Laplace and First uniqueness theorem proof: Why potential difference?

Question: On Griffith's "Introduction to Electrodynamics" page 120 the author states that when proving the First Uniqueness theorem: The solution to Laplace's equation in
some volume V is uniquely determined if V is specified on the
boundary surface S....
We suppose there are two solutions to Laplace equation: $$\nabla^2V_1=0\\\nabla^2V_2=0$$
the key to the proof is to look at potential difference: $$V_3 \equiv V_1 - V_2$$.
Why do we choose the potential difference. I understand the rest of the proof, how you get that $V_2=V_1$ and prove that it's valid... but why the difference? When you know the rest of the derivation, it's obvious, but what if I didn't know the rest of the proof.
Answer: If you want to prove that V1=V2, and you know that V1 does equal V2 at the boundary, a nice way to do it would be to prove that their difference V3=V1-V2 is equal to 0 everywhere. Proving that V3=0 is evidently much easier than proving uniqueness by itself, because you can exploit the only known fact from the problem - V1 equaling V2 at the boundary, and get a strong statement that V3 is 0 at the boundary, which you can use in conjunction with tha fact V3 also satisfies the Laplace's equation! On the other hand, using summ or product of V1 and V2 doesn't exploit the boundary condition in any way, so it's not in any way useful.
Answer: You can find so called 'happy ideas' all the time when reading the proof of any mathematical statement. And even if that key idea was somehow motivated by some sort of intuition, in general authors tend to omit mentioning that when writing the proof. A famous quote attributed$^1$ to Gauss is "A good building should not show its scaffolding when completed".

$^1$From W. Sartorius Von Waltershausen, Carl Friedrich Gauss: A memorial, see it translated here.
Comment: Thanks for nothing.
Comment: Sorry, it was not my intention to offend you in any way. I posted my answer because your question (v1) doesn't seem to be about the proof of the theorem itself; *I understand the rest of the proof*, you say. But rather, as you also say: *When you know the rest of the derivation, it's obvious, but what if I didn't know the rest of the proof.* From this last sentence, it seems that you are not used to mathematical proofs, hence my answer.
Comment: No problem. I only wish to know why we looked at the potential difference and not potential summation or product or anything like that. "The key is to look at the potential difference" is the quote from Griffiths and my question is "Why?"
Answer: To make it short (the type of argumentation already well explained by "vucko" ) I will give a formal summary of vuckos post:
If $V_1$ and $V_2$ fulfill the same boundary conditions, therefore $V_3 = V_2 - V_1$ has zero value as boundary conditions. Furthermore
$$\Delta V_3 = \Delta (V_2 -V_1)=\Delta V_2 -\Delta V_1 =0$$
Therefore $V_3$ fulfills the Laplace equation with zero boundary conditions. Therefore $V_3=0$. Consequently $V_2=V_1$. Q.E.D.


Formula to calculate the frequency of gravitational waves emitted by two bodies inspiral?

Question: Is there a small straightforward equation which can give the approximate frequency of the gravitational waves emitted by the inspiral of two bodies?
Comment: Do you mean frequency as a function of time?
Comment: @RobJeffries    More like the starting frequency from where I should look for the Gravitational wave signal assuming I was LIGO. I'm sorry if this question is trivial I am new to Gravitational waves.
Comment: I think this is possibly a duplicate of http://physics.stackexchange.com/q/174333/
Answer: To leading order, the angular frequency of radial motion in a binary inspiral obeys the equation
$$ \frac{d \omega}{dt} = \frac{96}{5} \left( \frac{G \mathcal{M}}{c^3} \right)^{5/3} \omega^{11/3} $$
where $\mathcal{M}$ is the so-called chirp mass, defined by
$$ \mathcal{M} = \left( \frac{m_1 m_2}{(m_1 + m_2)^{1/3}} \right)^{3/5}. $$
The frequency of the gravitational wave signal will be twice the orbital frequency. That is, the observed frequency of the signal is
$$ \nu = 2 \frac{\omega}{2\pi} = \frac{\omega}{\pi}. $$
You can use these equations to estimate the chirp mass of the system $\mathcal{M}$ using $\nu$ and $\dot{\nu}$, or vice versa. Given a sensible initial condition, you can solve for $\omega(t)$ or $\nu(t)$ analytically and find the evolution of the signal frequency.
Note that the differential equation for $\omega$ implies that $\omega$ increases monotonically ($\dot{\omega} > 0$). This equation also predicts that $\omega$ will diverge in finite time, which corresponds to the collision event and the end of orbital motion.
Comment: Could you please add a reference source for these formulas?
Comment: @magma The differential equation for $\omega$ is stated explicitly in the beginning of section II C of [this paper](http://link.aps.org/doi/10.1103/PhysRevD.49.2658), for example. See _The Classical Theory of Fields_ by Landau and Lifshitz for a complete treatment of the subject.


Is there a pejorative word to say "things relating to latin"?

Question: In French, the word "latineries" relates to the latin language in a pejorative way. Is there an English equivalent to this word?
Comment: Writing or speech may be said to contain an excessive number of **latinisms**, but the term itself is neutral
Comment: I've come across some highbrow usage of that word in English, but it's very unusual, and I'm not aware of any commonly-used or commonly-understood equivalent.
Answer: English doesn't have the same relationship to Latin that French does. In English, words from Latin (often introduced via French) tend to complement Germanic words from Anglo Saxon.
So there is "bird", but the adjective "avian"; we can "begin" or "commence"; we drink "water", but make an "aqueous solution of sodium chloride". The Latin term is associated with sophisticated language, particularly scientific or intellectual language. There are also borrowings of Latin phrases, like "quid pro quo". Most of these started as expressions used by lawyers.
We sometimes call these words or phrases "Latinisms".  Some authors use such words whenever they allow us to be more precise. Other authors (notably George Orwell) recommended always choosing the Anglo-Saxon over the Latin. However, the term "Latinism" is not itself pejorative. A negative sense could be suggested by saying:

...the unnecessary use of Latinisms, such as "quid pro quo"...


What happens to the blocks that do not pass the acceptance rules?

