my sheep.

Paul prayed 3 times in
2 Corinthians 12:8

Three times I prayed to the Lord about this and asked him to take it away.

There are many occasions in the Bible where three is the number for completeness as it is in the Western culture today.

Why would God have to tell Joshua three different times to “be strong and of good courage”?


Supersonic Fuselage design

Question: What should be the ratio of the fuselage and wings in a maximum dimension limited condition supersonic delta wing aircraft?
Also can anybody explain clearly the "supersonic area rule" or "Whitcomb area rule"?
Is there anything else I need to take care of while designing a fuselage for supersonic conditions?
Thanks.
Comment: you could start by taking some lessons in aerodynamics and fluid dynamics.
Comment: Gaurang, you seriously cannot expect to rock up to some internet forum and find out everything you need to know to design supersonic planes. Very probably, nobody here has that kind of knowledge. Even if they did, it's the sort of thing you'd need to write a whole book to explain, if not a series of books. Nobody is going to invest that kind of time.
Comment: Way above my pay grade, but a short answer is that if you plot the length of the aircraft on the x-axis and its cross sectional area on the y-axis, you want that curve to be as smooth as possible. If it isn't, you're going to have a large drag rise during transonic flight. So, if, say, as was true with the first F-102, you got a sudden increase in cross sectional area at the point of the fuselage and wing juncture, you could fix it (and they did) by narrowing the fuselage at the point to produce the now familiar "coke bottle" shape, which avoided that large drag rise.
Comment: @DavidRicherby Also, this is not a forum.
Comment: A continuation of my previous comment: As I remember it, the area rule was necessary in an era of engines with insufficient power to overcome the transonic drag rise without it and then push aircraft through the transonic range. It's not necessary and not used today with our much more powerful engines.
Comment: @Terry: [Wikipedia says](http://en.wikipedia.org/wiki/Area_rule) the area rule is still important today, citing examples such as the A380 and Citation X.
Answer: There is no ideal ratio. Make the wing as big as it needs to be for your take-off run, your cruising altitude or your approach speed, whatever demands the biggest wing, and make the fuselage just big enough to hold the pilot(s) and all equipment, payload, fuel and whatever you choose to carry around. Asking for a specific ratio without any details on the mission or the design parameters is silly.

The comments tell me I should say something about area ruling, though. Terry correctly mentions the x-wise gradient of the aircraft's cross section, but this is only helpful at Mach 1. At higher speeds, you need to look at the area which is covered by Mach cones which run along the longitudinal axis. Sum up the area of the intersection between those Mach cones and your aircraft's structure, and make the lengthwise distribution follow a Sears-Haack-body. Please note that the inclination of those Mach cones varies with angle of attack, so there is not a single optimum shape, but only a best compromise for all expected flight conditions.
Area ruling is only helpful around Mach 1, but there (or slightly above) is the transsonic drag maximum, and area ruling helps to get across this quickly, even with the high thrust of modern engines. If your task is to get to this strange radar blip at 16 km altitude in a hurry, every second counts. Also, modern airliners need to employ area ruling techniques to make cruise Mach numbers above Mach 0.8 economical.
And this is just scratching the surface ...
Comment: Am sorry for the question.Should have broken it into multiple specific questions.Thanks for the answers though.

Supersonic Fuselage design


Sharepoint server with mysql server optimal hardware configuration

Question: I have installed SharePoint Server 2013 with mysql server 2013 on a windows server 2012 on a dell server with Intel Xeon Processor on about 2Ghz and 4 GB ram,(the server is about 7-8 years old) the problem is that the sharepoint website is running extrelly lagged and slow. What you will suggest as an optimal hardware for thoose software products to be running smoothly inside a intranet?
Answer: Microsoft posts server limit on its TechNet site.  A Single server running SharePoint 2013 with all services and SQL server should have at least 24GB RAM and 8 CPU cores. That's just for basic functionality.  If you want to deploy MySQL as well you need to plan for that workload.  You won't get a satisfactory experience with your setup.
Answer: See the official  requirements here: http://technet.microsoft.com/en-us/library/cc262485.aspx
As Jesus said, 4 GB RAM is not enough. Search alone eats 8 GB RAM.


What is the exact reason that compounds such as carbon dioxide cannot interact with other compounds through dipole-dipole interactions?

Question: Whilst I understand that $\ce{CO2}$ has a linear structure, resulting in a vector sum of the dipoles of 0, I do not understand why the dipoles formed at the two oxygen atoms as a result of their increased electronegativity cannot interact with other molecules via dipole-dipole forces. Do the vectors equaling 0 somehow negate the present dipoles?
Comment: Do realize that there are other attractive forces besides dipole-dipole interactions. So two carbon dioxide molecules could be attracted to each other by a London dispersion force.
Comment: The attractive induced-dipole, induced-dipole interaction exists even when the molecule has no overall dipole. It is proportional to the product of average polarisabilities and mean of  ionisation potentials and inversely as the sixth power of separation of centres of mass. ($\ce{CO2}$ has an asymmetric vibration and two bending vibrations that produce transient dipoles.)
Comment: Of course it depends whether you consider the *chemical* reactivity of carbon dioxide as something that can be explained by dipole dipole interactions when other molecules are very close. Most people think of the chemical reactivity as something different even though the reactions are explained by the movement of electrons.
Answer: As the other answers have indicated, CO2 has no NET dipole moment. However it does have two dipoles pointed in opposite directions (as OP keeps mentioning). This means that CO2 can possibly interact through higher moments, such as the quadrupole moment. The mathematical procedure behind this is known as the multipole expansion. It is important to note however, the higher order multipoles get weaker and weaker by a factor of 1/R approximately, so quadrupole interactions are usually ignored for example.
Answer: Carbon dioxide is more than linear. It's symmetric, and the axis of symmetry perpendicular to the bonds also applies to whatever dipole moment it has. The only vector that looks the same after being rotated 180 degrees is the null vector, so the molecule has zero overall dipole moment.
Therefore, dipole-dipole interactions are not possible because carbon dioxide does not have a dipole.
Comment: Because in the ground state, the time average is zero.
Comment: But assymetric stretch of $\ce{CO2}$ creates a non-zero dipole moment. So why we say that it has zero dipole moment?
Comment: You mean the time average of the excited state right?
Comment: No, I mean the ground state...
Comment: This would be the same also on excited states right? I mean for every $υ$ the time average must be zero.
Comment: If you reasonably assume symmetry, then yes.
Answer: Exactly that.
Dipole-dipole interactions are caused because the positive end of one dipole is attracted to the negative end of another dipole. This macroscopic attraction can only occur if the centre of positive charge and the centre of negative charge do not coincide.
In molecules like carbon dioxide that contain a centre of symmetry, these charge centres do coincide. They fall on top of each other. So macroscopically, a neighbouring molecule cannot see any positively or negatively charged centre and therefore sees no possibility to attach via dipole interactions.
Comment: so basically the charges are distributed equally within the molecule? But how can that be ? Aren't the 2 o atoms suppose to deprive the c atom os electrons and make it positively charged?
Comment: Well yes, each of the oxygens is negatively charged and carbon is positively charged. But these microscopic charge centres both fall together in the molecule's centre of gravity when viewed from a greater distance. That's the power of symmetry.
Comment: so even though the oxygen is negatively charged, it cannot interact with the positively charged C atom of another CO2 molecule
Comment: Thank you Jan. So due to the symmetry the overall dipole is 0? Wouldn't the electrons still be more attracted to the oxygen, resulting in dipoles? And wouldn't these dipoles be able to interact with other dipoles? I.e a positive carbon atom
Answer: Consider the CO2 molecule as a superposition of two dipoles, one for each C=O bond. A nearby polar molecule will feel a dipole-dipole interaction with each C=O bond, but since those two dipoles are equal and opposite, those two interactions will almost cancel out. They may not completely cancel since the interaction with the closer of the two bonds will be stronger.
The resulting net interaction will therefore be much weaker than the interaction with a polar molecule such as water.
Note that a superposition of two equal and opposite dipoles in different locations forms a quadrupole. So, instead of electing to treat the interaction of a polar molecule with CO2 as the interaction with two equal and opposite dipoles, we can treat it as the interaction with a zero dipole but nonzero quadrupole. As user157879 points out, quadrupole interactions are much weaker than dipole interactions.


