Under what circumstances does TAM charge their $75 rebooking fee?

Question: I'm looking at a flight on the TAM (Brazil) website. It's a decent price, but it lists "Rebooking" at $75 more. Does that mean that if weather or their screw-up or something else interferes with the flight that they will take $75 more to put me on a different flight?
Comment: This should be laid out in their Conditions of Carriage, an English version of which is available [on the LATAM website](http://www.lan.com/en_us/sitio_personas/conditions-to-the-contract-of-transportation/).
Comment: No mention of such a fee there, but at the bottom was a link to "optional and additional fees" which also did not list such a fee, but it did have a fee with a name that sounded similar.  The amount of that one was $35.  Then I thought I would go through the booking process but stop short of payment to see whether I could get more details.   The flight search results had a completely different layout and the price had more than doubled!
Answer: Rebooking fees are usually only for changes you request them to do, not for involuntary changes caused by delays, weather, mechanical issues, etc.
Comment: @WGroleau the fee only applies for voluntary rebooking (at your own request). IIRC, LAN does not apply non-resident "fees" inside Chile, instead it price-discriminates on point of sale. However to be sure of this check the "Sales Restrictions" category in the fare rules, which can be seen on the Matrix on the final page.
Comment: Yes, the "rules" say sold only in Peru to Peruvian residents (as I expected).  If I change the sales city from default to Dallas, $258 is added.  The difference is odd, considering LAN and TAM are really the same company.  Matrix doesn't even show TAM, but that's probably because the TAM website said that LAN was operating the flight.
Comment: Hmmm.  Well, I can't risk $75 on a "usually."  Star Peru offers a lower price, but the Visa Verification has crashed three times already.  Matrix says LAN has a great price, but I know they'll add a huge non-resident fee to that.
Comment: Hmmm.  Went to TAM again, same search.  This time, it's in Portuguese, and what was "rebooking" does translate to "changes by you" and is US$50 instead of 75.  But the fare is now in reales, and converts to the higher price.


$X:=\{z \in \mathbb C : |z|\le 1\}$ ; is $\{p(z,\bar z) | p(x,y) \in \mathbb R[x,y] \}$ dense in $C(X , \mathbb C) $ under sup metric?

Question: Let $X:=\{z \in \mathbb C : |z|\le 1\}$ , is it true that any element in $C(X , \mathbb C)$ can be uniformly approximated by polynomials , in $z, \bar z$  , with real co-efficients ? If we wanted uniform approximation by polynomials in  $z, \bar z$  , with complex co-efficients then I know it would be true by Complex version of Stone-Weierstrass theorem . But I have no idea what happens if we want real-coefficients ( I can't see whether the set $\{p(z,\bar z) | p(x,y) \in \mathbb R[x,y] \}$ is a subalgebra over $\mathbb C$ or not ) . Please help . Thanks in advance
Comment: Unless I'm mistaken, the function $p(z, \bar{z}) = iz$ is a good example to have in mind.
Comment: @CameronWilliams : what to do with that polynomial ?
Comment: Can you approximate that function?
Answer: To expand on the counterexample $p(z, \bar z)=iz$ given by Cameron Williams, note that  when restricted to a subinterval of real line, such as $[-1,1]$, the set $\{p(z,\bar z) | p(x,y) \in \mathbb R[x,y] \}$ reduces to $\mathbb{R}[x]$. Clearly, any function one can approximate by the elements of $\mathbb{R}[x]$ must be real-valued on $[-1,1]$, and this does not include $iz$.


Bumblebee not working on Ubuntu 14

Question: Recently I installed Ubuntu on an Asus N46VM, which contains NVidia Optimus technology. Before using Ubuntu, I had used Fedora 20 with Bumblebee due to the support for Nvidia board, it worked without any problem. However I've tried to use Bumblebee on Ubuntu 14, but I've been getting no success until now. I've followed the steps to install Bumblebee described at https://wiki.ubuntu.com/Bumblebee#Installation, I can install successfully Bumblebee on Ubuntu, but it seems that nothing happens when I try running glxgears with a optirun call, the FPS I get is the same as glxgears running without optirun. I have also tried updating the NVidia's drivers, but nothing happens as well. What is going on ? I think Bumblebee should be supposed to work on Ubuntu as well as on Fedora, but I'm getting some troubles with Bumblebee on Ubuntu 14.
Answer: Instead of messing with bumblebee, I would suggest using the nvidia-prime package. It works seemlessly to allow you to choose which GPU you want to use for your desktop session. If you use prime-indicator as well you get an indicator in your tray (on the top right) which gives you quick access to change which GPU is being used.
Bumblebee is a bit outdated now and more or a pain to set up than the benefits it grants you.

Bumblebee not working on Ubuntu 14


What is the easiest way to spare Mettaton EX?

Question: I'm stuck on the Mettaton EX fight. Is there an easy way to beat him in a Pacifist run?
Answer: While it is not required to stay on the Pacifist path, the fastest way to defeat Mettaton EX would be to do so without blowing off his arms and legs, which means you will need to reach a Show rating of 12000 points, as explained in the Mettaton battle wikia page:

To defeat Mettaton EX without killing him, one must survive until his arms and legs are blown off and achieve a show rating of 10,000 or more; if his limbs are not blown off, a show rating of 12,000 or more will end the battle.

The page also lists ways to increase your rating. Here are some easy ways to do it:

Eating food sold by Burgerpants will give 300-500 rating points, but the Steak in the Shape of Mettaton's Face will give 700 points instead.
Early in the battle, Mettaton will ask the protagonist to write an essay about what they like most about him.

Writing "LEGS" earns 350 points, which is the highest amount, being the 'correct answer.'

Equipping a different piece of armor will give 1,500 points as long that piece has not been worn previously during the fight.
Using the Stick will cause it to be thrown at Mettaton. He will catch it in his mouth and ratings will boost by 700 points. Repeating this action will give 1 point. Using the stick after Mettaton has lost his arms and legs will instead give 1,400 points.

Thus, the absolute fastest way to get to 12000 Rating is to give the correct answer ("LEGS") and to constantly switch your equipped armor (keep in mind that they have to be pieces of armor that you have not already equipped during the battle).

In addition, having low health seems to be a fast way to boost your Rating, though if you have difficulty surviving this fight, you may want to restrict yourself to the previously mentioned methods.

Getting hit gives a violence boost of 10 to 50 points.
Using the Pose action will give 100-1000 points, inversely proportional to the amount of health the protagonist has left.
Comment: Boasting gives a good boost as well on attacks you can successfully avoid all the attacks on.  Even when someone is having difficulty, the order of the phases is always the same so you can time your Boasts for things like the break time for a bunch of free ratings :)
Comment: @Fambida thanks, originally I had them together, but I guess when I was copying more things over, I accidentally inserted something in between them
Comment: Blowing off his arms and legs doesn't kick you out of the Pacifist path.
Comment: @Powerlord, I knew that, but I can see how it was unclear in my answer. I've edited it to make it clearer, thanks for pointing that out!
Answer: In addition to the answer up there, if you're very good at avoiding the attacks, there is the "Boast" option in the Act menu which steadily increases the ratings during Mettaton's turn
But beware, if you get hit, you lose a lot of ratings
Answer: The only way to spare Mettaton in a pacifist run is to "fight" him using the Act menu. Your heart cursor will be the yellow shooter type, and you have to shoot Mettaton's heart when it is available. After enough hits, his arms and legs will fall off, and you'll be able to end the battle. There is no avoiding this fight - it's required if you want the pacifist or true pacifist ending.
You can of course fight him normally with the Fight menu, but that will actually kill him and pull you from the pacifist route, so you probably don't want to do that.
Comment: This isn't quite true, see the [wikia page](http://undertale.wikia.com/wiki/Mettaton/In_Battle).


Drop Oracle data file with 0 bytes and in offline (needs recovery) state

Question: What can I do to drop a data file mistakenly created and currently with 0 byte and in offline (needs recovery) state?
Answer: You can not drop the datafile. You can move it to a better location and keep it there. If you want to get rid if it, re-create the tablespace and copy the data to the new tablespace. Drop the old tablespace and rename the new tablespace. Don't forget to give quota to your schemas similar as they have on the old tablespace.


Videos playing very fast without audio Linux Mint 17

Question: In Linux Mint 17, videos are playing what looks to be 4x normal speed.  This includes all of youtube on both Firefox and Chrome.  I believe it to be 4x because if I set the youtube speed to be 0.25x it looks about right.
Answer: Open up sound preferences by going to Menu > Preferences > Sound

Once there, click on the "output" tab and read the list of devices.  For me, the problem was that Mint was trying to use the first choice (my graphics card which only has DVI) as the audio output.  Clicking on other selections fixed the problem.


Find when device was activated

Question: I am trying to figure out when my devices (tablet, tv dongle, etc) were activated. Is there a place (in google account settings, device files, anywhere) this data can be obtained?
My devices are rooted, so I can use terminal access to read internal files if needed.
Some suggestions in the net say to find the first day of data usage in the logs, but I want to find an accurate date (preferably a timestamp) of the device activation.
I can't seem t be able to type dialer codes in a tablet, there's no phone app. Also this question is about android devices in general, not only phones.
Comment: I can't find anything in the build.prop myself, but as a workaround you could try: If you connect the handset to a PC and mount the internal storage as a removable drive you can check the last-modified date on its directories. Check ".temp" and "ShareViaWifi, as I believe both of those are created during initial setup and aren't modified later.
Comment: @Krampus its not only for phones, its for android in general (android TV, tablets, etc) - so its a related question, but not a duplicate.
Comment: The other question doesn't need to be specific to phones, either.  I think it's a dupe.
Answer: If the date of activation matches the date of adding your Google account to the device, there's an easy way to check (unless you did a factory-reset in the meantime, and had to re-add the account):

with your Google account's credentials, log in to the Google Dashboard
Scroll to the section labeled "Android" and expand it (by clicking its title / the LGM [little green man])
Check the entry for the device you want to know the date for

At this place you should find the date you registered your device. That doesn't reflect the first time a device was added, but the last time the Google account was activated on the device. So if you activate your Google account following a factory-reset, this field is updated and thus reflects this point-in-time.
Comment: Yes, ANDROID_ID is recreated when device receives a new firmware. And it has no guarantees that it will be unique. I ended up with someone else's farm on a game because of duplicated android_id.
Comment: I think its as good as it will get, because if the device has been wiped / firmware updated, it will show the same date anyway. My P1000L has had a firmware update, and it shows in google only the date of the update, not of the first activation. It also sheds some light on how G indexes your devices, it seems to be by IMEI number. (otherwise it would list two P1000 in my account.
Comment: Both correct: 1) it's the day you initially connected your device to the Google account (most likely using the `ANDROID_ID`, if I remember correctly, which is "overwritten" (or re-generated) with a firmware upgrade), and 2) [it's the IMEI](http://android.stackexchange.com/a/49425/16575) as well/additionally.