Question: One of the acceptance rule for the blocks is that the block timestamp cannot be more than 2 hours in the future. What happens to blocks whose timestamp does not meet this rule, i.e. blocks that generally do not meet the acceptance rules (not consensus rules)? Are they rejected as if they had never been seen?
Answer: A block with a timestamp too far in the future is invalid. This seems to be discovered during ProcessNewBlockHeaders when learning about the block header. The node discards the invalid block header.
// Check timestamp
if (block.Time() > now + std::chrono::seconds{MAX_FUTURE_BLOCK_TIME}) {
    return state.Invalid(BlockValidationResult::BLOCK_TIME_FUTURE, "time-too-new", "block timestamp too far in the future");
}

via src/validation.cpp


Unity Build Automation

Question: My current project has some editor scripts that I run just prior to run/build, which dynamically creates some game objects in my main scene based on configuration. I don't want these objects to clutter up version control however, so I delete them prior to saving the project and committing changes to the repo.  Currently I'm doing both of these steps manually, but this seems like the kind of thing that should automated.  Is there a way to automate running these scripts prior to a Build and then deleting the constructed objects (presumably can create another script to help with this) prior to saving the project?
Answer: I don't know if it'll work for your specific use case, but note that there's the PostProcessBuild attribute for editor scripts that run automatically after a build. I use that to edit the XCode project generated by an iOS "build".
Another approach is to write a script that executes the build, and that script can probably do cleanup after a build too. Like, I routinely write editor scripts that add a "Custom Build" option to the File menu, and then that script does a bunch of stuff before calling BuildPlayer. Assuming the BuildPlayer method is synchronous (I don't know if it is, I've never tested this) then that script could also do more stuff after BuildPlayer.
Comment: A post-build for cleanup could help, though people might also run this generate script manually before running a scene in the editor.  I should look into whether there are editor methods to Save the project, as I could potentially create custom Build and Save menu items, then just tell people to use those instead of the defaults.  Still semi-manual, but getting people to consistently use one click rather than a few clicks is probably easier.
Comment: It sounds like you want to do the second thing then, either by itself or in addition to the post-build script. That is, you have the editor script that generates objects *also do the build*. Then it's impossible to run the generate script without doing a build.
Comment: Hmm, possibly, though I sometimes run the generator, play around in play mode for a bit, tweak the generator settings and regenerate, play some more, etc.  The Build process isn't too long, but not ideal to run it if not necessary.  Failing to generate before Run is also a possible mistake, but it's lower impact (only impacts the developer testing and not any downstream people impacted by a bad build or a bloated commit)

Unity Build Automation


Is it (theretically) possible to reduce inertial mass without reducing matter quantity?

Question: Ok, please bear with me because I only have a very little (or no?) understanding of physics outside Newtonian laws... So I was playing a video game called "Mass Effect" in which a particular compound ("Element Zero") can create a field in which objects lose what I understand is inertial mass without being shrunk down or losing matter (with this I mean that they are still physically formed by the same particles), thus rendering possible to accelerate them with a smaller force. Is it theoretically possible for such a thing to happen? Like interfering with Higg's Field?
Comment: No. This is utter nonsense, and only possible in video games. I'm sorry to burst your bubble! Physics is *damn awesome* though, much more so than these 'simple' modifications people keep on coming up with.
Answer: I don't think a big effect like the one in the game is possible, but a small effect is possible...
The mass of a proton plus an electron is greater than the mass of a hydrogen atom (by a little tiny bit) because $E=mc^2$, where $m$ is the inertial mass. The energy of the electron and proton in a hydrogen atom is lower than the energy of the hydrogen atom and the electron which are separated from each other. So the reduction in mass $\Delta m$ is given by  $\Delta E=\Delta m c^2$ and $\Delta m =\Delta E / c^2$. The value of $\Delta E$ is $13.6 ~eV$, which gives a value for $\Delta m$ of $\sim 2.4 \times 10^{-35} ~kg $, which is about $1.5 \times  10^{-8}~m_H$ where $m_H$ is the mass of the hydrogen atom.
So the mass of a hydrogen atom is slightly lower than the combined mass of its components. This is the only example I can think of relevant to your question.
edit after good comment from - Francesco Bertolaccini
The release of energy in fusion and fission processes can also be calculated with $\Delta E=\Delta m c^2$ - the energies released are much larger than the energy mentioned above in the formation of the hydrogen atom from an electron and a proton. For more information the wikipedia page on fusion as a power source may be a good place to start.
Comment: I see, and if I'm not wrong that's the principle behind nuclear fission and fusion, right? That's a different perspective from what I was thinking :D So Higg's field has nothing to do with the inertial mass of a particle?
Comment: Yes - good point - this is the fundamental principle between fission and fusion - I will edit answer for this..... I'm afraid that Higg's field is beyond my expertise... I am not sure about that.
Comment: I see... I guess I'll wait to see if someone has more experience on that field to see what else can be said.


Why can the most general state of a qubit be written as $|\Psi\rangle=\cos(\frac\theta2)|0\rangle+e^{i\phi}\sin(\frac\theta2)|1\rangle$?