I cant delete my question

Question: Sorry I have been unable to delete due to lack of knowledge.
Comment: The delete button should be right under your post (except in the extremely unlikely case of being on the app, in which case you need to open the post in a browser).
Comment: ironically, an author cannot delete their question if it has been answered and that **answer has been upvoted**. See [Can I delete my own post with answers?](https://english.meta.stackexchange.com/questions/4295/can-i-delete-my-own-post-with-answers)
Answer: It's always a good idea to include a link to the post you're asking about.
Normally there's a list of possible actions under the tags, opposite your user card. Here's the moderator list, and not everyone will have all these options; but delete is included where it can be.

While the Delete button/link should be under your post using the website [using the app it probably won't be], there are restrictions.
In particular, people with less than 20k rep need to wait two days to remove a question. If a question is worth asking, it's worth answering. Since you asked the question, it must be worth asking. If you have a change of heart, you need to wait and see if anyone will in fact answer it. If there aren't answers which disqualify it from deletion, you can delete it once the time limit has expired.
After the time limit, with less than 10k rep you can only delete your own question if there is at most one answer and that answer has no upvotes. That restriction is there because people have worked on answers which the community have thought to be worthwhile. Just because you're no longer interested in getting answers to your question should not allow others to lose the rep they have gained through their own work.
Members of the community who have the post-delete privilege (at least 10k rep) can still vote to delete the question if it warrants it. It takes at least three votes, possibly more.
Note that vandalising a question is very bad practice and will normally result in the question being reverted. That's far too complicated to do here (where the edited question has been migrated, and the migrated question is useful), but this question should not be regarded as a precedent.
Comment: Actually the OP was probably talking about this very question. Just look at the revision history on the main site :p
Comment: @Laurel Aha. Thanks: answer amended accordingly. (Restrictions aren't easy to find; feel free to put anything right I've misremembered. This question is useful.)
Comment: The delete button does exist on the mobile site but I thinkt deleting posts using the SE Apple app is more problematic. If I'm not mistaken there is no delete button.  Not 100% sure though.
Comment: [Delete questions/answers in iOS app](https://meta.stackexchange.com/questions/220639/delete-questions-answers-in-ios-app)
Comment: @Mari-LouA You can't delete via the app: the only options are Edit/Share. It appears the API restriction has not been reworked. Answer updated again.
Comment: The same is true for the android app, is that what you meant by API? Anyhow:  [Allow us to vote to delete from the Android app](https://meta.stackexchange.com/questions/216962) If the OP is using an app, it's not their fault they cannot delete an off-topic question, which it appears it was, by looking at its history.
Comment: Android is the same (I use that, not iOS) but it's not the primary interface for SE: it relies on an Application Programming Interface to make changes to the database. The primary means of doing that is the website. Mobile view should work, but most smartphones will also cope with desktop view, if that's different. [Looks like the app can't edit comments either!]
Comment: +1 for "In particular, people with less than 20k rep need to wait two days to remove a question." this probably explains why there are so many users on SO banned from posting questions. Even if they realise their post is off-topic and they want to delete it, they cannot. That post attracts downvotes and maybe an easy upvoted answer, and that user is stuck with a downvoted question but with an upvoted answer.

I cant delete my question


PHP MGRS Conversion Formula/Library

Question: Currently, the formula I'm using to convert MGRS to Lat/Long and vice versa is giving me slightly different results than most other conversion utilities.  It's usually between 10 to 30 feet off.  This isn't a very big difference, but it matters for the particular application of data.  I'm wondering if anyone knows of a simple library that does conversions between MGRS and Lat/Long or UTM.
Answer:
http://freegeographytools.com/2007/mgrsutmlatitude-longitude-converter
Comment: I'm not looking for a utility to perform the conversion.  I wanted to know if there was a PHP class out there.
Answer: http://www.ngs.noaa.gov/TOOLS/usng.shtml
which is also downloadable (including the FORTRAN source). USNG is a simplified version of MGRS. Datum is usually NAD83 and you don't have to handle the different letter schema based on the datum/ellipsoid. In MGRS, NAD27 data would have a different letter schema than NAD83.
Geotrans 3.0
http://earth-info.nga.mil/GandG/geotrans/index.html
which is in C or Java also has source code available.
Answer: I would recommnend you to use USNG library to convert
LL to USNG (function LLtoUSNG(lat, lon, precision))
LL to MRGS (function LLtoMGRS(lat, lon, precision))
LL to UTM  (Note: UTM calculations are an intermediate step in LLtoUSNG conversions but can be used as seperate function if required)
and more, plus reverse

PHP MGRS Conversion Formula/Library


Are cross-database queries expensive in SQL Server?

Question: Are cross-database queries expensive in SQL Server? All of the databases are in the same instance.
Comment: I have never found any evidence that they are, no. The only exceptions I could even think of (but haven't tested) are (a) that the optimizer can't benefit from relationships that can't be explicit (e.g. it can benefit from foreign key definitions, that can't exist cross-database) or (b) there are issues with statistics being a problem over linked servers, but I imagine there may be cases where even across database a user running a query can't see stats. Don't know if (b) is real or not - haven't tested, just know that can be a serious issue across servers.
Comment: I'm with @AaronBertrand on this one, I haven't seen anything to definitively say there's a huge hit.  The only thing I could *think* would add something to the equation would be possibly some form of authentication operations, but I would doubt it would be significant.  That's just me thinking out loud, I'm not sure.  Your best bet here would be to set up an automated test/benchmark with a cross-database, then with the same query inter-database and run that test thousands of times.  Get the average, max, etc. and make your decision based off of that.
Comment: @thomas could you see a difference definitively by looking at the actual query plans?
Comment: @Jonathan: the definition of "expensive" is relative to alternatives. So, what are you trying to do and what are you deciding between? Without more info it is quite possible to get an answer that appears correct to this question at face-value, but at the same time is absolutely wrong for your situation. Everything has _a_ cost, but that cost could be either cheap or expensive depending on what you are comparing it to. And query costs (i.e. time, contention, etc) need to be weighed against maintenance costs, hardware costs, etc.
Comment: I would like to know the cost in comparison to a query in the same table.
Comment: Hi Jonathan: well, yes, I figured that the other option was a table in the same database. What I was asking was: why are you considering data in another DB? Usually people have a good reason for wanting to separate the data into another DB, whether it be usage pattern, the type of data, a desire to have different DB configuration. Would the other DB contain mainly lookup data? Could it be data that doesn't change and hence the DB can be marked ReadOnly? Things like that.
Answer: Sorry, I don't have enough reputation to comment on the question, but from my experience if the client application raises a transaction for a query which uses cross database joins then it would promote the transaction to distributed and have the overhead of a DTC transaction.
The DTC overhead in this case could be viewed as a negative to performance. Generally the difference would be negligible although Microsoft describe DTC transactions as such:

Distributed transactions typically consume significant system resources
Transaction Promotion