Find when device was activated


Help understanding John Lee's definition of curvature

Question: On page 3 of his book Riemannian Manifolds, John Lee states the following
If you want to continue your study of plane geometry beyond figures constructed from lines and circles, sooner or later you will have to come to terms with other curves in the plane. An arbitrary curve cannot be completely described by one or two numbers such as length or radius; instead the basic invariant is curvature, which is defined using calculus and is a function of position on the curve.
Formally, the curvature of a plane curve $\gamma$ is defined to be $\kappa(t) := |\ddot{\gamma}(t)|$, the length of the acceleration vector, when $\gamma$ is given a unit speed parameterization.
Two Questions:
(1) Regarding the first paragraph, why isn't length sufficient to study curves?
(2) What is a "unit speed parameterization"? Does he mean velocity or speed (ie $v$ or $\|v\|$)?
Answer: 1- If length were sufficient a circle of length (perimeter) $2\pi$ would be the same as a square whose side is $\frac{2\pi}{4}$.
2- a unit speed parametrization is defined as follows: if $x(t)$ is an arbitrary parametrization take $s$ to be defined by $\frac{ds}{dt}=\vert\vert x'(t)\vert\vert$. You check that $\vert\vert \frac{dx}{ds} \vert\vert=1$. $x(s)$ is the unit speed parametrization of the curve


How can you tell between what is a "distributed" system versus "grid" computing versus "cluster" computing?

Question: These terms seem to get thrown around rather vaguely, in my opinion, and was wondering if there were some hard-lined facts about what accounts for which category in these fields. If there aren't any, then, any heuristics for determining how to categorize a system would be helpful :)
Comment: @KaitlynMcmordie: You can simply delete your own question and re-post it elsewhere.
Comment: Related: [What is "distributed computing" as a field of computer science?](http://cstheory.stackexchange.com/questions/8144/what-is-distributed-computing-as-a-field-of-computer-science)
Comment: I think the question @JukkaSuomela links is barely on topic, and this one is off-topic. I don't see a research level question in *theoretical computer science*. I would suggest asking this on a website like programmers.SE, but I am not completely familiar with their scope, so check the scope before asking.
Comment: @KaitlynMcmordie: Only the moderators can migrate a question to Programmers.  I do not know if the moderators will migrate this question or not, but if not, please read [this post](http://meta.cstheory.stackexchange.com/questions/1285/to-migrate-or-not-to-migrate-just-close/1292#1292) for a possible explanation why the moderators often choose not to migrate questions.
Comment: Can you move it over to either Programmers or one of the other S.E. sites?
Answer: The grid computing paradigm emerged as a new field distinguished from traditional distributed computing because of its focus on large-scale resource sharing and innovative high-performance applications such as:

distributed supercomputing;
on demand computing;
high throughput computing;
data-intensive computing;

Distributed supercomputing applications require multiple supercomputers to solve problems otherwise too large or whose execution is divided on different components that can benefit from execution on different architectures. This class of applications present a number of challenges to be faced, like resource discovery and scheduling, coordinated startup, configuration at multiple sites, wide area message passing and fault tolerance. An example of such application is SF-Express, a distributed interactive simulation of a military battle.
On demand computing refers to the possibility of dynamically acquiring online instruments (e.g. microscopes, satellite sensors and telescopes) connected by high-speed networks to gather and process the data generated. An experiment of microtomography at photon sources in 1999 demonstrated the feasibility of on demand computing.
The aim of high throughput computing (HTC) is to schedule many independent jobs for parametric studies or data analysis; in this case a measure of efficiency is the number of jobs processed per unit of time. The two most important tools for HTC, namely Condor and Nimrod are now grid-aware.
Data-intensive applications extract new knowledge from geographically distributed data archives or digital libraries; issues related to this class of applications include scheduling and configuration of multiple data flows through several hierarchy levels.
Finally cluster computing refers to the use of a cluster, which is a parallel computer, for running parallel scientific simulations. Therefore, when using just a single cluster you are doing parallel computing; when using more than one simultaneously but for the same application, may be with different architecture, you are doing distributed supercomputing, so this is grid computing. Platform computing, the company producing the LSF scheduler, promotes its LSF multi-cluster capabilities as grid computing.
Comment: @Kaveh: think of grid computing as a subset of distributed computing. If you believe that distributed computing belongs to TCS, then there is an obvious relation, the same that occurs, for instance between analysis and numerical analysis. The latter not only provides a practical way to compute things, it is based on theory that formally guarantees the correctness of results. The same happens with grid computing: the underlying theory (from distributed systems) allows practical use of very large-scale distributed applications.
Comment: interesting answer, but I don't see the relation with TCS.
Comment: are there different theoretical models associated with each of these or has this distinction been considered only in practice? It would be nice to mention them if there are different theoretical models for them.
Comment: No, there are not distinct theoretical models. It's a distinction that arise in practice, since, once you solve a problem theoretically, when you move to actual implementation you will often face new problems strictly related to the one you started with, that must be solved in order to make things a reality. Here is an example, taken from a different domain: say you want to go from Earth on the Moon. From a theoretical perspective, the problem is solved by computing the escape velocity, that  must be >= 11.2 km/s. However, this is clearly not enough: now you have to actually build a rocket...


Is treated bamboo, kosher for schach please? (Treated to prevent ruin by water.)

Question: Can anyone please tell me if bamboo that has been treated to withstand water, is kosher to be used as schach? I've heard divergent opinions.
Many thanks and G-d bless.
Comment: Why would you think that if it's treated that makes it invalid? If you could cite your sources that would greatly improve this post.
Comment: https://judaism.stackexchange.com/questions/53098/pressure-treated-lumber-for-schach
Comment: @DanF The concern for painted wood [or other materials] is whether or not once painted it is considered a new object and no longer maintain its original, natural character. However, R. Ovadiah Yosef (Chazon Ovadia Sukkot, pg. 32) rules that such additions are permitted altogether and in no way presents a problem, nor a problem of chatzitzah (barrier).
Comment: Hi Jacqueline R and welcome to Mi Yodeya. You've asked an interesting question, here. I think Gerhson's link would provide an answer by deduction. It seems that one of the main requirements is that the schach be made from a naturally growing material, which is what bamboo is, of course. Treated lumber may be used, so I can't see why treated bamboo would be any different. I gather that painted wood may not be used, but, I'm uncertain about that, or why the painting would invalidate it.
Comment: Don't forget that, like any library, Mi Yodeya offers tons of great information, but [does not offer professional advice](//meta.judaism.stackexchange.com/q/1734), and does not take the place of [seeking such advice from your rabbi](/q/9146).
Answer: The issue with materials that are "treated to withstand rain" would be if gezeirat tikra (no differentiation from a roof; see Sukkah 14a) applies to it. Some are of the opinion that only material that are structurally similar to a roof (e.g. wide planks of wood) are subsumed in this gezeirah (see responsa Shaarei Yosher 1:28 and works cited there). As indicated in the question, there are different opinions and so one should consult their LOR.


Is flap down the same as flap extended to 25degrees?

Question: Is Flap down command is same as setting the flap angle to 25degrees. Recently I am reading the information related to Flap and got confused with different terminology. Trying to get relation between flap angle and Down/Up commands. On the same line I read that EGPWS uses the flap down signal, and why EGPWS need this signal.
Comment: It isn't clear to me what you are asking.  Most aircraft, even simple GA airplanes, have several different angles they can be set at.  So, for example if the flaps were up, you could command them down to 10 degrees, or 25 if there was such an increment on your airplane.  I don't know what this has to do with EGPWS other than the tone might be inhibited with flaps down under the assumption that landing is intended.  What specific airplane are you asking about?
Comment: Hi @MichaelHall, just trying to understand the impact of flap signal on EGPWS behavior and at the same wanted to get my self clarity on what angles are flaps down and up. Thank you for the information. Now I got clarity.
Comment: Hi @ManuH I corrected the tittle, thanks for pointing that out.
Comment: Thanks, I was able to discern that from the answer.  Freeman is smarter than me!
Answer: Depending on the aircraft, flaps can be set at one of several different settings anywhere from 5° to 40° (or more, from what I understand). All of these settings are considered "down". (The exact flap angle depends on the aircraft, current weight, air temp and other factors and more than one is "appropriate" for take off or landing depending on these conditions.)
The EGPWS (Enhanced Ground Proximity Warning System) needs to know whether flaps are up or down (among other things) so that it knows if the plane is configured for landing. If you're 50' AGL and configured for landing, the EGPWS is going to assume that your intent is to land, so it won't tell you "pull up". If you're not configured for landing, but you're still 50' AGL, it's going to yell at you in an attempt to keep you from flying into the ground.
Comment: Thank you @FreeMan for the answer, this clarified my doubts.