Question: Why we can express a most general qubit as $|\Psi\rangle = \cos{\left(\frac{\theta}{2}\right)}|0\rangle + e^{i \phi} \sin{\left(\frac{\theta}{2}\right)} |1\rangle$? Is there any formal proof for this?
Comment: Generally the state of s qubit can be written  as $\alpha |0\rangle + \beta |1\rangle$ (a bit of $|0\rangle$ with some probability and maybe some  $|1\rangle$ with some probability ).  Since we're talking probabilities here, so the sum should be 1, that means $\alpha + \beta = 1$.  Maybe If you want to work it out yourself, you can try to check trigonometry rules and show that $\cos{\left(\frac{\theta}{2}\right)}  + e^{i \phi} \sin{\left(\frac{\theta}{2}\right)} =1$ (knowing that we can have complex numbers in the coefficient)
Answer: The most general pure state of a qubit can be written as $|\Psi\rangle=a|0\rangle+b|1\rangle$ where $a,b\in\mathbb{C}$. The amplitudes $a$ and $b$ can be written in polar form as $a=re^{i\alpha}$ and $b=se^{i\beta}$ where $r,s\in[0,\infty)$ and $\alpha,\beta\in[0,2\pi)$. Thus, $|\Psi\rangle$ is described by four real parameters $r,s,\alpha,\beta$ as
$$
|\Psi\rangle=re^{i\alpha}|0\rangle+se^{i\beta}|1\rangle.\tag1
$$
However, there are two constraints. First, the squares of the absolute values of the amplitudes are probabilities and therefore sum to one
$$
|a|^2+|b|^2=r^2+s^2=1.\tag2
$$
Consequently, $r,s\in[0,1]$. Now, for any real number $x\in[0,1]$ there exists a unique $\varphi\in[0,\pi)$ such that $x=\cos\frac{\varphi}{2}$. Let $\theta\in[0,\pi)$ be such that $r=\cos\frac{\theta}{2}$. Then from $(2)$ we have that $s=\sin\frac{\theta}{2}$ and substituting into $(1)$, we obtain
$$
|\Psi\rangle=e^{i\alpha}\cos\frac{\theta}{2}|0\rangle+e^{i\beta}\sin\frac{\theta}{2}|1\rangle.\tag3
$$
The second constraint arises from the fact that the global phase is unobservable. This allows us to fix the phase on $|0\rangle$ to be a positive real number. We can force this by dividing the amplitudes in $(3)$ by $e^{i\alpha}$
$$
|\Psi\rangle\equiv\cos\frac{\theta}{2}|0\rangle+e^{i(\beta-\alpha)}\sin\frac{\theta}{2}|1\rangle\tag4
$$
where $\equiv$ denotes equality up to global phase. Defining $\phi=\beta-\alpha$ we have
$$
|\Psi\rangle\equiv\cos\frac{\theta}{2}|0\rangle+e^{i\phi}\sin\frac{\theta}{2}|1\rangle\tag5
$$
where we can take $\phi\in[0,2\pi)$.
Answer: We know the general state of a qubit $|\psi\rangle$ is:
$$ |\psi\rangle = \alpha|0\rangle + \beta|1\rangle $$
$$ \alpha, \beta \in \mathbb{C} $$
Since, we cannot measure global phase, we can only measure the difference in phase between the states $|0\rangle$ and $|1\rangle$. Instead of $\alpha$ and $\beta$ to be complex, we can confine them to the real numbers and add a term to tell us the relative phase between them:
$$ |q\rangle = \alpha|0\rangle + e^{i\phi}\beta|1\rangle$$
$$ \alpha, \beta \in \mathbb{R} $$
Now, since the qubits state must be normalised,
$$ \sqrt{\alpha^2 + \beta^2} = 1 $$
and using the trigonometric identity:
$$ \sqrt{\sin^2{x} + \cos^2{x}} = 1 $$
to describe the real $\alpha$ and $\beta$ inn terms of one variable $\theta$:
$$ \alpha = \cos{\tfrac{\theta}{2}}, \quad \beta=\sin{\tfrac{\theta}{2}} $$
From this we can describe the state of any qubit using the two variables $\phi$ and $\theta$:
$$ |q\rangle = \cos{\tfrac{\theta}{2}}|0\rangle + e^{i\phi}\sin{\tfrac{\theta}{2}}|1\rangle $$
$$ \theta, \phi \in \mathbb{R} $$
If we interpret $θ$ and $ϕ$ as spherical co-ordinates ($r=1$,since the magnitude of the qubit state is
1), we can plot any single qubit state on the surface of a sphere, known as the Bloch sphere.
Example: the state $|+\rangle$ is the same for $\theta=\pi/2$ and $\phi=0$


Are these real WWII Nazi patches?

Question: Quick question for anyone knows about historic WWII Nazi artefacts. These patches were found in my grandfather's basement after he died. He served in WWII in Germany. Are these actual Nazi patches or just some crazy biker's patches?
Comment: Crazy biker's patch seems likely.
Comment: Right click on the image, do search google for image, it leads to Harley Davidson shops.
Comment: I'll cheerfully admit I'm not an expert on Nazi regalia but that patch doesn't look 70 years old to me.
Comment: that's what I thought... its old.. but doesn't seem that old!
Answer: That seems to be a Harley Davidson biker's patch from a few decades ago, on sale as a collectable here. The Third Reich use of the Totenkopf symbol had a quite different design.
Answer: Addressing the "Nazi" part, that patch combines two symbols which predate the Nazis by decades. The Iron Cross and Totenkompf (death's head) were Prussian symbols. When Germany became unified, they adopted many Prussian symbols because the Prussians were dominant in the new country. When the Nazis took power, they adopted Prussian symbols such as the Iron Cross, the Totenkompf, Goose-Stepping, and so on for a boost in credibility.
Here is Generalfeldmarschall August von Mackensen showing off his Best Hat Evar. This photo was taken in 1914 just prior to WWI. He's in his full Hussar dress. Note the Death's Head and Iron Cross.

So, no, having a patch with an Iron Cross and Death's Head on it doesn't make him a Nazi...
...in his time. One must also consider the time he probably bought it. While we've become much more sensitive to anything remotely associated with the Nazis in the last two decades or so, the Iron Cross or Death's Head wouldn't have gotten that kind of reaction then. It would be seen more as a symbol of strength associated with bikers.
Alternatively, it would have been considered captured war loot that the US was awash in post-WWII. Though that patch is probably just some biker patch.
Wear it today at your own risk.
Recall that in your grandpa's day we would have been watching a TV show about the wacky hijinks in a Nazi POW camp! Post-WWII attitude towards Nazis was more about a defeated enemy to be ridiculed than an association with white power to be feared (also it was the 60s so white power was the status quo).

SCHULTZ!!!

Are these real WWII Nazi patches?


Testing the integrity of cable shielding for suitability

Question: I am currently working with an ECG cable, is there a way for me to test the integrity of the shielding of the cable without using it directly on an ECG monitor to check for interference.
Comment: There are  several ways to test cable shielding effectiveness, but to be relevant they should apply to your equipment's needs, otherwise you may be wasting your time running inapplicable tests, or worse miss something because of test methodology.  Does the monitor have any sort of specification on the "standard" cable itself?  To give you an idea, for low frequency you typically have the 50/60Hz line noise, which shielding really doesn't do much to get rid of in an ECG situation (unless you put the patient into a Faraday cage), you have to do a differential signal measurements.
Comment: Are you testing for assembly faults, or for cable suitability?
Comment: for cable suitability
Comment: It would be good to edit that into your question, as comments may vanish at any time.  I'm afraid I don't have an answer, I suspect there is no easier way. But maybe someone else will come along.
Comment: On the other hand the input circuitry may be sensitive to high frequency RF signals (to some extent), but without knowing the range and reference cable performance spec you have no idea what specifications your cable needs to meet.  That said you can search for cable shield reduction factor and transfer impedance for some of the techniques used (aside from anechoic chambers with RF signal generators etc. $$$ and difficulty in getting numbers that are accurate on the latter.
Answer: Cable shielding depends on the % coverage by the woven braid with its gaps. Thus a solid shield (aluminum? foil) with a contact wire is the better type of coax cable.
For excellent control of electric fields, the external fields must NOT be able to find a path to the center-conductor. Hence the need for a solid(foil)shield, not a woven shield.
All this will get you some Electric Field shielding.
Magnetic fields are a different matter.