...which would suggest a performance degradation if your server cannot offer the resources it requires.
Just to clarify, the article above describes local transactions being promoted when remote systems are introduced, but I have seen this become the case for transactions on the same server when using cross database queries.
As Thomas Stringer points out in his comment, there will be extra overhead in authentication although I think as this will be SID-driven there will be minimal overhead there unless you have to use separate credentials to access the other database.
If there were difference in database settings which caused additional overhead in the join that could impact larger than the previous suggestions - for example database collation. Database collation could manifest as a functional difference, not just a performance difference.
I think Aaron has the strongest argument for performance with the optimizer not having the advantage of using relationships for cross-database queries whereas self-contained within a database you could use relationships to your advantage.
Comment: [`If a local transaction spans two or more databases on the same instance of the Database Engine, the instance uses an internal two-phase commit to commit all of the databases involved in the transaction.`](http://technet.microsoft.com/en-us/library/ms190295.aspx) -- If you've found otherwise, please report a bug on Connect with clear reproduction steps.
Comment: The transaction promotion article is for CLR transactions.
Answer: In SQL Server 2014, memory-optimized tables do not support cross-database transactions. You cannot access another database from the same transaction or the same query that also accesses a memory-optimized table. You cannot easily copy data from a table in one database, to a memory-optimized table in another database.http://msdn.microsoft.com/en-us/library/dn584627(v=sql.120).aspx Distributed and cross-database transactions are not supported for database mirroring/AOAG. http://technet.microsoft.com/en-us/library/hh393530.aspx.  If you are going to use ownership chaining, be aware of the security risks http://msdn.microsoft.com/en-us/library/ms188676.aspx
Comment: My bad...I missed the part about it being within the same instance
Comment: That only applies to scenarios that "resources that span multiple instances or servers". In this case we're talking about a single instance with multiple databases.


Install a new SSD but keep all the files and everything

Question: I bought a new SSD for my Macbook Pro Retina 2014. I want to be able to insert the new SSD but keep all my files, setting and etc. I am not sure what to do how can I clone my existing SSD to the new SSD?
Answer: You need to make a complete duplicate ("clone") of your existing internal SSD module onto the new SSD.
You can do this with Disk Utility, but the third-party apps  SuperDuper! and Carbon Copy Cloner are actually better at it.
However, the real problem is having both SSDs attached at the same time to make the transfer. You either need to buy a USB enclosure for one of them, or you need to have an additional USB drive to copy the clone to as an intermediary. You also need to have MacOS on this USB drive, because you'll need to boot up to it.
(Having a standard USB drive may be more use to you in the long-run,; or you may already have one.)
First step is to copy the clone onto the USB drive. You can either make the entire disk a clone, or copy the clone as a disk image (file) onto the USB.
Next, you need to install the new SSD in your Mac so you can copy the clone back onto it, but of course you need to be booted to another volume in order to run the cloning app!
Boot to the USB drive, and then run the cloning software in the other direction.
Answer: Connect the SSD externally and install a clean os on it. Then run migration assistant to restore files to it from the internal drive (or from backup).
Once yours sure the SSD is viable and tested, then do the installation.
This was the opposite when drive swap was easy SATA and batteries were removable with a coin. Now you want to delay things that open the Mac until you're sure your install is good.
Comment: how can I connect the SSD externally? the ssd is Aura Pro X2, I can't find an enclosure to put this drive in it and use it as external drive to clone my files
Comment: OWC sells kits that should work, @user263291 - https://eshop.macsales.com/item/OWC/S3DAPT4MB05K/
Comment: Also, good luck if this is your first repair / upgrade. OWC has amazing documentation, quality and support. Well worth the cost if you're not super sure how to research and do DIY repairs.
Comment: that enclosure is only compatible with original mac ssd and now owc ssd.


Everything missing in Elementary OS

Question: I was attempting to fix a full root drive today and when I restarted my computer, the background was black, all of my settings are missing, my taskbar is back to the original installation options (though my apps seem to still be there), and all my files are missing (home folders all empty). Does anyone know what I might have done and how I might fix it (or if it's possible)? My main concern is retrieving the files, as much as I'd love if I could avoid a clean reinstall. I have been using Elementary OS and Ubuntu for quite a while but am still quite basic when it comes to tinkering under the hood.
Thanks!
Comment: Can you be more precisse? What was you issue? How you solved, step-by-step? Have you seen your /home folder? How many users have your system? Have all your users lost their files?
Answer: I think you (or the system) have been created a new user. The following command list all users:
<code>sudo awk -F':' '{ print $1}' /etc/passwd
</code>

Everything missing in Elementary OS


Employer asking for online access to bank account - Is this a scam?

Question: I just recently got a freelance job as a virtual assistant. The employer I have looks like a reputable person with his own agency and everything. I'm new to it, so he asked for my bank account number, the swift number, my name phone number and the address of a bank. I however was a little irked when he said "if you don't mind I'd like online access to keep track of the account." It's only a $600 a week job, and I mean he wasn't forcing it and didn't ask for my Social Security number or anything like that. What should I do?
Comment: I bet part of the "job responsibilities" would be receiving money to the account then sending them out by Western Union, bitcoin and such.
Comment: By "_online access_", you mean they were actually asking for your online banking password?
Comment: Why should they want track your account?
Comment: Are both you and the employer in the United Sates? You mention swift code, but most Americans never deal with swift codes.
Comment: "The employer I have looks like a reputable person with his own agency and everything" - well, yes, criminals usually *try* to look like reputable people. Copying a professional website belonging to an agency that *is* reputable, and changing a few names and contact details, only takes half a day to set up. If you have never seen this "virtual employer" in person, you don't even know if the reputable looking person in the picture on the website is him/her, or somebody else!
Comment: If you give this employer access to your bank account, could you give me access to it as well? I pinky promise not to empty out your life savings and rack up thousands in credit card debt. I even have an agency!
Joking aside...run far, run fast, it's 100% a scam.
Comment: I'd be interested in knowing what the nature of the job is. It may also help improve other answers.
Comment: Comments are not for extended discussion; this conversation has been [moved to chat](https://chat.stackexchange.com/rooms/94071/discussion-on-question-by-rc-kelly-employer-asking-for-online-access-to-bank-acc).
Comment: Hi @RC Kelly - Welcome to SE. Sorry that you have to be in this situation. Have you already started working/contributing there?
Comment: So, are you a freelancer or are you an employee? I first thought that by "employer" you meant "client", but then people below are talking about you being an employee, and you have the "employment" tag on. People below are talking about the employer withholding taxes and whatnot when that shouldn't happen with freelancers so it's probably a detail that should be clarified.
Answer: In the United States, an employer is required to verify identity and citizenship as part of the hiring process.  The way that they do that is that they ask for documents.  There are three lists.  One list has documents that verify identity and employment status at the same time, e.g. a passport.  A second list verifies just identity, e.g. a driver's license.  A third list verifies employment status, e.g. a Social Security card.  You either provide one document from the first list or two documents, one each from the other two lists.
Beyond that, an employer also has to withhold taxes.  To do so, the employer has to tell the IRS your Social Security number.  Even if you are being hired as a contractor and not a W-2 employee, your employer still needs your Taxpayer Identification Number, which for an individual is ... your SSN (businesses may have an Employer Identification Number instead, but as an individual you can use an SSN).
Not asking for your SSN is a red flag, as a real employer would.
As you already expect, asking for access to your bank account is also a red flag.  A real employer doesn't need access to your bank account.  For the kind of job that you have, it would make more sense for you to have access to the employer's business account.  Obviously the employer would also have access to that.
If you have already given this person bank account information like your account number, then you should call the police and tell them that you suspect an attempt at identity theft.  You should probably also close your bank account and create a new one.  The bank could help with that.  But I would call the police first.  Perhaps they would like to operate a sting or something.  You can start with local police, but they may refer you to someone else.
It is barely possible that the person is just an idiot and not a scammer.  But if so, then a visit from the police will help the next assistant hired.  I'm assuming that your employment offer would be rescinded.
I take it that all your communication has been online or by phone.  So you don't actually know where this person is located.  I would not have high hopes for an arrest in that situation.  But the police might at least burn some of the scammer's accounts.
Comment: Minor nitpick, but social security card does not prove citizenship - it only proves eligibility to be employed, unless it is a restricted card ("Valid for employment with DHS authorization only").
Comment: @void_ptr To give a specific example, I am not a US citizen but I have a Social Security card.
Comment: I would ask that you edit this for SSN or EIN.  Since this person was hired as a independent contractor, either would be suitable for a 1099.  Otherwise it is a great answer.
Comment: A US employer asking for a SWIFT code in this situation is potentially another red flag. Even ignoring the part where they wanted online access, the employer would need a Routing number instead of a SWIFT code for domestic direct deposit payments (the rest of the data minus the online access seems relatively normal for direct deposit).
Comment: "As you already expect, asking for access to your bank account is also a red flag." - That's the crux of the answer, but this post takes too long to get there IMO.  For anyone who stopped reading after the first couple of paragraphs, yes it's quite normal for a U.S. employer to verify your identity/citizenship/tax details, but **no, they absolutely don't need access to your bank account(s) to do that, and no, it's NOT normal for them to ask**.
Answer: An employer has absolutely no business in looking at the private bank accounts of their employees. Even suggesting this is a pretty outrageous demand.
Maybe it's a misunderstanding and the account he would like access to is not actually your personal bank account on which you receive your personal salary, but a bank account with his money which you access on his behalf as part of your personal assistant duties? In that case, joint access would be pretty reasonable.
But in that case, the account should be in the employer's name, not in yours. If the account is in your name, you will be personally liable if that account balance goes negative. The debt collectors will then pester you personally, not your employer. Your personal credit score will be damaged, not his. And if your employer asks you to do illegal stuff (which you might not recognize as illegal), then you will have a much harder time to get out of personal criminal liability if the account is in your name.
Also, if the account is in your name, then you would actually have a pretty easy time to just steal the money. Your employer should be aware of that. So it would be in their interest for the account to be in their name. That means if they insist on the account being in your name, that's a red flag. They obviously want to avoid being connected to that account. The only reason why they would want to avoid that, is if they are doing something shady. And they want you to take the fall if it blows up!
Answer: This is 100% a scam
This is how it works:
You recently got a job online, as an "assistant". You don't know your employer personally (you say he "looks reputable" which means nothing), you've never seen them in person, in a way that let's you 100% confirm their identity or genuineness. What you have is someone who has set things up to appear genuine online - some social media, an agency, a website, whatever. Then they advertise for an employee - maybe to process payments, maybe an "assistant". From there, it goes one of 2 ways (yours is the second of these)