jquery datetime picker

Question: Hi
I attach the jquery datetime picker to a textbox using the following (please note that I have specified the format to be dd/mm/yy):
<code>$('#SupplierSearch_StartDate').datepicker({ dateFormat: 'dd/mm/yy' });
</code>
This all works fine on the form. But when I submit me form back to the server, the date that comes back is in the format mm/dd/yy (set on a property in my model. I am using ASP.Net MVC)
Below is my Html help code to generate the textbox:
<code><%: Html.EditorFor(model => model.SupplierSearch.StartDate) %>
</code>
In my model I have applied the following attribute to the property:
<code>[DisplayFormat(DataFormatString = "{0:d}", ApplyFormatInEditMode = true)]
public DateTime StartDate { get; set; }
</code>
Can anybody please advise me on how I can get this fixed or point out what I am doing incorrect or missing out
Thanks
Answer: Try this, and here is some reference material http://msdn.microsoft.com/en-us/library/8kb3ddd4.aspx
<code>[DisplayFormat(DataFormatString = "{0:dd/MM/yy}", ApplyFormatInEditMode = true)]
public DateTime StartDate { get; set; }
</code>

jquery datetime picker


Relativistic generalization of Quantum Harmonic Oscillator

Question: I am trying to find out relativistic description of a quantum harmonic oscillator.
For a classical relativistic oscillator mass is a function of co-ordinates(http://arxiv.org/abs/1209.2876).
$$m(x)=m-\frac{m\omega^{2}x^{2}}{c^{2}}.$$
I want to get a quantum mechanical model for this particular hamiltonian
$$H=\sqrt{p^{2}c^{2}+m(x)^{2}c^{4}}.$$
First, I thought of re-writing this hamiltonian with the help of $\alpha$ and $\beta$ matrices. I got
$$H=\vec{\alpha}.\vec{p}+\beta m(x).$$
But by adopting this prescription implies wave function should be $4$-component spinors. How can I get $4$-component spinors which would reduce to $1$-component wave function in non-relativistic quantum mechanics?
If someone can suggest some other methods to solve for the hamiltonian that would also help.
Comment: Just a terminological comment, not just questioning your wording but the wording in the aforementioned paper as well. "Harmonic" is something that is composed of sines and cosines (the solutions to the motion of the oscillator in this case). If the solutions aren't sines and cosines, e.g. because the energy isn't simply quadratic, one shouldn't call it "harmonic" oscillator. It's a relativistic generalization of the harmonic oscillator.
Comment: You are right.Changed it.
Comment: Your hamiltonian should have $m^2 c^4$. Also I would suggest that you can try to find (and I believe you will succeed) the eigenvalues and eigenstates of the operator under the square root. On the first thought, $H=\sqrt{Q}$ and $Q$ should have the same eigenvectors and correspondingly related eigenvalues. Note the reasoning behind the Klein-Gordon equation. You should also keep in mind that you should be carefull with physical interpretation of your results -- relativistic QM is not totally physically consitent and should be replaced with QFT.
Comment: Moreover, I think that your hamiltonian in $\alpha$ and $\beta$ matrices is not obviously the correct square root -- $m$ and $p$ do not commute. Finally, these matrices are some non-commuting operators, not initially present in the theory -- you are introducing new degrees of freedom, specifically spin. Just thought that it can be not exactly what you want.
Answer: One could include harmonic potential in Dirac equation in the usual way (see, e.g., here):
$$
\left(\gamma^\mu\left(i\hbar\partial_\mu - eA_\mu\right)-mc\right)\psi=0
$$
or in more mundane notation (and stationary in time):
$$
\begin{pmatrix}
mc^2-E+e\phi & c\mathbf{\sigma}\cdot(\mathbf{p}-e\mathbf{A})\\
-c\mathbf{\sigma}\cdot(\mathbf{p}-e\mathbf{A}) & mc^2+E-e\phi
\end{pmatrix}
\begin{pmatrix}
\psi_+\\
\psi_-
\end{pmatrix}
=
\begin{pmatrix}
0\\
0
\end{pmatrix}
$$
where $$e\phi = \frac{kx^2}{2}$$
Remark
Although relativistically one might be justified interpreting the potential energy as mass, one should not be misled by the usual  notation for the spring constant $k=m\Omega^2$, which is just a matter of convenience.
Answer: The answer is simple, you need to use the Hamiltonian
$$H=mc^2(u_0^2-\sum_{k=1}^3u_k^2)+\frac{k(x_0^2-\sum_{k=1}^3 x_k^2)}{2}$$
Then we get 4 independent standard problems about the harmonic oscillator
Comment: The formula $m(x)=m-\frac{m \omega^2 x^2}{c^2}$ is not relativistically invariant and therefore not correct, the correct formula is the dependence of energy on velocity, which is done in my formulas.


Retail Algorithmic Trading

Question: Is it possible for the retail algo traders to take the same approach to risk management as the larger quant funds? is there a risk management budget imposed on the trader beyond that which they impose on themselves, or is there a compliance or risk management department that enforces this?
Answer:
Is there a risk management budget imposed on the trader beyond that which they impose on themselves, or is there a compliance or risk management department that enforces this?

Usually there's multiple layers of risk checks that gets looser or more general as you move more distance away from the trader. Depending on the size of firm, risk and compliance may refer to the same group, but there's enough differences between the two for the groups to be separate - compliance can just be involved with legal work (e.g. preparing quarterly and annual filings to regulators) whereas risk would be involved in the trading (e.g. setting exposure limits). Depending on the firm's strategy style, a "trader" might only be a warm body to keep an eye on screens and make small tweaks to risk parameters live based on market activity.
As for whether it's possible - well it's not really something to aspire to. Generally large quant funds have these multiple bureaucratic safeguards because they need to protect themselves against rogue actors. If you are a rule-abiding and successful portfolio manager, researcher or trader at a quant firm, compliance and risk are often viewed with a derogatory mindset.
Likewise, if you're working on your own, there's no one to protect against besides yourself. If I were a retail trader, I would take advantage of the nimbleness of the operation and figure out where I can have additional efficiencies here that are not possible in a bank.

Retail Algorithmic Trading


Does the sentence imposed by God, during wilderness wanderings, fit the crime?? The crime: wood gathering on the Sabbath-Numbers 15:32-36

Question: God's no nonsense judgement against working on the Sabbath of being "Stoned to death" seems awfully harsh on the post Exodus, most likely long suffering, wilderness wanderer, who was merely gathering wood for the burning thereof. Surely he should have been allowed to at least atone for his seemingly forgivable sin/error and yet he wasn't so allowed.
Comment: Sounds a lot like Rom 6:23, "For the wages of sin is death, but the gift of God is eternal life in Christ Jesus our Lord."  Perhaps the man refused to repent.
Comment: Related: [Why were the elders of Israel uncertain about the man picking sticks on the Sabbath?](https://hermeneutics.stackexchange.com/questions/37703/why-were-the-elders-of-israel-uncertain-about-the-man-picking-sticks-on-the-sabb/37737#37737)
Comment: @Dottard - He may well have refused to repent, especially if it happened some 30 plus years into the sojourn.
Comment: **Harsh?**-Similarly,  how long should a leak on a wooden sail boat  be allowed to go Unchecked? If not dealt with, no matter how small the leak, would not the lives of the whole ship's passengers and its crew be jeopardized?
Comment: @raygrant - I'm not sure that this is a fair analogy, Ray, but you have a point, at least as far as an unchecked leaky boat is concerned.
Answer: The punishment does not fit the crime by human standards. The traditional justification for the punishment often involves the idea that the law was plain and there can be no excuse for violating it.

The law of the sabbath was plain and positive, and this transgression
of it must therefore have been a known and wilful sin. And from the
connection of this verse with the former it may be justly inferred
that this man had sinned with a high hand, despising the word of the
Lord, and the authority of his law.
Benson

However, there is little if any evidence that this standard of justice was imposed generally in Israelite courts. In that sense, some observers consider the incident as an object lesson intended to emphasize the seriousness of keeping the sabbath holy, rather than as precedent to punish every minor violation of the sabbath law. Indeed, later prophets often had to remind Israel and Judah of the law because it was not enforced (Jeremiah 17:21,Isaiah 56:2, Ezekiel 44:24 etc.). Thus Keil and Delitzsch affirm that:

The History of the Sabbath-Breaker is no doubt inserted here as a
practical illustration of sinning "with a high hand." It shows, too,
at the same time, how the nation, as a whole, was impressed with the
inviolable sanctity of the Lord's day.

By way of contrast, the prophet Isaiah's approach to Sabbath observance accentuated the positive:

Thus says the Lord: Observe what is right, do what is just,
for my salvation is about to come,
my justice, about to be revealed.  Happy is the one who does this,
whoever holds fast to it: Keeping the sabbath without profaning it,
keeping one's hand from doing any evil. (Is. 56:1-2)

Conclusion: the punishment here fits the crime only in the sense that a willful public disobedience of God's law cannot be tolerated. However, the purpose of the story is to impress the people of God that the Sabbath must be kept holy, not to impose a rigid system of justice in which anyone who prepares or lights a fire on the Sabbath must be put to death.
Comment: Of course, we are not told how far into the 40 years this supposedly willful act of disobedience occurred. There would be little excuse, if any, if this *gathering* had been done within say a year or two of the 40 year sojourn, but what if it was 30 plus years in. Patience is a virtue but .... Good answer though, + 1.
Answer: Before answering this question, we should recall that the ultimate penalty for ALL sin (no matter how minor) is death according to Rom 6:23 -

For the wages of sin is death, but the gift of God is eternal life in
Christ Jesus our Lord.

We also have this in Heb 6:4-6

It is impossible for those who have once been enlightened, who have
tasted the heavenly gift, who have shared in the Holy Spirit, who have
tasted the goodness of the word of God and the powers of the coming
age— and then have fallen away—to be restored to repentance, because
they themselves are crucifying the Son of God all over again and
subjecting Him to open shame.

Further, almost all the sin and guilt offerings as described in the Torah are for unintentional sins, Ex 21:13, Lev 4:2, 13, 22, 27, 5:2, 3, 15, 17, 18, 22:14, Num 15:24, 25, 27, 28, 29, 35:11, 15, etc, etc.
The punishment for intentional Sabbath-breaking was execution:

Ex 31:14 - Keep the Sabbath, for it is holy to you. Anyone who profanes it must surely be put to death. Whoever does any work on that day must be cut off from among his people.
Ex 35:2 - For six days work may be done, but the seventh day shall be your holy day, a Sabbath of complete rest to the LORD. Whoever does any work on that day must be put to death.

However, these laws were only VERY rarely enforced.  Benson observes this:

Numbers 15:35. The man shall surely be put to death — One reason why
the breach of the sabbath was punished with such severity by the
Jewish law is, that it was an implicit denying of God to be the
Creator of the world. For the sabbath being a sign, (Exodus 31:13,)
whereby the worshippers of the one true God, who created the world,
were distinguished from the idolatrous nations, who believed the world
was eternal, and who worshipped the sun, moon, and stars, and a
multitude of nominal gods, the violation of this institution implied
or led to a defection from the true religion to polytheism and
idolatry.