Why does an Arduino MEGA 2560 have different PORTS

Question: I'm currently looking at my Arduino MEGA 2560 board and was just wondering why there are different ports to it e.g. ANALOG IN, DIGITAL, Communication? in fact I've only seen this in some microcontrollers. Not many microcontrollers have ports dedicated to certain things. Is anyone able to give me an answer as to why there is different ports to the microcontroller? is it making it easier for the chip to handle etc. A detailed but not too detailed answer would be great!
Comment: Most microcontrollers have a lot of pins with various functions, but **extremely complex** patterns of what is and isn't supported on each pin, often appearing to derive from internal details known only to the designers.  In short, you look at the data sheet with care, but don't (productively) wonder why.  A great way to shoot yourself in the foot (if you neglect to check) is to design a PCB connecting a signal to a "general purpose" pin which happens to uniquely not support the *one* special mode you actually need it for...
Comment: "Not many microcontrollers have ports dedicated to certain things." [citation needed]
Comment: Probably because it means less logic which means less upfront development cost which means lower cost to the system integrator.
Comment: I remember using an Attiny chip, not sure of the exact model, which basically just had ports A,B,C,D and didn't have any labels such as PORT A = Write PORT B = Communication etc. Basically from my experience, what I think is i've seen is that Not many microcontrollers seem to have dedicated ports to them
Comment: Most ATtiny MCUs do have peripherals fixed to specific pins. The ATtinyX41 have reassignable pins for OC, but they're also fairly new.
Comment: Why is it that they have specific pins for them? what's the reason behind it? could you please give me an explanation
Comment: That confused me a lot. Maybe dumb it down a bit?
Comment: Actually, most of the ATmega2560 pins have two or more functions. The ATTiny crams a lot of functions into one pin because it has a low pin count and to make it versatile, and therefore marketable.
Answer: Thats because Arduino has configured the Mega to do so. In microcontrollers you need to select what use will each pin have based on the availability. To make user interface easy arduino has done this based on what is commonly used by everyone. Even then some functions are still not disabled.
For example 50,51,52,53 are SPI pins on mega which use Pin change interrupt for working. Thus it not necessary we use them for SPI we can also use them for Pin change interrupt.
Similar is the case for pins 10,11,12,13. These PWM pins can also be used for Pin change interrupt but other PWM pins cannot.
Comment: This is not correct! On most atmega, attiny, some pins can serve a specific purpose (eg usart, spi, i2c, analog input) and although you can use them for digital input or output, you cannot chose other pins for these specific purposes. Hence, this is not "Arduino that configured the mega".
Comment: Well if you can look at the data sheet of atmega 2560 used on the mega you can see a lot of functions for a single pin. and also There are some pins having pin change interrupts on them but they do not work on the mega board.
I have seen working of ARM cortex-M4 microcontroller, many functions exist on each pin and you need to have a board configuration file programmed to enable disable some the pins of registers and ultimately the functions. A similar version exists at the ATmega controllers if you can see the proper programming in CVAVR.


WTFx1000 The ERC-20 Token Transfer might have Failed (ErrCode: unable to locate corresponding Transfer event log), check with sender

Question: im sending from one mew to another mew
my gas is doubled
why do i keep getting this
TxHash: 0xac2bdee062f62ede0a478ea6a60e4c4465d76f099e3764895b849c491baa5f75 TxReceipt Status:FailBlock Height:4941786 (129 block confirmations)
TimeStamp:34 mins ago (Jan-20-2018 05:28:05 PM +UTC)
From:0xe1201a1cdd447c684f435313bf27554fc628c3d3
To:Contract 0x0b76544f6c413a555f309bf76260d1e02377c02a (InternetNodeToken)
**The ERC-20 Token Transfer might have Failed (ErrCode: unable to locate corresponding Transfer event log), check with sender.Value: 0 Ether ($0.00) Gas Limit: 250000
please help
Answer: This looks like a bug in the token contract. It has this:
<code>require (balanceOf[_from] > _value);
</code>
which means you can never send all your tokens. You tried to send 97.288000 tokens, which is your complete balance. Try 97.287999 instead. This token uses 6 decimal places, so that's the maximum you'll be able to send.


cart total percentage discount in magento 1.9

Question: I need to give 20% discount of cart total for all registered users in my store if the user register at the checkout time also get the benefit of discount. For guest checkout no need any discount
Answer: Follow below steps.

Navigate to Promotions > Shopping Cart Price Rules
Add New Rule
Rule Information Tab > Select Customer's Group
Actions Tab > Apply > Percent of product price discount
Actions Tab > Discount Amount > 20
Answer: follow below step to give discount of 20%

open pramotion -> shoping cart price rule
add new rule
define rule name and all data select customer groped
in action define percentage


When was the paper of bitcoin announced?

Question: When examining about this, it is said that many are November 1, 2008, but on the other hand, there are sites that are on October 31st. Which one is correct?
Comment: Both might be true.  It can be simultaneously Nov 1 and Oct 31 in different timezones.  [An aircraft flight can arrive the day before it departed](https://www.independent.co.uk/travel/news-and-advice/flights-take-off-2018-land-2017-time-travel-auckland-hawaii-air-new-zealand-hawaiian-airways-taipei-a8136491.html)
Answer: According to the mailing list archives, the mailing list received Satoshi's email at Fri Oct 31 14:10:00 EDT 2008. However the mailing list is moderated so perhaps the email was not received by everyone until a day later on November 1st. The first response to the email was sent on November 2nd, so it may be that the email was not forwarded to the list for a day or so.
Answer: According to the original source of Bitcoin, On 31 October 2008, a link to a paper authored by Satoshi Nakamoto titled Bitcoin, as a peer-to-peer electronic cash system was posted to a cryptography mailing list. He implemented the bitcoin software as open-source code and released it in early 2009.
Answer: Archive.org say on dec 24 2008 : "Sat, 01 Nov 2008 16:16:33"
[https://web.archive.org/web/20081224103053/https://email@example.com/msg09959.html]
And Tim May (Cypherpunk) say (about the mail list creation in 1992) :  John Gilmore and Hugh Daniel hosted the mailing list. There was no moderation, no screening, no "censorship" (in the loose sense, not referring to government censorship, of which of course there was none.) The "no moderation" policy went along with "no leaders."
Comment: Some improvements needed in this answer: 1) Your link is broken. 2) You don't explain exactly why two different dates are given. 3) You could add a preamble to the last sentence to give it some context for people who read your answer first (the order is randomised in the main views)


How much value do publishers and editors place on informative/educational content in fiction stories?