The person's "job" is to receive money and make payments, keeping some as of it as salary. The money is often by cheque, the payments by untraceable methods such as bitcoin or Western Union. Time passes. The "employer" vanishes, and a while later the "employees" bank tells them that the cheques they had used to fund the payments were fake or bounced (unpaid). This takes weeks or months to happen, long past the time a bank allows you to draw against them, because the cheques are routed cunningly by the scammer. Result: the "employer" gains £7,000 of untraceable bitcoin, and the "employee" ends up £10,000 out of pocket because their bank eventually discovers the cheques were bad, and  deducts back from their bank balance, all the "cheques" which had been deposited while the scam was running. (Optional bonus feature - a police visit for suspicion of money laundering.)
The "employer" asks for personal information, or bank information, requires use of TeamViewer/AnyDesk remote software "for supervision", or provides a special "app" needed to do the work. Time passes. The "employer" vanishes and the "employee" discovers the information was used to hack them, withdrawn money from their account, the app was used to disable antivirus or to plant ransomware on their computer, or similar.

The giveaway here is the combination of a request for information+access that few if any legitimate employers ask for - but scammers routinely want to get, no evidence of a genuine employer, a common kind of scam setup, all corroborated by an online obtained "job" where you actually don't have any idea at all who the "employer" really is.
If you believe your "employer" may be genuine, please update your question to provide the details you've been told about them or which make them look "reputable", and add a comment to say you've done so (so I notice) - people may be able to look into it and give you a good idea what the info says. If you have received emails, look up for your email client how to display the full email headers for one or two of their emails, and add those as well, if you can (they are often informative in scam contexts, although often it's clear without them).
Comment: Even if meeting someone in person, that does not "100% confirm their genuineness".  Con artists exist, and in fact, wearing a nice suit and renting a nice office can actually contribute to successful fraud.
Comment: Worth noting, indeed.
Answer: This is a money laundering or advance fee scam
It's not a real job.  You will discover that your "job" is "money handler" - He wants to transfer money into your personal account, and transfer that money out again. Right off the bat, this would be money laundering, but it's worse than that.
The transfer out of your account will be good money. However the transfers into your account will have a problem.

It is stolen money, he stole by accessing the account of someone else, e.g. phished credentials or another "virtual assistant" like yourself.  Someone will want that money back, and they will take it, leaving you holding the bag.
It is fake money that never existed in the first place, e.g. from fake cashier's checks.

Any way it goes, it is rigged so he gets the money and you are left holding the bag (since you are reachable and he is not).  Further, because you willingly did things you should know better than to do (launder money, hello), you're an accomplice not a victim. This will be held over your head to assure you pay the money back.
How do fake cashier's checks work? Banks don't know how long checks take to definitely clear, and so they "put a hold" on deposited money for a certain number of days.  These checks are rigged to take longer than that to wind their way thorough the international banking system and be refused as a fake.  By then, the bank has released its hold and he has whisked the money away.
Answer: If you're getting your paycheck direct deposited into your account, they would need your bank's routing number and your account number.
However, there's no reason why an employer needs to access the details of your bank account, so they shouldn't need any more information. You should definitely not give them your ATM PIN or online username/password.
And if you're not using direct deposit, they don't need any bank information.
Answer: Please note that in answering your question there is some part that is applicable to all/any workplaces.
The employer would obviously know your pay because they're paying you, but no one legit can ask for access to your online account. This may or may not be a scam but this is certainly inappropriate in a workplace (online or on location) in any context.
So don't share your online account access info or any account information other than the bank account number and routing code for legally paying you for your work.
For the scam aspect, I'd recommend you that look up the prospective employer for reviews and any information you may get. My suspicion is (if you haven't already started working) that this is a scam and you should look for a different opportunity.
Answer: It's a scam! You'll never get paid. The Swift number is used for international bank deposits. Nothing else. If the scammer gets your online login, he'll have money wired to your account from some other country and use online access to send that money from you to himself or another criminal. If you give him online access, he can change everything in your bank's records including your password!   He can take complete control of your account. He can change your address and have credit cards in your name sent to him. He'll have new plastic, new CC account number and the security code on the back. Do not communicate further with this guy. He's not going to pay you, anyway.
Comment: @gerrit All online banking _should_ use 2FA. Many banks, however, do not.
Comment: Please don't SHOUT.
Comment: Surely all online banking uses two factor authentication?
Comment: @duskwuff There should be a plaintextoffenders counterpart for banks, 1FAoffenders...
Answer: Any legitimate employer will need to know the account number and routing numbers for your bank in order to set up a direct deposit. If we are talking about a chequing account, it is information printed on the face of the cheque, so many employers simply ask for a void cheque. That tells them everything they need to know to deposit money into the account but otherwise offers no access. You should never give anyone access to your bank account, online or otherwise - never share PINs or passwords with anyone, for any reason.
As outlined in one of the other answers, this smells like a scam.