Now, the narrative in the story contained in Num 15 does not record all the details.  For example, we do not know if the guilty man refused to repent, despite being given opportunity to confess before-hand. See appendix below for some other examples.
Lastly, at the establishment of the new nation of Israel, law and order was the primary concern and the seriousness of sin had to be emphasized.
APPENDIX - Death penalty for Sin
Here are a very few examples of people being executed for unconfessed and deliberate sin:

Achan for the sin of embezzlement as per Josh 7:16-26
Ananias and Sapphira for the sin of lying, Acts 5:1-11
Stephen for (supposed) blasphemy, Acts 7:54-60
Woman taken in Adultery, John 8:1-11, but Jesus prevented it.
Korah Dathan and Abiram and many others for the sin of usurpation of priests, Num 16

There are several more plagues sent among the people.
Comment: Another good answer for sure. + 1.
Answer: It is a good and difficult-to-answer question.
Approximately one and a half millennia after the Numbers stoning, the same immutable God was fully revealed in Jesus Christ (Hebrews 1:3).  The main NT narrative reveals a God willing to forgive, who loves enemies, and who is essentially non-violent.
Christ in John 8:1-11 handled analogous circumstances of an adultress due for death by stoning under the Law:
John 8:1-11:

1 Jesus returned to the Mount of Olives, 2 but early the next morning he was back again at the Temple. A crowd soon gathered, and he sat down and taught them. 3 As he was speaking, the teachers of religious law and the Pharisees brought a woman who had been caught in the act of adultery. They put her in front of the crowd. 4 "Teacher," they said to Jesus, "this woman was caught in the act of adultery. 5 The law of Moses says to stone her. What do you say?" 6 They were trying to trap him into saying something they could use against him, but Jesus stooped down and wrote in the dust with his finger. 7 They kept demanding an answer, so he stood up again and said, "All right, but let the one who has never sinned throw the first stone!" 8 Then he stooped down again and wrote in the dust. 9 When the accusers heard this, they slipped away one by one, beginning with the oldest, until only Jesus was left in the middle of the crowd with the woman. 10 Then Jesus stood up again and said to the woman, "Where are your accusers? Didn't even one of them condemn you?" 11 "No, Lord," she said.
And Jesus said, "Neither do I. Go and sin no more."

Events in the book of Numbers date c. 1450-1400 BCE toward the end of the bronze age in mid-east Asia.  Patriarchal Israel had emerged from Bronze Age nomadic culture with a primitive perspective about the character of God.  Despite Numbers being redacted and edited many centuries after its events, I posit that God left intact in inspired Scripture, at least in part, the primitive worldview of justice and punishment he used purposefully to engage with people of that time.  Not as God is, but as people were then culturally able to understand.  God is not limited, but the intellectual and cultural capacity of people would have been.
No doubt Scripture records what Israel's leaders, scribes, and folklore always held true.  But Christ corrected "an eye for an eye" to "turn the other cheek" (Matthew 5:38), for example.  To that point, Jesus clearly explained 'You have heard it said ..(..).. but I say to you..'
In Numbers, as elsewhere in the OT, I posit that God used primitive culturally-bound humans to communicate timeless truths, and imparted it to us from their perspective.
And so the Numbers stoning rightly challenges us in the context of God's character as revealed in Christ, as similar circumstances did for Christ himself in John 8:1-11.
Comment: Very nice! Usually, I don't see such a deep perspective from somebody this new to BH. Well done. It's an upvote from me at least.
Comment: @Olde English - thank you.


The levels in kashrut

Question: If a piece of a biblically forbidden food (such as a piece of pork), falls into a bowl of meat. We all know the rule, it's batul bshishim. How about a piece of food which is only rabbinically prohibited (a food that contains דברים מעמידים such as cheese). If the cheese would fall into a bowl of other cheeses, would the same laws of bitul apply regarding the shishim part. Or would we be even more meikel, because the food itself is only rabbinic?
Comment: Is the bowl hot? Is the rabbinic prohibition עיקרה מן התורה?
Comment: I don't understand your case. Is this Lach beLach on Min beMino or is it Yavesh beYavesh on Min besheEino Mino?
Answer: Assuming I understand your question correctly, batel b'shishim applies to both rabbinically and torah prohibited foods. It's basically the ratio at which chazal felt a tiny quantity of food became relevant.
However keep in mind it only applies to food when you can't separate the two foods. If a piece of non-kosher cheese fell into a bowl full of pieces of cheese, I see no reason why you wouldn't just pick it out.
Comment: That would be an issue of Rov, not batel b'shishim.
Comment: If they are all the same color. Then you wouldn't be able to tell them apart
Comment: Well if the issue is rov then the cheeses are forsure kosher because rov  of the cheese is kosher.
Comment: @Bachrach44 I'm really confused; I thought batel b'shishim generally only applies to bosor b'cholov (=kosher and kosher mixtures), not taruvos (=kosher and non-kosher)?
Comment: @SAH It applies to taaruvot. 60 is the quantification of how much taste spreads, so it is used whenever we have a case of ta'am k'ikar. (There are a few exceptions, I'm ignoring them for now). See Mishnah Chullin 7:4-5 and Yoreh Deah 98 (especially the rema in 98:1).

The levels in kashrut


Are there server with rank limit in battlefield 4?

Question: I always end up playing with people with ranks 100+. Are there servers that have rank limits?
Comment: not sure about BF4, but in BF3 there were some servers like "Noob only, max level 40" or so. And the server was configured that way, that it would kick all players above that level cap. But DICE didn't include that feature into servers per default
Answer: Unfortunately, no, there are no servers that have a rank limit on them. Which is a shame because I think that it would be a good idea, especially for new players.
The main reason you're getting into game with people rank 100+ is that the game is almost a year old, most people have got to rank 100, so many in fact that DICE have upped the level cap 3 times since the game launched, with it now at 130. Level cap was raised with each release of DLC's.
Comment: Thanks for the response, ughhh I was hoping that I just never noticed a cap limit or something :(
Answer: I think it's sucks. I've had the game since release and am only at rank 46. No I don't get to play a lot. I have a life, but when I get on I am immediately overwhelmed with people over 100 rank. I can't ever take them out no matter what I do. Maybe two kills a match against rank 100+. Or I constantly am getting killed over and over by one person. Annoying. I don't know how people just spot me out all the time and do that. One in every game almost. Can get killed on far end of map, yet when I spawn again no where near the last death spot here comes the guy that just killed me. When are games going to be fair instead of feeding people that have no life or job that somehow gets everything handed to them?
Comment: Yeah it is a shame that there is no effort to group people based on similar skill. Even after all this time EA still never gave us this feature :(

Your best bet might be to join a clan or recruit some friends to play with you. Also if you do play by yourself, be picky about your squad and stick with your squad-mates. Lone-wolves are pretty vulnerable in BF4 unless you are an expert player.
Comment: Also, there is a distinct possibility of people cheating.  I'm not one to claim "you cheated because you killed me"… but I also hate this uncertainty: am I just really this bad or are they really this good or are they just (low-key so it's harder to detect) hacking?


Copy and Paste Weight from one vertex to another

Question: Is there any way to copy the weight of all bone of a vertex (say A) of a Character mesh and then paste the same to a different vertex (say B) on the same Character mesh? So that Weights for both A & B can become same.
Answer: Go to edit mode, select all vertices you want to affect, shift select the vertex which has the correct weights (source, last selected) and click the Copy button in the N properties panel, vertex weights tab.
If you want to transfer the weights of some vertex groups and not all of them, click the relative folder button instead.
The applied logic is the same as many others functions in Blender: active to selected; active vertex will be white, while selected vertices will be orangy.
Be aware that this procedure will only add and/or overwrite weights, it will not delete existing ones which are not present on the source vertex, so consider if to clear existing weights before copying.
Comment: Thank you very much for your comment. However, could you please inform me that whether there is any scope to copy any left side vertex weight of a lefty side bone of a character to the similar vertex on the right side but of a right side bone. Say, vertex A is on Left hand and has a weightage of 0.5 for a bone Hand.L. Now how to copy this 0.5 weight on a similar vertex B but on the right hand, however, weight to be imposed by Hand.R. Concept is similar by copying pose from one side by pressing Ctrl+ C and then by Ctrl + Shift +V to paste the similar pose on the other side of a character.
Comment: As this site is intended, you should ask a new question about this different topic. Short answer: yes, there is.
Comment: As advised, above question is asked freshly. Kindly consider to give solution at this link please: https://blender.stackexchange.com/q/277268/143418
Answer: Select the target vertex in Edit Mode then hold the Shift key and select the destination vertex, and in the side panel (press N, Item section) choose Copy.


Is educating yourself much more difficult than being educated? Why? Is it just a motivation thing?

Question: Is educating yourself much more difficult than being educated? Why? Is it just a motivation thing?
I hope that's OK to ask. Anyone who uses this site frequently probably has an interest in learning, and not just reading, about philosophy
Answer: For me being here is mainly about educating myself. That motivates the reading, but I don't know what motivates the interest in the first place.
Being educated puts one on a career path. Because of that I imagine being educated would be more difficult. There are expectations to meet. It is not as easy to stop showing up as it is to stop checking this site.
Comment: philosophy is interesting, i wouldn't over think that one ha
Comment: @another_name Good point.
Answer: Both methods are useful in their time and place but I believe when it comes to philosophy you have no choice but to educate yourself. Guidance is helpful, but you need to be free to pursue your own interests and not be forced to follow someone else's agenda. Indeed. I'd predict most students would do better if they sat at home and didn't go to lectures. After all, these days we have the internet.
As for motivation, if this is lacking I doubt any method will work.
Speaking from experience, I will be eternally grateful I didn't study philosophy at university.
Answer: Everyone educates themselves. 'Being educated' merely means entering a context in which someone else structures one's self-education: useful for people at the beginning of their path, but ultimately a context that has to be abandoned before it becomes constricting. That is easier, sure, but less fulfilling.
People who don't have an interest in educating themselves will learn nothing no matter what educational context they are put it. People who want to learn will learn regardless. Half of a teacher's job is getting students interested in self-learning; the rest is giving students things to learn.
There's a vast difference between 'knowing' something and 'understanding' it. Knowledge can be force-fed through mere repetitive exposure, but no one understands anything unless they take the time and effort to think about it on their own. Facts of life...
Answer: Having your education directed has this advantage:  since you are not educated, you have not been educated in what is worth your while to study.  Hence, you may waste time, or worse, study erroneous matters and have to unlearn it if you are to progress at all.
The downside is that, of course, direction will direct down the common path.