Question: I have heard people say that 'good stories are educational as well as entertaining'. But how much does it actually matter in the eyes of editors and publishers? For example, if one is writing a story that happens to involve farms to some extent, is the author expected to enrich the story and provide readers with significant details about the realities of farms and crops, even if they are not crucial to the plot? (I notice that Stephen King does this quite extensively in his story 'Children of the Corn', to the point that it almost seems too excessive to me – like info dumping).
Answer: I think you mistake the meaning of "educational" in this case. "Educational" needn't be only about dry information that's related to what you're writing about. At the heart of a story, there is a moral choice, an ethical discussion. It's very rarely made explicit (and indeed, making it too explicit would often be considered "preaching"), but it's there. So a good story makes you think, consider how you view the world, maybe re-evaluate some ideas. In this way, it is educational.
Consider All Quiet on the Western Front: it isn't about all the specific information about trench war. It's about how we see war in general. The Lord of the Rings isn't about the info-dump on hobbits and Middle-earth (there's nothing educational there, since it's all fantasy) - it's about common people stepping up to the challenge.
Comment: Agree.  If the story can't teach you _something_, at least through self-reflection, it's probably not a very compelling story.
Answer:
How much value do publishers and editors place on informative/educational content in fiction stories?

No value at all.
Consider a Romance; When Harry Met Sally. What did you learn there, that you did not know before? New, factual, interesting things, mind you.
Consider a SciFi movie, Star Wars. What new facts did you learn about our world in the entire series? New, factual, interesting things? Nothing I can recall.
Fiction is for entertainment; period. If you insist on describing something real, then for the sake of entertainment you must do some research so you don't drop-kick a knowledgeable reader out of their suspension of disbelief.
If you put the jugular vein in the leg, I'm going to have to put the book down for a minute to get over laughing. Not with you. At you.
The same if your expert swordsman calls the hilt of his sword a "handle." (The hilt consists of pommel, grip, and guard.)
If you MUST have your farmer doing some farmer-only-thingy, do enough research to not make a fool of yourself. If she is harvesting apples, she's likely in a cool climate and apples are harvested in September and October. Not early July. If your story time line demands the scene be in early July, or someplace apples don't fare well, find something else for her to farm or do instead. Feed chickens or gather eggs. And spend thirty minutes reading something about those too.
That's all the "educational" you need, enough to not make a fool of yourself, shocking the reader back to reality and out of immersion in your story.
This is about as important as not using a word incorrectly, and not having a wild misspelling of a word. For the same reason, reasonably knowledgeable readers will catch it.
I'm not saying you need to write for experts in every field, but presume you are writing for typical college graduate with good grades; they know a little about a lot. Even an American math student like me had to take American and World history, sociology, psychology, art and art history, years of English, Mythology, Physics, Astronomy, computer courses -- and then all the basic math and algebra and geometry and statistics that everybody takes, then the advanced math courses.
I am also not saying "write what you know." That can make it easier, you don't have to do as much research. In some cases it leads to fiction nobody else could write, like getting into the nitty-gritty of lawyering or medicine. But it can also turn writing from fun into work, an extension of your working day, that slows you down. So "writing what you know" is a personal choice.
I am saying that all that matters is a good story. It doesn't even have to be an original plot; Star Wars isn't. But the elements of the plot are original and compelling, George Lucas did that with stellar imagination, enough to capture a generation of movie goers. The same goes for Harry Potter: Rowling's imagination filled a fantasy-mystery plot with something new and compelling. It doesn't really educate us in any way about things in the real world.
Think about sexual content in a novel: It is one form of entertainment, but it is not demanded and, as Harry Potter proves, sexual content is not a necessary component at all; you can write a bestseller without even an obscure allusion to it.
Likewise, being casually educational can be one form of entertainment, but publishers and editors and agents only care about overall entertainment value that translates into sales and profits. That's it. Like sexual content, educational content is not a necessary component at all. You can write without it.
Just do enough research to not make a fool of yourself.
Comment: -1 I can understand this as an initial reaction. It was my initial reaction as well. But it is demonstrably false. Publishers are interested in the educational aspects of fiction in many fields because book buyers (whether readers or not) are interested in them.
Comment: @MarkBaker Publishers are **interested** in anything that sells, including a book full of explicit sexual acts, like 50 Shades. Does that mean you have to get pornographic to sell books? Because this is clearly the spirit in which the OP is asking this question, whether they need to shoehorn in some kind of educational content or not. The answer to that is NO, they value *entertainment* and as I said, educational content is but ONE form of entertainment, and completely unnecessary. A lack of it will *not* change the value of the story being told, just as a lack of sex didn't hurt Harry Potter.
Comment: Agreed: "No value at all." In large bold letters. (My writing mantra: Just tell us a story.)
Answer: It depends on the genre. The purpose of a story is to give pleasure to the reader. Learning things is certainly one of life's pleasures. In historical fiction, for instance, readers often take the story as a kind of history text and expect to learn things about the period. In age of sail novels, a large part of the attraction is learning about the methods and the customs of the sea.
In thrillers, details of weapon systems and military procedure are often highly valued. Tom Clancy is a good example of this. John Grisham's readers in part take pleasure from learning something about the law and the operations of courts and lawyers. Forensic shows rely very heavily on the learning element, even though much of what they say about forensic procedure is bogus -- to the extent that it has become a problem for courts and police.
There are genres of cosy mystery in which the detailing of meals and their preparation is considered an integral part of the pleasure of the book.
Children, who are, for the most part, avid learners, often value the learning aspects of their novels.
In fact, the interest in learning things is so great that some readers will invest huge amounts of time and effort in learning made up things. Thus some successful fantasy novels get companion encyclopedias and bestiaries for people who just can't get enough of the world. Thus some readers obsess over the blueprints to the Millenium Falcon.
This has to be distinguished, however, from the novel of didactic intent. The didactic novel exists to teach, and the teaching is wrapped in novel form precisely because the reader does not believe that the reader will be interesting in learning this subject any other way. Such didactic novels are usually abysmal as stories.
But that does not mean that publishers are not interested in them. Book publishers are in the business of selling books. It is no skin of their nose if the books go unread. Thus they may well buy pedantic books to sell to grandparents or teachers who want to try to use them to force unpalatable learning down a child's throat. The aims of the grandparents and teachers are seldom satisfied by this approach, the the aims of the publisher are.
In short, publishers will publish what they think they can sell, and this includes both books that included teaching as part of the pleasures offered, and didactic books that are bought by intermediaries with the intent to covertly educate the intended reader.
Answer: Verisimilitude Not Education
To add to Galastel's great answer, and using your farm example, the details about farms and crops aren't there to educate the reader (at least, that's not their primary function), they're there to provide verisimilitude. To immerse the reader in a real farm and make the story seem realistic enough to be plausible.
Agents and Editors
Agents and editors will want to feel like you understand the world you have created and can write it knowledgeably with telling details. They don't want to read stories where the characters float around in thin air or in a setting that's so inaccurate it detracts from the novel.
Frustrating Readers With Lack of Research
Also, as a writer, you don't want your readers coming back to you and saying, that would never happen on a farm, you clearly haven't done your research. But you're right about info-dumping, you don't want to do that either. The secret is to strike a balance between enough information to create your world and avoiding info-dumps.
Immersive Entertainment
I believe the primary function of a novel is to entertain. If I want to be educated, I turn to non-fiction. But in order to entertain, the world I'm immersing myself in must feel real. And it's for that reason you need significant details about the realities of your world.
Best of luck with your book!
Answer: All experienced events change the experiencer.
Reading a book is an experience.
All books change their readers.
Good books have a greater impact on readers.
Here's a definition of education from Wikipedia:

Education is the process of facilitating learning, or the acquisition
  of knowledge, skills, values, beliefs, and habits. Educational methods
  include storytelling, discussion, teaching, training, and directed
  research.

There isn't a book in existence that does not change its reader in some way, because every experience experienced incrementally alters some aspect of the experiencer.
Editors and publishers are, in the main, smart people. They know the power of books to change (educate) people and so will seek out those that they judge will have a greater impact on their readers. Those are the kinds of books that will be memorable and will cause the reader to recommend them to others.
I'm not necessarily talking about the minutia of the running of a farm here. The education is probably more likely to involve something about the human condition of people on farms or the deeper aspects of the role of farms in the world. But still - that's education.
Good luck with your writing.
Answer: I agree that good stories are often educational, but not in the way that you're interpreting the phrase.  Typically, in a story, you see a character placed in a dilemma or a challenging situation and you learn either how they resolve it successfully, or what not to do (in the case that they fail).  Often this takes the form of a moral lesson, although it's never an explicitly didactic one in good fiction.  I'm not always a fan of the dictum "show, don't tell," but here's one case where it definitely applies.
You can also learn a wealth of other things from many good stories --science, history, culture, psychology, law, agriculture, philosophy --but that's probably best conceptualized as a "bonus" that enriches a story, but only in the case that it serves the needs of the story.  When the teaching overpowers the story it's neither a good story nor good teaching.
On the other hand, I don't think I'm alone, from my perspective as a reader, in demanding some level of depth from my reading.  I'm all in favor of entertainment, but something that is "only" entertaining isn't very satisfying to me for very long.
Comment: A great example is *Discworld* -- Terry Pratchett's series started out as straight humor/entertainment/parody, but then started getting pretty deep into social satire.  He had a world -- he could tweak variables to resemble our world, and humorously exaggerate the results.  A great way to explain "why don't the poor just make better buying choices" can be demonstrated by a character moment summarized here:  https://wiki.lspace.org/mediawiki/index.php/Sam_Vimes_Theory_of_Economic_Injustice


What is the most appropriate Natspec tag to use for struct properties?

Question: For each struct property, what is the most appropriate Natspec tag to describe what it is? Is it acceptable to use <code>@param</code>?
Answer: I don't have enough reputation to leave a comment. As much as conventions are determined by developers actually doing things in practice, I have been using @param for struct properties in production for several years now. I also use it for documenting events.


Use position data of one geometry object on another

Question: I am using Curve to Mesh to apply a profile along a circle. The profile curve is a line. I want to vary the length of the line according to e.g. the x coordinate within the circle. There are various vector math and math nodes that allow me to manipulate a 'position' input, but I do not know how to get the correct data flow. In the node setup shown in the picture, the 'position' input corresponds to whatever geometry I end up connecting it to, but a) I want to use the position data of one piece of geometry on another geometry, b) the input to 'curve line -> end' is not a field.
Any advice greatly appreciated.
Answer: I am not sure what you want to achieve, but if you want this:

you can use this node setup:

Be aware: There is a big difference between circle sockets and diamond shaped sockets. The circle sockets are single value only. They are constant over one frame. The diamond sockets are fields (aka multiple values). So you cannot do it as you did want to do it. You have to change the position of the lines after instancing.
Answer: Sometimes, it may be convenient to manipulate the Radius of the path curve, down which the profile is swept. That gives you a (XYZ) uniform scale of the profile, which can be varied as some function of the path:
Comment: nice one!! love it


Only .ftpquota and no other file on my ftp

Question: I had a problem logging in my ftp account with old login-password (I don't know why, I have changed it to try again, but there is no positive result). So, I made a new FTP account in my CPanel - and I successfully logged in in my Firezilla, but there is only file .ftpquota and no other files. My website is working, but I can't change files in FTP. How to solve it?
Answer: When you create a FTP user account in cPanel you assign it a ownership directory, if that folder happens to be a child directory of the master account then you won't be able to go higher. e.g

/home/your-username/public_html/new-user/ Will not be able to view the parent public_html.

This is a security messure and something you shouldn't need to turn off. Your issue is the old FTP account that you should try and resolve. You should be able to reset it via CPANEL or contact your hosting. Simply creating another user will not give you the access rights you need unless you have a good understanding of how chmod works.
Your options

Reset the main password to the FTP account.
Change the new FTP user to the root of where the HTML files are stored, while granting correct chmod premissions.
Contact your Web Host... after-all, that's what they are their for....
Comment: Thank yo so much for such a detailed answer! The problem is solved, your answer helped a lot!
Answer: While adding new FTP account, you need to put "public_html" folder in 'Directory' input field. So create new FTP account with Directory listing set to public_html, and the trying logging through FTP client. Worked for me.


Cannot create a private key with seahorse

Question: I'm trying to create a new private key (I suppose it will be an RSA key) using seahorse, but once I insert label and strength (the other fields are disabled) the create button is still disabled.
seahorse-screenshot
Answer: It looks like this may be a bug in Seahorse, as I just tried to generate an RSA private key on Xubuntu 18.04, and encountered the same issue.
You can instead generate the RSA private key using the command-line:
<code>openssl genrsa 4096
</code>
You should then be able to import this into Seahorse if you need to.