Should I self-impose a word limit for a journal article abstract

Question: The author guidelines for the Journal of Computational Physics do not specify a word limit for the abstract, saying only that it should be 'brief'.  But should I impose a word limit myself?  I have heard that some indexing services can truncate overly-long abstracts, though I've not come across evidence that this is this case.
Comment: If they're not imposing a *specific* limit, why should you? That being said, the goal of an abstract is to be a brief snippet that gets potential readers interested in your work. If someone can't figure out in 30 seconds to a minute what you're trying to do, your abstract is probably too long.
Comment: Besides the potential truncation, the closest answer might be a recent (correlative) study, which found that - in contrast to common advice - a larger abstract coincides with a higher amount of citations: http://journals.plos.org/ploscompbiol/article?id=10.1371/journal.pcbi.1004205
Answer: Yes, you should limit yourself because the goal of the abstract is to be short.
Now, where to put the limit?
A good idea would be to look at recently published articles in this journal and note their length.
Comment: Good answer.  Could be strengthened by looking at some abstracts and providing a suggested maximum.  Hertz: Additionally: try reading some abstracts on a small netbook.  If it doesn't stay "above the fold" it's too long.
Answer: An abstract should sell your paper, introduce the family of constructs/theies used and may indicate the key narrative/outcomes. It should end after that so that one needs to read the paper. The better journals usually ask for stricter limits. (Journals of ) Pure sciences probably have editors wanting to allow you to use complete sentences and sensible sentences even in the abstract so skipped the abstract word limit out of politeness..
Comment: Are you saying that one should write the abstract as click bait?


Who introduced the standard 8-bit punched tape, and when?

Question: Who introduced the standard that later became widespread for the 8-bit punch tape, and when? I think the 5-bit tape was an earlier standard.
Comment: It'll be a telegraphy thing rather than a computer thing.  FWIW, though, there were 5, 6, 7, and 8 channel paper tapes.  I skimmed the Teletype material at bitsavers, and it looks like the model 33 (1963-ish) was the first to support an 8-bit code, so that likely sets the approximate timeframe for 8-hole tape.
Answer: TL;DR:
5 hole (*1) tapes were developed as part of Murray's teletype system, the first (successful) to use typewriter keyboards as well as punch tapes. Wider (more holes) paper tapes are compatible enhancements thereof.
There is no sole 'inventor' per se, as the 8 (+1) hole punch with fixed width tape of 1 inch is the logical extension of existing 5 hole format. What can be said is that  the Teletype Model 33 made it a cross platfom standard

The Long Read
The first media to store serial transmitted data on a punch tape was the two hole format developed by Charles Wheatstone (*2) in the 1850's. It was meant to store Morse code. Unlike later formats it didn't store symbols but transmission elements. Dots went into one column, dashes into the other and spaces were simply not punched.
The 5 hole encoding was developed by Baudot in the 1870s, but it wasn't until the 1900s that Donald Murray added a paper tape to his modified Baudot system (*3). Murray's system defined the paper tape as

11/16 inch width
1/10 inch hole spacing (horizontal)
1/10 inch line spacing (vertical)
AWG 13 (0.072 inch) 'data' hole diameter (*4)
AWG 17 (0.046 inch) transport hole diameter (*4)
The 6 hole positions were arranged as

two 'data' holes
one transport hole
Three 'data' holes

Later 6..9 hole systems imply extended that tape with more holes, following the standard, to the 'left' - and of course used way different encodings.
Width of these tapes were specific to each extension. Manufacturer usually extended existing punch designs by adding more holes to the 'left', keeping all transport mechanic (and thus the transport holes) the same. It saved not only cost in design and production, but resulted in great compatibility with the existing 5 hole types. To enhance this some readers were made in a way to use no left stop, not limiting the tape width at all (*5).
In fact, some systems (like certain Friden Flexowriter punches) were intended to use arbitrary wide sheets to allow punching of data along files.
'The' Flexowriter is often mentioned as a first, but beside that there was no single model, but a family of different and often customer specific models, it did use many different formats. Even with an installed 8 hole punch installed, codes used were 7 bit, using the 8th hole at maximum for parity information - or special markings.
Examples with computers are:

LGP-30 Flexowriter
Alwac III Flexowriter
7-level Flexowriter

There were in addition many codes that used constant bit numbers, like 3-out-of-7 (More, CCITT#3) or 4-out-of-7 (CCIR 476).
It wasn't until the 7/8 bit ASCII TTYs, more specific the Teletype Model 33, that made a compatible 1 inch wide tape the  defacto standard for everything up to 8 holes. Not to mention that ASCII as well finally made numeric values the base of symbols.
While early computers used many various formats, next to everything after 1963 went for ASCII and 8 hole paper tapes for their control applications, new mini computers and later micros.