What is the difference between message age and maximum age in STP

Question: What is the exact difference between these two values in RSTP and what values it can hold. How is the switch's behavior changes as these values are changed.
Comment: Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
Answer: Message age - Root Bridge sends out BPDU with message age as 0. And the receiving subsequent non-root bridges adds the value 1 to it. Effectively, this values says how far a bridge is from the root bridge. It is not a fixed value.
Max age - If 3x hello packets (which is sent out every 2s) are missed, a bridge waits for the configured max age time (default 20s) to conclude link failure to root bridge and initiates topology re-convergence.
Answer: The Message Age field in STP is similar to a TTL field in IP packet and is supposed to
—prevent BPDUs being forwarded between STP-aware switches endlessly
—define a maximum lifetime for a particular BPDU to prevent processing old and possibly outdated information
The Message Age is set to 0 at the root bridge. Every other non-root switch will increase the Message Age field by one in the received BPDU when relaying it via its own ports. Moreover, on each non-root switch, the BPDU itself will age out in (Max Age - Message Age). This makes sure that the more hops this BPDU has traversed from the root, the sooner it will expire.


Why did the United States so aggresively attack Communism when they knew it wasn't going to work out in the long term?

Question: Didn't USA had the foresight that any country that embraces communism would eventually fail? Why did it go around spreading anti-communist sentiment to an extent where they defied the internal politics of countries and preferred to instill a dictator who would ruthlessly crackdown communist?
Comment: The idea that communism *must* fail seems like anti-communist sentiment. But apart from starting from a somewhat shaky premise, this question also seems way too broad. It would help if you could at least narrow down your question to a specific timeframe and/or anti-communist action; though "why did x do y" style questions - ie questions about motivation - are always in danger of being closed as opinion-based.
Comment: Because the long drawn-out process of failure creates a lot of human misery?
Comment: How exactly (and when) did they know it was not going to work out ?
Answer: US policymakers did not have total faith Communism would fail. They worried it would defeat capitalism, or at least coexist with it indefinitely.
For example, consider the "New Frontier" speech given by Senator (later President) John F Kennedy in 1960:

For the harsh facts of the matter are that we stand on this frontier at a turning-point in history. We must prove all over again whether this nation — or any nation so conceived — can long endure — whether our society — with its freedom of choice, its breadth of opportunity, its range of alternatives — can compete with the single-minded advance of the Communist system.
Can a nation organized and governed such as ours endure? That is the real question. Have we the nerve and the will? Can we carry through in an age where we will witness not only new breakthroughs in weapons of destruction — but also a race for mastery of the sky and the rain, the ocean and the tides, the far side of space and the inside of men's minds?

Furthermore, governments are concerned with the present as well as "the long term". The USSR lasted for 74 years (1917-1991). Cuba has been Communist for 58 years and counting (1959-). The US government wanted to have friendly regimes in other countries immediately, not many decades in the future.
Finally, the USSR collapsed in part because it was vigorously opposed by the USA. In particular, the Soviet government was unable to afford the costs of the arms race between the two nations. Of course there were other reasons for the USSR's collapse, but at the very least the arms race was an important contributing factor.
Comment: Re "_a race for **mastery** of ... the inside of **men's minds**_"?  An historic rivalry which has finally given way to a ministerial spirit of Russo-American collaboration.
Answer: The Cold War was not just a struggle between two economical systems, but also a struggle between two hostile power-blocks: The US-lead Capitalist power-block and the USSR-lead Communist power-block. Both sides were considering the other side a direct threat to their existence. Even if one assumed that the system of the other side is unsustainable and will eventually collapse (which wasn't undisputed, see the answer by Royal Canadian Bandit for details): Until then the other block was still capable of starting another world war and likely willing to do so if they were sure they could win.
It was therefor important for both sides to prevent the other side from increasing their power by gaining more supporting satellite states. When a country allied with one side, that side gained exclusive access to that country's natural resources and economical power.
And even more important: Allying with a country gives the opportunity to station military assets on their territory. When you look at the geographical locations of important cold war hotspots like Vietnam, Korea or Cuba, you see their strategic importance.
This is why both power-blocks supported any regimes which favored their ideology and opposed those which favored the other's ideology.
Comment: The U.S. followed the containment theory set forth by Kennan during the 1940s after the war ended.  This theory postulated that the USSR would collapse of its internal inefficiencies if stressed by the USA but not confronted directly as by war.  Vietnam was "containment" when the USSR was on a roll with China going communist.
The theory worked in practice with the final USA move being StarWars, a possible development that the USSR couldn't match.
Comment: Please provide a link to clarify the term "*power block*".


Transit Visa - Heathrow, London

Question: My nationality is India. I have been working in Belgium. Now I'm leaving Belgium permanently, so I surrendered my Resident Permit and got a Model 8 and Appendix 37 from commune. I am travelling from Brussels -> Heathrow -> Chennai and I am leaving Belgium permanently. Can I travel with Model 8 instead of a transit visa through Heathrow airport?
Comment: What is "model 8"? What is your nationality?
Comment: My nationality is India. I stay in Belgium for Work and i had Resident permit. Now i am surrendered my Resident permit in Commune of Belgium and got Model 8 and Appendix 37 from them which  states that i permanently leave the country.
Answer: I ran your parameters through Check if you need a UK visa, and it appears to me you do need a visa, with the type depending on whether you need to pass through immigration, for example to collect and recheck bags. I suggest you do the check yourself.
There are clear lists of documents that would allow you to transit without a visa. The ones in your question do not appear on the lists. "a common format residence permit issued by an European Economic Area (EEA) country or Switzerland", which you used to have, does appear, but to depend on that you would have to currently have one.
Comment: As i am transiting via same British airways, i don't have  to collect and recheck the bags. My flight is tomorrow. Would it be possible to get transit visa on Brussels airport?
Comment: @KarthickS No you cannot. You need to provide your Biometric details. Best would be to postpone your flight or get one which is from Belgium.
Comment: There may not be any direct flights, but @KarthickS could look over the possible connection airports to pick one that either does not require a visa for transit by citizens of India, or for which they can get one easily and quickly.
Comment: There are many flights to India from various airports in the Schengen area. It's not at all necessary to go via the UK.

Transit Visa - Heathrow, London


What is the optimum split of main versus GPU memory?

Question: I read that it's possible to allocate either 128MB, 64MB or 32MB to the Raspberry Pi's video memory.
Given that my primary use case for this machine is educational: I will be running lightweight python scripts and web-browsing. Is it likely to suffer any loss of features by switching to only 32MB of memory for me?
I do not anticipate wanting to use any 3D or playing any video.
Comment: Make sure you adjust your **swapiness** :D
Comment: Wait, this has 42 votes. It's not a question, it is _the_ answer!
Comment: I believe the range of  values is now wider than that, and, as per
@Krysztof Adamski answer it is possible to specify a specific split in the config.txt for different total memory sizes so that the **same** card can produce appropriate splits when placed in **different** RPis - which is useful when preparing distributions I guess...
Answer: I would expect you to not experience a noticeable difference unless you are doing graphically heavy tasks, such as playing video.
However, it's difficult to gauge the optimal settings, as performance limits will vary depending on what applications are executing and user expectations.
The best thing you can do is experiment.
If you do want to change the split, then there are two different methods, depending on the firmware you have. If possible then make sure you have the latest firmware. If it is not possible for you to update for whatever reason, then I have included both options below:
New Firmware (after October 2012)

Edit /boot/config.txt and add or edit the following line:
gpu_mem=16

The value can be 16, 64, 128 or 256 and represents the amount of RAM available to the GPU.

Old Firmware (before October 2012)
In order to alter the video memory you need to replace the start.elf file on the /boot/ partition of the image. The possible .elf files should be in the /boot/ directory on your Pi. You make the switch by replacing the start.elf file with one of the others.
There are three memory models, and here are the recommendations taken from the Raspberry Pi discussion board:

224MB RAM and 32MB VRAM for a Linux desktop distro, or a heavy (non
  GUI) applications that don't need to play video, nor render 3D.

192MB RAM and 64MB VRAM (default) for desktop distro's that want to
  play video or have 3D effects.

128MB RAM and 128MB VRAM for applications and games that do extensive
  multimedia or play 3D rendered games.

 And one more:

240MB RAM and 16 VRAM for almost zero graphical power. There is enough
  GPU memory to render the screen, but not much else. Use this when you
  need a further non GUI performance boost.
Comment: I have updated the answer for the new 512MB Models.
Comment: Subtitles [seem to increase memory needs](https://github.com/huceke/omxplayer/issues/140#issuecomment-15452919); for a 1080p video with subtitles, 64MB was strictly not enough.
Answer: RAM is very crucial for Linux performance for couple of reasons:

Caches. Linux runs without free memory for most of the time. If some memory is not used by applications, it is used for caches which speeds things up. So no memory is ever wasted. If applications needs more memory, caches are freed so caches won't ever prevent applications needing more RAM from running. But of course the more your can cache, the better.

Many applications (especially desktop ones) needs a lot of memory to run nowadays. This is no surprise - most desktop systems (or even smartphones) this days have a lot more memory than Rapsberry Pi. For example web browsers even lightweight) will use tens of MB of memory as soon as you open some bigger web site.

Each and every application needs some amount of memory. So the more applications you run, the more memory they use.

So you always want to have as much ARM memory as possible. But GPU has its own needs depending on what features you want from it.
256MB version
For older RaspberryPi, with 256 MB of RAM, 4 splits are available:

240/16 - The most RAM size ARM can get (240MB) with mimial GPU memory.   It's the best for general computing when you don't need 3D graphics or hardware video acceleration. It has enough memory to handle 1920x1200x16bpp framebuffer resolution. While 32bpp is also working, it's not recommended since it leaves very small amount of free memory for GPU.

224/32 - This one is deprecated in favor of 240/16 split. It has 3D
and hardware video decoding build in but since there is not enough
memory to actually use it, there's little point in using it. Unless
you have some problems with 240/16 split or want to use 32bpp
framebuffer at highest resolution. It is possible it will be deleted in the future.

192/64 - You need at least 64MB of GPU split to use hardware video
acceleration or 3D graphics. It may not be enough GPU memory for
hardware accelerated video playback at highest resolution or if you
need a lot of GPU memory for things like textures.

128/128 - The most RAM GPU can get. Use it when you need a lot of
memory for 3D graphic card and hardware accelerated video playback in
high resolution. This split is needed for RaspBMC to work properly or
to play fullHD video content with omxplayer without problems.