How to enable user themes in Gsettings (Gnome-Tweaks)

Question: I am working in a chroot environment, and I need a command in terminal to set gnome-shell themes. In this case I'm wondering how I switch the "user-themes" setting on the gnome-tweaks extension tab (see photo). Switching via the GUI is easy, but so far I don't know how to do it from the command line.
Comment: hi, what do you mean by little black screen? the terminal? if you want to change the look of your terminal, that is done from the settings menu in the terminal itself.
Comment: I need to a command in terminal to set gnome-shell themes.
Answer: First, to install the "user themes" extension using the terminal, if you haven't already, you can run the following command:
sudo apt install gnome-shell-extensions

This will install the "user themes" extension along with a bunch of other extensions.
You don't have to use gsettings, unless you need to. You can enable the extensions you want using gnome-shell-extension-tool. Specifically, to enable "user themes", the command you need to run is
gnome-shell-extension-tool -e user-themes

Parameter -e tells gnome-shell-extension-tool to enable the extension.
You can also disable an extension using the -d parameter instead of -e.
Answer: first get the enabled-extensions with below command
gsettings get org.gnome.shell enabled-extensions
example;
pratap@i7:~$ gsettings get org.gnome.shell enabled-extensions
['firstname.lastname@example.com']
pratap@i7:~$

then add your extension name to the output you got
example;
gsettings set org.gnome.shell enabled-extensions "['firstname.lastname@example.com', 'firstname.lastname@example.com']"

Answer: In new versions, the commands above are deprecated. The new command is:
gnome-extensions COMMAND

If you want more info about the commands type:
gnome-extensions help

And for enabling or disabling the extension "user themes":
gnome-extensions enable/disable email@example.com


Could two stars collide without a resultant explosion?

Question: Are there any scenarios in which two stars could collide, and simply fuse without triggering a supernova. Maybe a mid-sized star slowly spiraling in together with a smaller star? It seems like any time stars collide it's incredibly violent. It seems that one star always gets torn to pieces and accretes matter onto the other, or ends in a red nova. Is there any way this process could be calmer?
Comment: Binary interaction and merger is an area that we do not have much knowledge about, besides from simulations. There are many parameters playing roles in the scenario, and I am pretty sure there is a case of what you mentioned, especially if one of the companion turned into a black hole before merging.
Answer:
Are there any scenarios in which two stars could collide, and simply fuse without triggering a supernova

As said in the comments, binary evolution of stars is very uncertain, but there exist numerous codes (i.e. MESA, STARTRACK, BSE, etc...) which incorporate LOTS of physical processes over the course of millions of years (and if they evolve to black holes billions of years) of binary evolution. Ono Pols has a nice textbook free on his website here.
Stellar binaries usually experience at least a couple mass-transfer (MT) phases after the binary components fill their Roche lobes. To put it simply, MT is either stable or it is unstable, but either way it results in the donor star losing it's envelope so that it's core emerges (which will later turn into a compact object). In stable MT, the donor star transfers some mass to its companion and thusly loses rotational angular momentum (it's spin decreases) while the accretor's spin increases. In unstable mass transfer, the mass transfer rate is too much for the accretor to handle so that the binary is engulfed by the envelope, this is also called a Common Envelope (CE). The CE results in either 1) the envelope is ejected from the binary resulting in a drastic decrease in the binary separation, or 2) the binary merges leaving a single star which accretes the leftover envelope.
So, I think the case 2) is an answer to your question. Keep in mind that CE is very very uncertain, as Postnov explains, and it is a relatively quick process (less than 1 million years). It's worth noting that case 1) of CE is crucial for forming compact binaries that have small separations so that they merge within the lifetime of the universe (so we can measure they're gravitational waves).
For your nutrition, here in Postnov's review on compact binary evolution the section on CE is lovely (section 3.6 on page 35).
Of course, if you're willing to include black hole binaries in your question, then their merging due to the emission of gravitational waves is certainly a calmer scenario.
Answer: The merger of two stars may only result in a supernova if the merging stars are white dwarfs, or possibly a white dwarf with a neutron star, and even in these scenarios a supernova is not certain.
Other mergers do not cause supernovae. Examples are the formation of contact binaries (W UMa binaries), which may then fully merge to produce fast-rotating FK Com stars, or more generally, the merger of main sequence stars is thought to produce blue stragglers. It is possible that some of these events may result in some sort of outburst, but nothing like a supernova - e.g Tylenda et al. (2011).
Comment: I think it's been theorized that some hot subdwarf stars could also be formed from the merger of white dwarf stars.  See [O-type subdwarf](https://en.wikipedia.org/wiki/Subdwarf_O_star) and [B-type subdwarf](https://en.wikipedia.org/wiki/Subdwarf_B_star).
Comment: @AlaskaRon Yes indeed, a supernova is only one of the possible outcomes.


Should I focus on the notes of the song and less on the rhythmic notation when learning a new song?

Question: Curious about the best ways to practice songs written for piano, i pay too much attention to the written rhythmic pattern and then I get frustrated since the rhythmic value written is not what my hands can play yet, should I focus on the notes and then add the rhythmic notation later? Your thoughts?
Answer: Pieces are made up of two basic elements - rhythm and melody. If either one changes, it's a different piece. Take a very simple kids' song, and just change the rhythm, keeping the same notes in the same order. Or keeping the rhythm, but changing some notes. A different song.
Try tapping out the rhythm as you look at a new piece. That way, there's no tune attached. When you're happy with the correct timing, play a phrase at a time. Hands separate, then together. It's not a good idea to play the tune in the wrong rhythm, for reasons mentioned earlier. And do it all at a tempo you can sustain, as slowing down doesn't help the learning process. I don't mean don't slow it all down - I mean don't start at a speed too fast to keep at. And keep counting. Even seasoned pros do that. Most of the time!
Answer: I'm concentrating to the chords and harmony, singing the melody and playing the rhythm as fine as possible. Playing the tune is the last priority. But this might be an individual approach.
Answer: During the learning process I think it's appropriate to modify elements to gradually build up to a full performance and also to explore the actual musical effect of various elements.
Simplifying a melody to just quarter notes can help you get the fingering changes under control. Simplifying the left hand to just block chords on half notes can help you understand the basic harmonic structure. You can do a lot of simplification and reduction in that way to feel out the essential musical elements. Once you have the 'lay of the land' you can then focus on the detail. This kind of process helps both with learning the mechanical performance, but also the musical structure.
Also consider breaking out certain passages that need special attention. For example, if there is a rhythm and chord change combination that is particularly trick. Focus on it. Play just those few measures fast, slow, legato, staccato, transpose it, etc. Maybe even invert the parts between right and left hands. Do it in the morning, then the evening. Be sure to take break from it with other material, but then go back to the problem area. That kind of contrast should help reinforce the training. Hit it from every angle you can until it isn't a problem.
And like @Tim said count out loud with rhythm practice. Use a metronome.