*1 - At that time there were no bits. Holes represented a sequential value, not a numeric one. Numeric interpretation only came with computers.
*2 - Yes, the very same man who developed the bridge named after him, well known to anyone who ever peeked into electronics.
*3 - Baudot's original code was made to be entered using a piano like keyboard with 5 keys, with the letters arranged in a way to ease typing. All text was a continous stream, with out any control characters (beside erase that is).
Murray added a typewriter style keyboard, mechanical code conversion and format characters (LF, CR, BELL) making the output (more) like a typed letter. To improve thruput, typing and sending, as well as receiving and printing was separated b use of a punched tape. That way transmission speed was always maxed put be feeding punch tapes, no longer dependent on typing speed. Similar investment in printer could be reduced, as a single, faster printer could be used to print out paper tapes from several receivers.
To reduce stress on his 5 hole punch tape, he rearranged the letters in a way to give the most common letters the least number of holes, so Baudot's ITA1 became Murray's base for the later ITA2 (which includes changes from Western Union, the first adaptor of Murrays system as well as some CCITT ones).
*4 - Yes, these are not the usual fractional measurements, as the pins used to punch were made from 'wire', so AWG it is.
*5 - In this case usually two transport sprocket wheels were used to avoid misalignment.
Comment: [Wikipedia](https://en.wikipedia.org/wiki/Punched_tape#Encoding) supports the position that teleprinters switched to a 7-bit code when ASCII came along, and I suppose that would be the natural point for 8-hole tape (no reason before that).  Why 8 rather than 7? (Sprocket holes don't count).  Future-proofing. The [Teletype model 33 tech manual](http://bitsavers.org/communications/teletype/33/273B_Mod32_33_TechMan_Jan63.pdf) describes the device as supporting an 8-level code, though "at the present it utilizes only the first seven intelligence elements *(=bits!)*".
Comment: "8-bit bytes" is an entirely separate issue, and that has a well-known answer:  IBM System/360.   It is independent of 8-hole paper tape carrying 7-bit ASCII, and independent of transmission of 7-bit ASCII in 8-bit data frames (plus 1 start bit and 1.5 stop bits)
Comment: @Anixx No, the 8 bit byte was introduced by IBM with the system /360 - although the machine was originally intended to use ASCII - but ASCII design took way too long (as it's known with committees), so IBM went with EBCDIC. And yes, the while I would not call Teletype the 'inventor' of 8 hole paper tape, their Model 33 created the de facto standard everyone followed - not at least because it did so nicely fit the freshly established 8 bit byte.
Comment: Per [this link from Wikipedia](http://www.baudot.net/docs/house--teletype-corp-synopsis.pdf), the 8-channel M35/M33 appeared in 1961, though the quoted  "6 years of development" puts some fuzziness around the date that 8-hole tape was adopted.
Comment: @another-dave it was common with 7-bit ASCII to use the 8th bit for parity, giving you a rudimentary form of error detection.
Comment: Good, but who introduced that 8-bit tape?
Comment: @another-dave so, it is possibly Teletype Model 33 that introduced 8-bit bytes.
Comment: @another-dave [8 hole support] It as well comes rather natural to use a full inch when extending the width.
Comment: @another-dave The whole thing is blurry, as there have been tapes of any size from 2 to 24 holes at that time. Trying to nail it down to a single machine is fruitless.
Answer: The earliest contender I can find is the Friden Flexowriter, from 1958.  It was an electric typewriter with an attached paper tape reader and punch. You can see a video here.  This is a restored Flexowriter, but the visible sign lists 1958 as the year it was presumably built.  As you can see,  this particular model uses 8 channel tape.  The Flexowriter would have been suitable for offline preparation of source code for computer programs.
Soon after, Digital Equipment Corporation introduced the PDP-1 computer, the first computer built by Digital Equipment Corporation.  (prototype 1960).  The PDP-1 had 8 channel (fanfolded) paper tapes with a reader and a punch.
The PDP-1 was patterned after the TX0 computer built in the 1950s by Lincoln Labs.  I am not sure what kind of paper tapes the TX0 worked with.  It did have a magnetic tape drive.
As other answers have pointed out,  ASCII was a later development.
Comment: You can read the MIT produced operations manual for the Flexowriter [here](http://archive.computerhistory.org/resources/text/DEC/pdp-1/DEC.pdp_1.1963.102650623.pdf).
Comment: KDF9 used 8-channel paper tape with 6-bit + parity encoding (bottom of page [here](http://sw.ccs.bcs.org/STRSUBS/CCs/KDF9/orderCode.htm)).  We could use a Flex to prepare Algol 60 input tapes, so I suppose the Flex was producing the same code -- though this is not necessarily true, since device- and even application-dependent code was not unusual then.  Internal code, printer code, paper-tape code, and teletype code were all different.
Comment: If that particular model was indeed a 1958 model, and if the 8-hole tape unit was original on that device, then Friden has a better claim to 'first' than Teletype.
Comment: The first time I ever saw a Friden Flexowriter was in late 1962.  It definitely had 8 channel tape at that time.  The video looks to be of the same model, but I can't tell.  I think the one in the video is 8 channel tape.  For the year 1958,  all we have to go on is the sign in the video.
Comment: The [PDP-1 Handbook from 1960](http://bitsavers.org/pdf/dec/pdp1/F15_PDP1_Handbook_Apr60.pdf) has Flex codes, so 8-channel readers on Flex go back at least that far.  And the photoelectric reader on the PDP-1 is also 8-hole, as you mentioned.  So we're back to 1960.  It would be good to find original Friden docs to be sure that 8-hole readers were in use on the 1958 models.
Comment: While we're on the subject of Flexowriters,  [this document](http://web.mit.edu/digitalapollo/Documents/Chapter5/laningwhirlwind.pdf) shows that the Flexowriter was used in conjunction with the Whirlwind I computer.  Ken Olsen was one of the designers of that machine. Ken went on to found Digital Equipment corporation.  I have no information on the number of channels in the paper tape used with the Whirlwind.
Comment: Let us [continue this discussion in chat](https://chat.stackexchange.com/rooms/133044/discussion-between-walter-mitty-and-another-dave).


Get Praat version in script

Question: Is there a command in the Praat scripting language that will get the user's current Praat version? The reason I ask is that I want to distribute a script that uses vectors, but since vectors were only introduced in the last year, some users won't be able to run the script.
Answer: This line echoes the script version: echo 'praatVersion$'.
If you want to check for the current version you sound do:
versionPraat$ = left$(praatVersion$, (rindex(praatVersion$, ".")-1));
versionPraat = 'versionPraat$' #convert string to numeric

if versionPraat < 6
   exit Please download the most recent version of Praat.
endif

I stole this from a script written by Chun Chan.
Answer: DazedAndConfused had the basic answer: praatVersion$ gets you a string with the version. If you're looking for a way to get the major/minor/build numbers in a way that's robust to Praat's versioning convention (where 6.0, 6.0.01, and 6.1.17alpha are all valid version numbers), use the following:
dot1 = index(praatVersion$, ".")
majorVers = number(left$(praatVersion$, dot1-1))
nonMajorVers$ = right$(praatVersion$, length(praatVersion$)-dot1)
dot2 = index(nonMajorVers$, ".")
if dot2 > 0
    minorVers = number(left$(nonMajorVers$, dot2-1))
    buildVers = number(replace_regex$(right$(nonMajorVers$, length(nonMajorVers$)-dot2), "[^0-9]", "", 0))
else
    minorVers = number(replace_regex$(nonMajorVers$, "[^0-9]", "", 0))
endif

Get Praat version in script


What is the best approach to handle a project with different sub projects?

Question: I am currently handling a project. In the implementation phase of the project, they have 2 business units. the first BU is responsible for the technical requirements, the second is for the admin like billing, etc. From my company's perpective, we only see them as one and we will bill them as one but it looks like they have a conflict. How can I help them streamline their process? What is the best suggestion here?
Comment: Can you clarify please... are you delivering a single piece of work to them, or two separate components, and if two, how do they relate to each other?
Comment: I am delivering a single piece of work and I need to bill them afterwards. The project has 2 units, one is for the technical unit and other is for the billing. Once the project is accepted by this customer (technical unit) and my POC is the technical unit. However, the technical unit doesn't want them to be billed and wants it to go through the billing. I am not sure how to handle this scenario.
Answer: If I am understanding your question and clarification correctly, your company will deliver a technical solution and then you will bill the client for the work. Your concern appears to be that the people within the client's company who will accept the technical part are not interested in the billing: they only want the technical solution. On the other hand, the people who will pay for the solution may disagree with their colleagues about the technical need (or the technical solution). Please correct me if I have misunderstood.
It seems that you anticipate having to act as the mediator between the two parts of your customer's business: this cannot be a good place to be, and you risk being criticised from both sides, or worse. In reality, if the client has internal issues, you can help to guide them, but it should not be your responsibility to solve them or to suffer as a result of them.
Without knowing the specifics, it is difficult to give specific advice, but in principle, I would recommend that the customer should nominate a single point of contact for the entire project. This person would then take ownership of the end-to-end communication between your client and your company (from the client's perspective), and your company should consider providing a similar single point of contact.
Handling the situation may require considerable tact and sensitivity. Your client may not like you suggesting how they should organise themselves, and you may be accused of overstepping your remit, so think about the approach and tread very carefully, even if they recognise the issues themselves. If you can sell the concept in a way that will benefit the client (as well as benefiting you), you are more likely to get a positive outcome.
Answer: This actually sounds like a pretty straight-forward Change Management issue. Your contract 'should' identify both who's responsible for acceptance of the deliverable, and who's responsible for payment (and those terms).
In the absence of that language, I would respond to your clients wishes about who to bill by sending a Request for Clarification to your client identifying the issue and asking for specific guidance. Explain that there seems to be some disagreement or ambiguity about who's responsible for what, and you need to clarify it before you get too far.
In the end, as a contractor, your primary job is to make sure you and your company get paid for supplying the project to the client (and in a timely manner). And you need to make sure that any issues that might hinder that are out of the way -- BEFORE you deliver the project.
Answer: A precision: you have to deal directly with the department of the technical requirements (I understood that your are doing something technical for them, you do something for them if you bill them?) Let that department contact their own billing department and settle their tension/conflict; ask them to settle it before you bill them. Ask for a name and address to send your bill. But no matter what, when you bill, you charge the hole compagny (with the compagny name on your bill); but their accounting/billing department will make you the payment of your bill.


Where does "Let's roll!" come from?