512MB version
New versions of RaspberryPi has 512 MB of RAM. You have 4 additional splits designed for this version:

496/16 and 448/64 - like 240/16 and 192/64 split for 256MB respectively but with 256MB more RAM available for ARM. Since GPU has only 16 MB/64MB or RAM, all the limitations from 240/16/192/64 still apply.
384/128 - Similar to 128/128 split for 256MB - you should get all the graphic chip features but with reasonable amount of ARM memory. This will probably be the most universal split right now.
256/256 - Some bonus graphic card memory if you really need it, like for big textures and similar things. While 256MB of ARM memory is bigger than you could possibly have in older RaspberryPis, I don't think this one will be used on many occasions.

Dynamic splits
New versions of firmware package removes most of start_*.elf files, leaving only start.elf and start_cd.elf (also fixup*.elf files where added). From now on, you don't have to manually choose proper start_*.file. Instead, you should use gpu_mem* options in config.txt file to choose how much memory goes to GPU. The remaining memory will be allocated to ARM. It should be possible to use arbitrary number in the following range for this options:
**512MB of RAM**: 16M-448M
**256MB of RAM**: 16M-192M

While there is no hardcoded split values right now, most of the informations for the rest of this answer should still be valid when you're trying to determine how much memory should be allocated to GPU.
start_cd.elf and fixup_cd.elf files are cutdown versions of the respective files, only used when gpu_mem=16.
Using one SD card on both 256 and 512 MB version
If you are using one SD card image for different RaspberryPi versions, you can use gpu_mem_256 and gpu_mem_512 options. First one will only be used on 256MB version while second one only on 512MB version of the board.
Comment: Useful guidance. To clarify - what would be the optimal split for running headless? Does for instance the rpi camera module have a GPU requirement?
Comment: Yes, upgrade is worth if you want to use both XBMC and LXDE without boring manual config file change before eatch switch, or if Pi is your main computer which you want to use on a daily basis.
Comment: There is now a `gpu_mem_1024` for, not surprisingly the 1024 MB RPis - which currently is the RPi 2 B (and I guess A if one does come out?)
Comment: According to http://elinux.org/RPiconfig#Camera , the camera module requires the following lines in `/boot/config.txt`:  `start_x=1` and `gpu_mem=128` or more.  Also, `/usr/bin/raspi-config` has an option that enables the camera.
Answer: From my experience, 8-16MB for just terminal.
20MB for 800p GUI
24MB for 1080p GUI
32MB for 1440p GUI
64MB for <1080p youtube videos
128MB for 1080p youtube videos
256MB for 4k videos (more than likely the Pi can't do 4k very well).
Comment: That's true for 3B+ and earlier, for 4B don't set a value, let the system calculate an optimal value. Do not set a value over 256MB on a 4B.


How to fix apt-get install / update not working on my Kali Linux

Question: I'm using Kali Linux but the apt-get install command is not working to install any application on my system.
Mustafa1112221
I think it's got something to do with my Kali Linux repository sources.list file. The file may be empty or the current repo address is wrong or experiencing a problem.
Please, could someone provide me a link to the Kali official sources.list file?
Comment: Is it a freshly installed Kali Linux?
Comment: Hi, The last edit make your question unclear. please keep it as it.
Answer: Open a terminal and follow these instructions:

Type sudo -i
Type in your password.
Type touch /etc/apt/sources.list
Type chmod 644 /etc/apt/sources.list
Type echo "deb http://http.kali.org/kali kali-rolling main contrib non-free" > /etc/apt/sources.list
Type apt-key adv --keyserver hkp://keys.gnupg.net --recv-keys 7D8D0BF6
Type apt update
Now install whatever it is that you want to install.
Comment: @TomaTomov : `apt-get update` does require an internet connection because it downloads information regarding the latest packages and their versions from the repository. You won't be able to take update unless you have an internet connection. To check if you are connected try pinging a website. for example, `ping duckduckgo.com` and see if it replies. This is a simple but not a sure shot to check your internet connectivity. Depending upon your environment and firewall configurations, you may or may not get a reply.
Comment: I upvoted this answer because Google led me to it when I searched for why 'apt-get update' didn't do anything.  It was the 2nd answer from the google-gods and quickly fixed the problem.
Comment: What i get on step `6 ` is `gpg:keyserver failed: Network is unreachable`. How should I solve that .. installed Kali and all I see is the bash shell mode.
Comment: @TomaTomov : Are you able to access the internet? Are you working behind a proxy?
Comment: I think I am not able to access it. No. I am not behind proxy. How to check if i am connected to internet through the command line? What I found is that I need `wpa_supplicant` but I can't install it without the net connection I guess because when I try I get `Network connection failed`. I am really sorry for my confusing answers but I am total newbie. For now :)
Comment: I am at work now but will try it as soon as I get home and will give a feedback. Thank you very much !
Comment: got error gnupgX not installed error at step 6 but I moved on and it worked anyway,


How do I cut 1968 cast iron rough sewage pipe?

Question: I have a rough in bathroom that was done in 1968 for the homeowners to finish. Cast iron pipes for shower and toilet are about 4 in. out of floor. How do I go about cutting them to install a toilet and a shower? To be flush with the concrete floor.
Comment: Why is it important that you cut them flush to the concrete?
Comment: can you provide a pic... not exactly sure what you are trying.
Comment: Cast iron ( grey) cuts easily with something like a hacksaw; it cuts more like oak than steel. I once identified an item as cast iron ,not steel, by how easily I drilled a hole in it.  Concrete does not cut easily.
Answer: Usually, the easiest way to cut cast iron pipe is with a snap cutter, but I don't think you could cut it completely flush with one. A sawzall with a long diamond tip blade might work for you. A diamond wheel in an angle grinder is another possibility. Alternatively, you could frame a wood floor to the height of your pipes.
Answer: If they really need to be flush, I'd recommend using a 4½" angle grinder.  Cut the pipe close to the concrete with an aluminum oxide abrasive cut off wheel.  Spring for the good ones, and get an extra.  Finish the job with a flap disc, which will allow you to get in nice and close with good control.  Your application won't require anything finer than 40Grit, which will give you a decent rate of material removal.  Use a type 27 zirconia alumina disc.  And PLEASE USE EYE PROTECTION.
Answer: Had a half bath where the 61 yr. old commode had a very slow constant leak/refill issue. Replaced all the internal parts possible, some twice. When I pulled it up to replace  wax seal, noticed that the cast iron pipe was rigid and uneven/wavy with 1/2 inch or more differences in height.
Took my angle grinderwith 40 grit ferrous metal cutter to it, after attempt with oscillating tool with metal cutting attachment. In 45 minutes, it wore out cutting just two inches. The angle cutter took 10 minutes, never wore out blade.
*SPARKS! Tip: because of limited space, location, and sparks, I soaked old, large work towels dripping wet. I spread them out covering newly installed beadboard, baseboards, and flooring. I stuffed pipe to prevent metal from going down drain.
IMPORTANTKeep a large bucket of water and a fire extinguisher within arms reach. I used nearly all the water to cool the  wet towel in pipe, and rewet others.
*There is alot of "dust", but a lot it got caught up in the towels. Still had to clean up from ceiling down. Use rags you can throw away, there will be dirty greasy wax in the "dust". Use the widely known name brand, blue grease cutting dish soap full strength on wet disposable rags. Don't bother rinsing out, gets gummy and rusty, just throw away. Will clean up easier and faster.


Nuts soaked and dried. Fridge or not?

Question: I usually soak nuts overnight. I usually do this with walnuts, almonds, and hazelnuts. Then I dry them at low temperatures. After that, is it better to store them in the fridge or outside if I want a long shelf life?
Comment: There have been comments asking why the OP is soaking the nuts. It turned out to be a health-related reason, which devolved into a discussion on the correctness of their health beliefs. Please do not ask this again - we don't need this discussion, and it isn't really needed for an answer anyway.
Comment: Are they fully dried again, or just enough to get the surface water off (you can check the weight before and after the process)?  This could affect the keeping properties
Comment: They are fully dried
Answer: A study ("The Effect of "Activating" Nuts on Fat and Mineral Content" https://ourarchive.otago.ac.nz/handle/10523/9017) which soaked and then dried nuts at low temperatures found that these activated nuts (except for whole almonds) do have less fat, and we know that the oxidation of fat produces rancid flavors and odors. However, there is still a significant amount of fat left to go rancid after soaking and drying, so there seems to be no reason not to refrigerate them, when that is known to increase their life.
Comment: There's like 1 sentence in here that actually answers the question, buried under discussion of (off-topic) heath benefits. Would be nice if it were inverted to bring the relevant part to the fore.

Nuts soaked and dried. Fridge or not?


Where can I find Majors and Ultras for each species?

Question: I'm doing bounties where I have to kill ten Majors/Ultras of each species (Hive/Fallen/Vex). What are the most reliable and accessible areas where these forms spawn on each planet?
Answer: I have found that Devil's Lair is the best place to farm Fallen Ultras. In the first area with the Fallen Walker event, there is a dropship that will periodically drop off 4 Fallen Vandals, one of them being a Major.
Shows up in this area, behind the building:

For the Hive, I like doing the Sword of Crota mission. There is 5 Ultras in there, including the Swarm Princes. Plus running around with that sword is always awesome.
If you're a high enough level however (I believe level 15 is required?) you can do this during Earth Patrol missions. From spawn, head left and you will see a small shack on it's own. In there, there are 3 Hive Majors (2 Acolytes and a Knight). After beating these three, head back up the way you came, and they should respawn.

The other alternative is the Summoning Pits, during the Phogoth the Untamed mission. Possibly more efficient, but I tend to steer clear of.. that... filthy no good -omitted-
Ahem. Sorry. Where was I?
Vex! There is a Minotaur Major in the Ember Caves you can find in the Venus patrol missions, all the way over next to the Pillar of Light past the bridge in the back corner of the structure. If you jump on your Sparrow, cross back under the bridge and cross to the other side of the area and return, he should have respawned. If not, try again and he will have the second time around (possibly just needed a second).

And finally, Cabal. Again, during the Mars Patrol, jump on a sparrow, and head straight. Go through the big tunnel straight ahead, then when you come out the other side, you should see an egg-shaped building. To the left of this should be a building marked "Clovis Bray". In here, you'll find an Imperial Phalanx Major. To make him respawn, simply run back outside, and across to the end of the walkway directly across from the entrance of this building, and he should respawn.
Comment: Excellent answer. I always cackle when I see the hive one pop up because it is so quick to do. :)
Answer: The best way to do those "kill 10 majors" bounties is by going to the appropriate planet. Earth for Fallen, Moon for Hive, Venus for Vex, and mars for Cabal. Do missions on hard difficulty to get nearly all majors in one go.
For example, the Moon's Sword of Crota mission on hard has 10+ Hive Majors in it, and it's a very quick mission.
A Major/Ultra is any creature with a yellow health bar.
EDIT: Since a recent patch, the Sword of Crota mission now only has about 5 majors in it, but that is still better than wandering around in patrol I guess.
Comment: It may not be exactly 5, but it is definitely less. The sword room on its own used to have enough ultras for the bounty, now it really only has the 3 princes and maybe an ultra acolyte or 2.
Comment: Are you sure there's only 5? Last or remember, there was three Swarm princes in the sword room, there is one Swarm Prince out in the patrol region, and in between there is the small room with the Wizards, and knight, and the acolytes, (at least 3 of those are ultras). There is also two ultra Knights down a side corridor that leads to a separate area. I may just be mixing two separate missions together though
Comment: Turns out you were right. Just re-did it, and yup; only 5. I was just mashing two missions together. Edited my answer.
Answer: For the fallen, do the "Scourge of Winter" mission; the Kell-Guard or whatever they're called count as ultras, and you fight a decent number early on.
Answer: I've found that doing the strikes, especially on heroic if you're high enough level, nets you Majors pretty quickly since they're the sub-bosses in each area. And the end boss is an Ultra with usually a Major or two as well.
Answer: For Fallen majors, go to the entrance to the Winter Kell on Venus. The Captain there is always a major. And the respawn on that group is six seconds. It will always be a major. This was where I farmed for 200 Fallen headshots, too. The whole thing took me 15ish minutes to obtain. Just stay on the upper level, and then move back and forth from on top of the bigger group to the cave entrance.
For Hive majors, load up the patrol for Earth. Go in the first building on your left. Hidden in the bottom is three Hive majors (two acolytes and one knight). All three will always spawn as majors, and the respawn time is also six seconds. Any time I have the hive bounty, I am done in a couple minutes.
I have rarely spent time hunting the Vex, so I defer to other information.
The Cabal spot for me is the same listed above, but with a twist. I go into the dome, wipe out the 3-4 enemies, and then come outside to wipe out the respawning 3 Cabal foes just in front. Each groups spawn in six seconds. I was able to grind up 200 tears in minutes there.


How can I easily share the output of a command or a text file with others?

Question: I recently asked a question on Ask Ubuntu (or somewhere else), and a comment or answer asks me to paste the contents of some file or the output of a command to troubleshoot my problem.
How do I do that?
Answer: How to easily share all or part of some text file (or the output of a command)
You'll paste the file or command output to Ubuntu's "pastebin" service, and then allow others to look at it by simply sharing a link, or use it yourself to further copy and paste a few lines into your question.

Note: If your problem involves the Software Center, Update Manager or apt-get not working, you may have to use the Alternate Method near the bottom of this answer.

1. Start the terminal

The terminal allows you to interact with your system by typing commands, instead of using the mouse/cursor. It's not hard to use when you have instructions, and can make some tasks very easy to accomplish.
Press the Ctrl+Alt+T keys together (at the same time) to start the terminal. This will work for everyone using regular Ubuntu, but if you're using...

Lubuntu: Click on the Start button, go to Accessories, and click on LXTerminal
Xubuntu: Right-click anywhere on the desktop, and click on Open Terminal Here
Kubuntu: Click on the Start button, type konsole in the Search box, and click on Konsole

You will get a window with a solid background, showing your username and your computer name, like the below (size/colors may differ!):

2. Install pastebinit, a small tool to paste text from the terminal

In your terminal, type sudo apt-get install pastebinit and press Enter
Enter your password and press Enter (what you type won't show, don't worry)
When asked Do you want to continue?, type Y and press Enter, and wait for the prompt (user@machine) to return.

3. Pastebin the file or command and paste its URL on AskUbuntu

Go back to your browser, select the exact filename (or command) whose content you were asked to paste, and press Ctrl+C to copy it to the clipboard.

A filename may look something like /etc/apt/sources.list, while a command may simply be something like dmesg or grep -i failed /var/log/auth.log. The answer or comment will tell you explicitly if it's a file or a command.

Now, for a file, type pastebinit in the terminal, follow with a space; then right-click on the cursor and click on Paste to paste the filename you copied.

Press Enter, and after a few seconds you will see a link of the form http://paste.ubuntu.com/ as shown below. Move your mouse over it, and it will be underlined -- then right-click and click on Copy Link Address to copy it link to your clipboard:

But for a command, copy and paste the command in the terminal first, and then type
| pastebinit after it, and press Enter (see screenshot below)

The | symbol may be obtained by pressing Shift+\, which is usually below the Backspace key.
Move your mouse over the link--it will be underlined--right-click and Copy Link Address as explained above.

4. Paste the link into your AskUbuntu question/comment

Go back to your browser, and add a comment (or edit your question) with the Pastebin link - just press Ctrl+V to paste it. Save the edited question or add the comment, and that's it!

5. Optional: View your pastebin link, and only paste selected lines from it into your AskUbuntu

You can also paste the link into your browser's address bar, or click on the link after you've added/edited it to AskUbuntu, to view it on pastebin. It will look similar to this:

Notice the line numbers. You can also copy and paste selected/requested lines from here directly into your AskUbuntu question.
Please enclose the pasted lines with the
 and
 tags as shown below, so that it appears as separate lines in the question instead of being jumbled together:

Alternate way to pastebin a file using an editor
This may be useful if you are having a problem with Software Center, Update Manager or apt-get and cannot install pastebinit; it only works for files, not commands.

Select and copy the filename from AskUbuntu into your clipboard with Ctrl-C.
Open a terminal as shown in Step 1 and type gedit, followed by a space, and then right-click to paste the filename as in Step 3 and press Enter

Lubuntu users type leafpad instead of gedit; Kubuntu users type kate

The editor will open with the file:

Click anywhere inside the editor window, and press Ctrl+A. All the text should now be highlighted in another color:

Now press Ctrl+C (or the Copy button, if you know where that is) to copy the entire file to the clipboard.
Go to your browser, and open paste.ubuntu.com. Type your name/nickname in the Poster: box; then click in the Content: box and press Ctrl+V to paste the text you just copied from the editor in there:

Click on the Paste button, and in a few seconds you'll see the text you pasted:

Select the address shown in the address bar (highlighted orange above), copy it with Ctrl+C, and then go back to AskUbuntu and paste it in as explained in Step 4.
Alternately, select a few lines, copy them and then paste them in AskUbuntu as explained in Step 5.
Comment: I don't like to use the ubuntu pastebin for these forums because it is impermanent. Older questions and answers are left with holes because the old pastebins get deleted.
Comment: Then what should I write when it asks me for the password??

Also, someone might want to share the complete file, for that is `cat file | pastebinit` ;)
Comment: There are few problems, pastebin can't be edited / deleted once it's uploaded. This is pretty important if company sensitive info / password is accidentally posted. And as @Insperatus mentioned after a while old pastebin expires and old questions have broken links
Comment: What tool did you use to take the terminal window screenshot with a drop shadow? And, how did you add the arrows and comments? Very stylish, great work!
Answer: Usually, bash has a utility called "script" that creates a sub-terminal that writes to file. For example, if you do:
script ask-ubuntu.txt

It will create a new bash prompt, and all input and output will be stored in the file ask-ubuntu.txt when you exit the sub-shell.
You can then obviously copy the contents of the file, or upload the file, wherever you need it.
Comment: That's pretty neat! (use Ctrl-D to exit the subshell, btw)
Comment: Using the "exit" command also works.
Answer: If you don't necessarily want to share the output on Pastebin, another option is to use xclip. It takes what you give it on the standard input and puts it optionally in the X selection, or the clipboard.
First, install xclip with this:
sudo apt-get install xclip

By default, xclip puts copied text into the x selection instead of the clipboard. Since standard copy and paste functions use the clipboard, we'll have xclip use it too instead of the default.

To copy the output of a command to the clipboard:
command | xclip -sel clip

To copy the contents of a file:
xclip -sel clip < file

To paste, use the standard shorcut Ctrl+V, or right click and select paste.
Answer: I needed something to share terminal output even when X server wasn't loaded so I created this service: termbin.com. The only thing you need is netcat, then you can easily share with anyone anything that can be shown in terminal, there's example:
cat /etc/fstab | nc termbin.com 9999

After running this command you'll get in response url address with text file.
To make your life easier you can add such alias to your .bashrc file:
echo 'alias tb="nc termbin.com 9999"' >> .bashrc

Now sharing will be much simplier:
uname -a | tb

You can get saved ones for example by using curl. You'll find more examples on termbin.com.
You can host your own server as well, there is github repository: https://github.com/solusipse/fiche. If you want to make it private, don't forget to set whitelist parameter.
Answer: I may have missed something in the excellent answers above, but if what you want to copy is relatively small, all you have to do is get it displayed on your screen (e.g. cat filename in a terminal or opening the file in a editor), highlight the desired text with the mouse and copy it into your clipboard.  Once there, you can just paste it into your reply on the web or into an email, etc.
Using pastebin or pre tags is important with longer material so that it gets separated out from the rest of the question/answer so that it doesn't distract people from the rest of the post.
Answer: I may have misunderstood the question, but I've found that in the terminal you can use the following after selecting what needs to be copied with your mouse:

Ctrl+ Shift+ c     (Copy)
Ctrl+ Shift+ v     (Paste)

Obviously once in AskUbuntu or whatever forum with your browser you use your regular Ctrl + C and Ctrl + V.
This might be a new addition in 12.04 seeing as I'm a relatively new user and noone else seems to have mentionned it beforehand, but to me this seems an easier and quicker option rather than having to install software or running command lines, especially for relatively new users. Hoping this helps complement Izx's incredibly complete answer.
Answer: I wonder why no one mentioned this SOLUTION.
If you wanted the output to be compiled into a text file you just have to add " > filename.txt" ( without quotes) to your terminal command
examples
eightnoteight@mr:~$ date > date.txt
eightnoteight@mr:~$ echo yes! it really works with echo too > echotest.txt