Question: Where does the idiom "Let's roll!" come from?
Answer: The phrase "Let's roll" has been in use as early as 1908, but achieved a special connotation on September 11th, 2001, as they were Todd Beamer's final words before rushing the cockpit on United Airlines Flight 93.
Wikipedia cites a possible origin of the word from 1908:

The phrase may have its origins as early as 1908 in the cadence song now called "The Army Goes Rolling Along", which likely extended into tank usage.

The same Wikipedia page also provides several other uses of the phrase in pop culture, including the 1950 Hemingway novel Across the River and into the Trees and its use in 50s and 60s police television shows such as Adam-12 and (the original) Dragnet.
Comment: I have never heard about Todd Beamer saying it.. and I'm sure many people haven't. I just associate it with like generic action movies.
Comment: @Claudiu: I'm sure many people don't associate it with Todd Beamer. I mean, some kid in India is going to have no clue. But I'd think it would be pretty common knowledge among Americans - especially East coast Americans - who were adults when 9/11 happened. I've updated my answer accordingly.
Comment: @HorusKol: Read the Wikipedia page about Flight 93 - http://en.wikipedia.org/wiki/United_Airlines_Flight_93. The passengers had called various people from the airplane phones and from cell phones prior to the passenger revolt. The last words heard by a phone operator talking to Todd Beamer were, "Are you guys ready? Okay. Let's roll!"
Comment: hm maybe. I mean I lived in NYC when it happened.. but I also generally don't watch the news so much
Comment: since everyone died on flight 93, how does anyone know who actually rushed the cockpit and what they said? surely this is just something they put in to make the film worth watching?
Answer: This Glossary of Colloquialisms mentions "Let's roll" as:

A term to move and start an activity, attack, mission or project.
A symbol of heroism and initiative in danger.

Example:

The phone line from Flight 93 was still open when a GTE operator heard Todd Beamer say: "Are you guys ready? Let's roll!".

It defines its history as follow:

"Let's roll" was in common use on 1950s and 1960s police television shows such as "Adam-12" and (the original) "Dragnet".
But its usage on 9/11 changed its meaning.
Todd Beamer, a passenger on the hijacked United Airlines Flight 93, called his wife on the plane's seat back GTE airfone after the flight was hijacked. Through that phone and other phone contacts with the ground, the passengers learned that two other hijacked planes had been crashed into the World Trade Center.
  As a result, some of the passengers apparently decided to storm the cockpit. Beamer spoke his last known words to the group, overheard via the phone connection: "Let's roll".
The catchphrase became especially known and popular after being used by President George W. Bush in a speech to AmeriCorps volunteers and during his 2002 State of the Union Address. Profiteers soon tried to lay claim to it as a trademark, even though the phrase was in common use long before September 11.
In early 2002, United States Air Force Chief of Staff Gen. John P. Jumper ordered that one airplane in each USAF squadron and all USAF demonstration planes would bear an image of an eagle on an American flag with the words "Let's Roll" and "Spirit of 9-11", to remain until the first anniversary of the attack.
The phrase was also used as the title of a Neil Young song about the flight. It was also used by Lisa Beamer, widow of Todd, in a 2003 book titled "Let's Roll: Ordinary People, Extraordinary Courage".
Comment: Heh commenting again, I was not aware of this at all.

Where does "Let's roll!" come from?


Libgdx change to other screen and dispose current one

Question: I am trying to switch to a GameScreen when the LinearVelocity of a Box2D - Object is x = 0.
If it is so, I am using the setScreen() - Method by calling the main class. This works perfekt, but when the screen should change, it just flickers, which is most likely caused by the render() - method in the screen class.
So my question is now, how to dipose that render method or how to dispose the whole screen class, so that I only the new screen appears!
Thanks
Answer: The libGDX Game class delegates to a single Screen so if your game is flickering between the new Screen and the old Screen after you call Game.setScreen it is likely because the new Screen sets the old screen back.
Comment: Thanks, that was what I supposed to be the problem. Do you know how to fix that?
Answer: I found the solution.
The problem was, that the new Screen was actually empty resulting in an emtpy render - method aswell.


Are multiple reviewers allowed for the same question needing review?

Question: If you are working on a review, are you the only one working on it, or can others be reviewing it at the same time. If so is it then down to whoever gets it done the fastest? Or, once you pick a question to review is it solely yours to review until you either finish or skip? Is there some time-limit in the review process also, I was wondering?
Answer: The system is designed to balance reviews getting done in a timely manner and reviewers not getting too often frustrated by reviews that get completed by others while they already invested time in them.
The short is that at least for certain review tasks  the task is set aside for one reviewer for some time after they start it (If I understand correctly, this is by now the case for all but close and reopen reviews.) You can read up on details starting on this post on the main meta site:
The robo-approvers are killing my will to review edits
See in particular the answers by gnat and Shog9.


Does a raised-cosine filter have infinite support?

Question: I recently asked a question here regarding limiting the bandwidth of a digital baseband signal and was introduced to pulse-shaping using a "raised-cosine filter". At the time, I mistakenly assumed this to be a sinc function combined with the Hann window. This is because the Hann window is also known as the raised cosine window, and that seemed logical to me. However, after reading on Wikipedia, I see that the "raised-cosine filter" is actually another beast entirely. This function appears to be a sinc function windowed by another sinc-like function, with two parameters, β and T. The extent of this function, like sinc, appears to be infinite, for all values of β and T. This thoroughly confuses me, as I thought the whole point of windowing the sinc function was to give it finite support. What am I missing?
Answer: An ideal raised cosine impulse is band-limited with cut-off frequency $f_c=(1+\beta)/(2T)$, where $\beta$ ($0\le\beta\le 1$) is the roll-off factor. Since it is band-limited, it must have infinite support in the time domain. The important difference with a sinc impulse is that its envelope decays much faster (for $\beta>0$, of course).
Note that even though an ideal raised cosine filter has infinite support, in a practical implementation you would use a filter with finite support (implemented as an FIR filter) that approximates the ideal raised cosine impulse in some interval around its maximum.
Comment: So in practice would you window the raised cosine filter with yet another window (e.g. Hann)  when creating the FIR?
Answer: https://se.mathworks.com/help/comm/examples/raised-cosine-filtering.html
" Ideal raised cosine filters have an infinite number of taps. Therefore, practical raised cosine filters are windowed. The window length is controlled using the FilterSpanInSymbols property. In this example, we specify the window length as six symbol durations, i.e., the filter spans six symbol durations."


Not able to Reinstall MacOS in Internet Recovery Mode

Question: I have a mid 2013 Macbook Air.
Without realizing the effect of my actions, I accidentally typed into my terminal a sudo -rm -rf "Xcode" command, where I intended to delete Xcode but in actuality deleted all files that contained anything to do with Xcode. As a result, my Mac software was corrupted, and I am unable to login to my computer (it goes to login screen, after entering credentials it crashes).
After countless hours with Apple Support, I:

Tried reinstalling Mac OS in normal recovery mode (Command + R) but it doesn't let me because it keeps asking me to enter login info for App Store purchases and never closes that screen.
Then I tried Internet Recovery Mode, which for some reason isn't able to reinstall MacOS because of an Internet issue (although it works in normal recovery mode) and gives the error "Could not connect to Recovery Mode server." I read up on this on Stack Overflow, and it was advised to use an Ethernet cable connection, which I have tried, but it still doesn't work. In the Safari in the Internet Recover Mode, it gives me the error "Localized String not Found".
Answer: Do you have access to another Mac? If so, I'd download the OS installer to that machine, and use it to make a bootable USB stick via the createinstallmedia tool which is inside Contents/Resources in the installer's app bundle. Then, boot your Mac from that USB stick and all should be well.
Alternately, connect your Mac to another Mac via target disk mode and install it from a fully-booted macOS installation that way.