What is the minimum font size for an Ecommerce header

Question: I'm dealing with my art director to redesign one of my website and more specifically the header of one of those. My websites are in web responsive and i'd like to know if there is any specification for the size of the font?
Comment: It's not really clear what you are asking... Depends what's in the header - what specifically do you refer to in the design? There is a base font size of 100% by default which equates to the default browser font size.
Answer: Recommended font size for paragraphs in web is at least 16px (somewhat depending on which font you're using, serif fonts suffers from poor readability in lower res).
A good rule to follow is to let the H1 header be 1.5em in size, meaning 150% the size of your paragraph font. So if your paragraph font is 16px then let the H1 header be 24px.
Note that the font specs are also affected by browser preferences.
You can read more about it here.
Answer: Not knowing specifically which typographic element are you asking about (page title, navigation, company branding/logo etc) I would echo AndroidHustle's comment and advise that you do not go below 16px for your smallest size. This 'should' hold up on both mobile and desktop but ultimately you will need to test thoroughly on various devices.
More from this old smashing magazine link here:
http://www.smashingmagazine.com/2011/10/07/16-pixels-body-copy-anything-less-costly-mistake/

"16-pixel text on a screen is about the same size as text printed in a
  book or magazine; this is accounting for reading distance. Because we
  read books pretty close — often only a few inches away — they are
  typically set at about 10 points. If you were to read them at arm's
  length, you'd want at least 12 points, which is about the same size as
  16 pixels on most screens"

and some other general guidance from iA on text sizes:
http://ia.net/blog/100e2r/


Wiring problems on the TP4056 Module

Question: I have been thinking of putting a 18650 Li-ion battery salvaged from an old laptop power battery to use so, I bought a few of those cheap TP4056 battery protection and charge circuit modules and I am a bit confused. I accidentally bridged the two B+ and OUT+ solder pads and now I am having a tough time removing the solder from them so I was wondering if I could directly connect the battery to the B+ and B- charging pads while the OUT+ is still connected to the B+. Will it be ok if I do so and will it affect the charging circuitry?
Comment: Does the module have a part number? I'm asking this because there are different types of 4056 modules: Some of them has DW01 supervisor IC, some of them does not.
Answer: There is a reason OUT+ and B+ are separated. There is a battery protection between OUT+ and B+. If you short it you effectively disable this protection, and that is definitely not a good thing. I wouldn't do it.

Wiring problems on the TP4056 Module


Installing Tar file

Question: I have downloaded the iredmail tar file iRedMail-0.9.9.tar.bz2, and placed it ia a folder named iRedMail. Can some one give ma step-by-step instruction how I install it on Ubuntu 18.04? I will very much appreciate help from an expert.
Answer: The install instructions for Ubuntu are found here.
to extract the file you go to the folder you extracted in and type the command from a terminal tar -xvjf <file>.tar.bz2 replace  with correct name. It will extract the file in the same folder.
Answer: you can find the step by step guide for installation and also the setup here.
Click here!
after downloading the tar file you can extract the file using the command
tar xjf iRedMail-0.9.9.tar.bz2

and this will generates the directory containing all the files .
change your directory into that directory and you can add and modify the configurations in the iRedMail.sh file .
Comment: Consider adding more details to your answer before including the link for further assistance.

Installing Tar file


Advanced textbook for vector fields

Question: I am currently reading Spivak Calculus on Manifolds and Munkres Analysis on Manifolds.  I am looking for a more advanced text, especially on vector fields as they relate to the great conserved fields of physics.  Thanks for your help.
Comment: Welcome to Mathematics Educators! A more appropriate place for your question could be [Mathematics Stack Exchange](https://math.stackexchange.com/). You could delete your question here and post it over there. Good luck!
Comment: I'm voting to close this question because it appears to be seeking personal advice about a course of study.  As such, it is not really on-topic here.
Answer: This may not move in the directions of your greatest interests, but it is one option.
(Not my field so I cannot assess comparable books.)

Eschrig, Helmut. Topology and Geometry in Physics. Vol. 822. Springer Science & Business Media, 2011.
  Springer link.
  Review by Jan Jerzy Sławianowski.
  Publisher summary: "Written as a set of largely self-contained extensive lectures, the book gives an introduction to topological concepts in gauge theories, BRST quantization, chiral anomalies, supersymmetric solitons and noncommutative geometry."

Note: Not to be confused with Topology and Geometry for Physicists by Charles Nash and Siddhartha Sen.

Advanced textbook for vector fields


Is there a way to identify number of wallets opened by an exchange

Question: This is precursor to a data science question.
Essentially, I am trying to map bitcoin's price to the number of wallets opened today on an exchange.
It's quite common that the users would put in a buy request tomorrow, so a time series can be applied.
Answer: You won't be able to see the number of wallets, but you could get quite an idea by looking at the number of adresses an exchange generated.
( Exchanges such as poloniex only have 1 wallet for all their customers. )
See https://www.walletexplorer.com/wallet/Poloniex.com/addresses
I'm not entirely sure if walletexplorer also tracks coinbase adresses, but in the case it doesn't, you can use http://matbea.net
They currently track over (28.500.000+) coinbase adresses.
https://blockonomi.com/matbea-trace-bitcoin-address/
^^ (I wouldn't recommend using their webwallet though, and i'm also not saying that this is 100% indexing all adresses.)
For example, using matbea.net, i was able to identify 1DUiLkf34q7MVxtRkMAXVtP42UNGRrbftY  as a coinbase adress.
https://www.walletexplorer.com/wallet/000001e522b362b7?from_address=1DUiLkf34q7MVxtRkMAXVtP42UNGRrbftY
Answer: No.
No one ever creates Bitcoin wallets, they instead just use an empty one. The process of starting a new Bitcoin wallet is by cryptographically generating a private key. From the private key, you can derive a public key and a public address which you then distribute as the recipient of a potential Bitcoin transaction. These keys are generated randomly, so there is a chance, despite its minimal possibilities, that the key your wallet produced ends up the being the same key that someone is using today. This key could have funds that are spendable, and you just got access to the private key to their wallet. Now, the chances that someone ends up generating a private key that is, or was, being used at all is 1/(2^160).
Imagine every bitcoin private key was a grain of sand. There are, approximately, 2^63 grains of sand on earth. Now replace each grain of sand on earth with the earth itself, making there be 2^63 grains of sand in place of each grain of sand there already existed. Now, if you add up all this sand, you will have 2^126 grains of sand, which is still 0.0000000058% of 2^160, being the total amount of possible addresses.
Now with all this information, try and find out which of these addresses Coinbase has chosen. It's impossible.
Comment: It's not entirely true what you're saying. There are services that index adresses from coinbase / poloniex
Comment: If Coinbase declares that they are using address X or Y, then, of course, it would be possible, however with no declaration, there is no possibility.


Identify the shrub

Question: Please help me identify the bush?  this looks like Spirea cantoniensis but the flower is different
Answer: It's a Deutzia variety, most likely D. scabra. Your photo does not magnify well so I can't see the flowers clearly, but a variety of D. scabra is pictured here.
If this Deutzia does not have pink tinges anywhere, it won't be that particular variety, but Deutzia it is nonetheless.

Identify the shrub


Why does android use up and down arrows in expandable list view?

Question: In Android Ice Cream Sandwich an up arrow is employed to indicate an expanded group and a down arrow to indicate a closed group. Both seem like non-standard choices, most user interfaces use a right arrow to indicate a closed group and a down arrow to indicate an expanded group. Is there a reason why the Android designers didn't follow convention?
Comment: I suppose the down arrows says something like: "click here to expand this down below, while the up arrow says: "click here to move the children back up again!"
Answer: I don't know the actual reason, you'd have to ask someone on the dev team there but here's my understanding.
Using right arrows to show a closed menu and a down arrow to show an expanded menu is a visiblity of system status indicator. It indicates that a group is either open or closed.
The android arrows on the other hand, are not meant to show the status of the folder (Open or Closed). They're meant to show the user that clicking on it will either open or close the folder. They're action buttons. The down arrow indicates that this group CAN be clicked on and something will go down, and when open the group can be clicked on to close that group.
You can change the arrow's direction if you'd like to follow conventions.
Answer: I believe in this case the arrow down is a signifier that gives the user 2 pieces of information:

"Hey, there's more content"
"Ah, and that content will appear below here"

In my opinion, the arrow down is in this case more accurate than an arrow right because it shows where the new content will actually be displayed (i.e., below).
The arrow up is again a signifier.
Its opposite direction maps an opposite effect compared to the one the user has just experimented: while the arrow down opens the panel, the arrow up closes it.
Arrows up and down are in my opinion intuitive, mainly because:

They let the user create the right conceptual model of how the interface will operate (see "The design of everyday things", Don Norman, 2013, pag. 25)
In the physical world we are already familiar with this kind of ON/OFF, vertical interfaces (e.g., light switches).


Mean decrease in accuracy ranks variables differently than mean decrease in Gini Index?

Question: I created a random forest model with 17 predictor variables and a categorical outcome with 3 classes (650 data points). When I look at the variable importance plot, I see that the ranking of variables is quite different depending on whether accuracy or gini index is used as the ranking metric. What might be the reason behind ranking discrepancies between these two methods?
Answer: there can be many different combinations of reasons at play. I'll try to list them down and perhaps you can check which one is valid for your problem as the details for the data(features, target) etc. is not known

Mean Decrease in Impurity(MDI) can be biased towards categorical features which contain many categories
Mean Decrease in Accuracy(MDA) can provide low importance to other correlated features if one of them is given high importance

As you can see the definition of both the metrics is very different and both are susceptible to some kind of errors as per the data distribution. You can read more about them here and here


proving the stability of the equilibrium point with a Lyapunov function

Question: The below nonlinear system is considered as a pendulum with a nonlinear damping coefficient:
$$
\ddot y+(a+b\cos(y))\dot y+c\sin(y)=0, \qquad a\geq b\geq 0
$$
Use the energy of the whole system as a Lyapunov function to check the stability of the equilibrium point.
Comment: What is exactly your question? What have you done so far?
Answer: You have the actuated system
$$
\ddot y+c\sin(y)=-(a+b\cos(y))\dot y
$$
The sum of the kinetic and potential energy is given by $$V(y,\dot y)={\dot y}^2/2+\int c\sin y\,\mathrm{d}y.$$ The derivative of $V(y,\dot y)$ is $$\dot V(y,\dot y)=\dot y \ddot y+\dot yc\sin y=-\dot y c\sin y-{\dot y}^2(a+b\cos y)+\dot y c\sin y=-{\dot y}^2(a+b\cos y),$$ by substituting $\ddot y=-(a+b\cos(y))\dot y-c\sin(y)$. From $a\geq b\geq 0$ and $\cos y\geq -1$ follows $$\dot V(y,\dot y)\leq 0.$$