How to improve rhythm timing on off beat rhythms

Question: I am practicing Rock You Like a Hurricane by Scorpions for a long time, but I mess up on the timing in one or the other place. In that song, the snare comes at 3rd beat (or 7th if you count 8). And the chorus rhythm is not on the beat. What is the best way to practice such songs? How to keep count and not mess up on the guitar rhythm when listening to drums? (PS I don't have timing problems on 4/4 rhytms where the rhythm falls on beat, it only happens with off beat songs like even Highway to Hell when I play along with other friends)
Comment: I'm a bass player - can't give you enough technical detail to post answer, but in my role I'm continually faced with such problems. For example, take a listen to the opening of BB King's The Thrill is Gone - how and when the bass comes in... At any rate, the best way for me is to listen to the part that I need to play and figure out how that guy is counting in his head relative to the drummer - count off that part within its own context The part your interested in will have it's own time relative to the drums. That means the drummer's 3 might be your 1 etc.
Comment: Counting and practice, they really are key. Even count out loud when practicing so you know you'll hit on the 3rd beat.
Comment: @DrMayhem count on 1st or the 3rd beat?
Comment: Or do I count 1 to 4 each bar?
Comment: Also the strum comes in between beats too twice, in the & between 3rd and 4th beat. I get confused there
Answer: I tell you a few ways how I, as a drummer, would approach this.
a.) You can practice the rhythm you play to a metronome.
The best way is to slow it down and start at a languid tempo where it is easy for you to process (really slow :D I can't stress this point). Playing it up to speed will get easier once it is internalized.
b.) If you do not like the metronome approach, tap quarter notes with your foot and practice your rhythm to it, this will tighten up your internal clock.
c.) You could also use an online drum machine, get the beat played there in it (seems to be quite simple), and practice to that. Also, start slow.
d.) Sing the song and play your rhythmic pattern in the air.
I usually do this to get jazz tunes down.
I sing the main pattern and snap 2 and 4 with my fingers, and a similar approach could work for you. I usually do this on my commute.
e.) Do basic rhythmic practice, take a metronome and try playing on the beats, the off beats- just basic patterns, this will strengthen your time and will help you develop the skills for the song :)
I hope one or more of those approaches appeal to you.
Answer: There's an app called rhythm trainer, I used it to train my rhythm. You can find other similar apps, choose the ones you like and practice everyday. App developers have gamificated rhythm training very well, they have made such kind of training addictive. Have fun with them and become a rhythmic master!


Introductory campaign for Interface Zero

Question: I would like to run an Interface Zero campaign, but it is my first contact with the game and I have played Cyberpunk 2020 years ago. Could you describe some good concept for introductory campaign, that would allow me and my players to gently embrace the concept of the game?
Answer: There are two systems for interface zero. Savage Worlds and True20. In order to provide an answer that fits both concepts, I shall be vague on mechanical details.
The genre of Interface Zero is summarized thus:

What is Interface-Zero?
Interface-Zero is the True20 cyberpunk game from Reality Deviant Publications, combining elements of classic cyberpunk, post-cyberpunk, bio-punk and a touch of Japanese anime.
The world of 2088 is filled with adventure from the formerly-United States of North America, to the state run business arcologies of the New Chinese Mandarinate, from the Deep Net, to deep space and beyond.

The first commandment when running a game that you and your players are unfamiliar with is: Simplicity, simplicity, simplicity. Blatantly steal a plot from a novel. Looking at the TAP, my first thought is to steal the plot of Vernor Vinge's Rainbows End. It's trivial to adapt that world into cyberpunk, and offers a refreshing take on augmented reality separate from the one that the more traditional books (Shadowrun, for example) offer.
Don't start with a campaign. Start with a series of 1-2 session games designed to familiarize everyone with the rules and the settings. Run a game with the TAP, run a game with Corporate Espionage, run a game in the Sprawl. Encourage your characters to make throw away characters for each of these.
Once you've calibrated the groups' expectations and formed either an implicit or explicit social contract, sit down with your players and ask /them/ what they want their game to be about. If you've run vivid (yet simple) games, there will be aspects that have "hooked" them. If they want to be part of an organization, let them forge the history of the organization through a game of Microscope. Especially with cyberpunk (with its many options for "naah, we don't want to do that", you are at your players' mercy, and that's the best way to have it.
Edit
Rough Campaign Map
The Curious Case of Edwin Rashomon
Roughly modeled after Rashomon, it's a great way to have the players explore a scenario with different characters (across multiple games) but a consistent theme. Watch the movie first.
Act 1: The court case, an exploration of a TAP enabled crime scene.
This first game, players will be corporate types, hackers, tagging crime scene for a criminal case. They will chat with the various investigating officers, and play with the TAP rules to create a scene for the court. You'll want a hacker, an icon, and a technician at least, and play them off the competing lawyers.
Act 2: The Husband's tale
The bodyguards and troubleshooters of the husband. They'll explore corporate politics and some troubleshooting. End the act with the encounter with the bandits.
Act 3: The Bandit's tale
Gritty gangland. A gang, taking the role of the bandit, doing shady work with their own views on what really happened. End the act with the encounter with the husband and wife. Make sure that the players can play the encounter out the way they want to, identical results are not very interesting.
Act 4: The Wife's tale, reconstructed
TAP and hacking. Get a visual reconstruction of the event from the wife's implant and associated devices. Lots of hacking, some palm-greasing. End the encounter with explorations from different feeds, presenting the wife's view.
Act 5: The court case. Let each player take a different side and have them argue it out.
This is designed so that you can show the players many different and vital aspects of the setting around some consistent narrative, but it may be a bit ambitious.
Comment: I am using Savage Worlds edition.
Comment: Are my suggestions sufficient? Or do you have any more direct questions?
Comment: Well, I'd love to see a simple, one paragraph premise of a short campaign (even 2 - 3 session long) you would use.
Answer: Here's an old favorite.  It assumes the PCs are on the wrong side of the law, and are a group.
The PCs come across a top-flight industrial espionage guy dying in an alley.  There's another such guy there, too; they've just killed each other.  In between them lies a dataROM/diskette/whatever; it's obvious they fought over this diskette.  The cops are coming. Like, NOW.
Obviously, that ROM is worth a lot of money.  If the PCs don't pick it up, they are stupid and you should not play with them any more.  Once they do pick it up, the following things become evident:

The ROM is protected by some totally high-end black ICE, and it'll take some real chops to crack it;
A lot of people are looking for this thing -- everyone from fellow hustlers and shady-types to expensive corporate retrieval teams;
The ROM came straight from the desk of a high-ranking corporate VP in some corp's weapons/ICE/etc. division; and
They're gonna hafta hustle to keep this thing, open it up, make a buck from it and not get killed.

Let your players plot and scheme and call in favors and have cool fights, which gives you a handy excuse to lay out your cyberpunk setting for them.  In essence, they take a mostly-self-guided whirlwind tour of whatever dystopian near-future you've devised for them.  Amp up the stakes, ramp up the danger...you know.
Finally -- finally...!  Finally, the PCs manage to crack the ROM, preferably during/immediately following the biggest fight in the game (I always pictured the end of Leon aka The Professional, with a healthy heapin' dose of the subway chase from The Warriors).  What's on this?  What cost so much blood, so much money?  What caused all these betrayals, all this mayhem?
It's the corporate guy's home videos of his grandmother, the last time he saw her, before she died.
Comment: I played similar thing, but with ritual scorpions of some corporation for CP 2020. I remember some lost limbs and calling favors to get hospitalized in some private clinic. It was nice.