How to alpha-beta pruning to expectiminimax

Question:
I have this problem above and I'm trying to think of how to apply alpha-beta pruning to the above. The algorithm states that on you're opponents turn the (expecti turn) you just return the lowest value, but does that mean you apply the probabilities to those values? So for the far left you'd get 2 as the largest value then multiply that by 0.5, but then that set's $\\beta$ in the expecti node to $0.5*2=1$ and when it goes into the branch to the right it's comparing values without the probabilities applied to it when updating $\beta$.
Comment: I don't understand the current title "How to alpha-beta pruning to expectiminimax" because a "verb" is missing. Please, edit your post to fix this.
Answer: The internet doesnt seem to have easily accessible resources on this topic. In my newbie opinion, it should not be possible to prune any nodes. Expectiminimax takes the weighted average of the children, so it would need to consider all leaf values in order to do so. There is no node which can be ignored, as it will an operand in a sum. Now lets say the left subtree was bigger, in the right subtree you evaluated the first min node and it is smaller: well since the second min node can contribute to the sum making it bigger, it cannot be ignored. Now lets say the first min node on the right subtree was bigger, well since the second term adds or reduces(reduces in this case) weight it still has to be considered to get the final value.
Since all the nodes are checked and added together with appropriate weights under chance nodes, I highly doubt there is any advantage of alpha-beta pruning here since it would take the same number of steps as regular expectiminimax. Perhaps if the tree was larger and the chance nodes were central, more nodes could have been pruned out.
Comment: This is dead wrong. If we know the maximum and minimum value a node can have, we can estimate the max/min expected value of a chance node and prune there.
Comment: @thecoolwinter I am talking about the question at hand, not in general. If you have an answer, please provide it. Except you cant because this question cannot be solved by pruning
Comment: @thecoolwinter I am talking about the question at hand, not in general. If you have an answer, please provide it. Except you cant because this question cannot be solved by pruning
Answer: I know I'm a bit late, but here is a response from the book "Artifical Intelligence, A Modern Approach" from Russel and Norvig: "It may have occured to you that something like alpha-beta pruning could be applied to game trees with chance nodes. It turns out that you can. The analysis for MIN and MAX nodes is unchanged, but we can also prune chance nodes, using a bit of ingenuity. At first sight, it might seem impossible to find an upper bound on the value of a chance node before looking at all its children, because the value of a chance node is the average of it's children's value, and in order to compute the average of a set of numbers, we must look at all the numbers. But if we put bounds on the possible values of the utility function, then we can arrive at bounds for the average without looking at every number. For example, say that all utility values are between -2 and +2; then the value of leaf nodes is bounded, and in turn we can place an upper bound on the value of a chance node without looking at all its children".
Comment: This does not provide an answer to the question. Once you have sufficient [reputation](https://ai.stackexchange.com/help/whats-reputation) you will be able to [comment on any post](https://ai.stackexchange.com/help/privileges/comment); instead, [provide answers that don't require clarification from the asker](https://meta.stackexchange.com/questions/214173/why-do-i-need-50-reputation-to-comment-what-can-i-do-instead). - [From Review](/review/late-answers/22146)
Comment: As it's currently written, your answer is unclear. Please [edit] to add additional details that will help others understand how this addresses the question asked. You can find more information on how to write good answers [in the help center](/help/how-to-answer).
Answer: alpha-beta-prunning algorithm is using for improve performance and reject the options, which not consist the condition - or it's possible to set some factors of probability to take into consideration or not.
This algorithm work on tree structure - and if there are a lot of levels (10-20) - It allows you to eliminate paths - which will logically not be used - saving memory and computing resources.
In this particular case - for finding the minimum value it works like this:
First branch:

Go to B
Go to D - and there is 2 and 3 - so return the min 2
Go From B to E - and choose 5 - the minimal value in B points is actually 3 - so there isn't need for checking the next - cause everything below E, will be higher than D (3)

Second branch:

Go to
Go to F - and check 0 and 1
If C have 1 - than not necessary to go to G - cause 1 is the smaller.

The given sequence is a simplification - and in the case of this algorithm there are also layers - min & max

Source:
https://www.javatpoint.com/ai-alpha-beta-pruning#:~:text=Key%20points%20about%20alpha%2Dbeta,values%20to%20the%20child%20nodes.

Implementation, however, is a more complex matter - unless we do copy and paste :-)
https://gist.github.com/exallium/1446104/5109388cfc21578f555dcac0ba54da680326af7b
Comment: This answer describes the application of alpha-beta pruning to minimax, not to expectiminimax.
Comment: Alpha-beta pruning for minimax doesn't specify what to do with chance nodes.
Comment: Hi, Whats the difference?


What is the fundamental reason why information is on the event horizon of a black hole?

Question: I don't know the mathematics behind black holes and information theory but is there a simple explanation of why information is on the event horizon. Why can't it be elsewhere?
Comment: I guess you're talking about holography. Well, the basic reason why people say that is because the entropy of the black hole is proportional to its surface area, not volume. But then again, there are many different theoretical considerations on the black hole information paradox, but they're all ideas at this point, and we don't know which one is true (or maybe we haven't even found it, yet).
Answer: I'm going to answer the question you actually asked, but as you'll see, this is probably not quite the question you really wanted.
A possibly intuitive argument for it is information conservation. The basic idea behind this is Laplace's idea that if you knew the state of the universe perfectly at one instant of time, you could predict its entire future and deduce its entire past from that slice by applying the laws of motion. All the information about the entire 4D history of the universe is encoded in every 3D instant-of-time slice.
In general relativity, the slices can curve and bend, but the same general principle applies. If we take a surface that intersects (cuts) every worldline running from the beginning of the universe to the end exactly once, then the entire past and future of the universe can be determined from its state on that slice. This is called a Cauchy surface.
And if we have a region of spacetime surrounded by a surface such that every worldline passing through that region has to cross the surface, then what happens on the surface determines everything that happens inside the region. This is close to being the defining property of the event horizon of a black hole. All the information about what happens inside the black hole has to pass through the event horizon, and all the information that passes through the event horizon ends up in the black hole. If we know the state of the universe on the event horizon, we can use Laplace's 'Clockwork Universe' principle and the laws of physics to deduce everything that happens subsequently, inside the black hole. So all the information in the interior of the black hole is on the surface - or more precisely, passes through the surface.
That is certainly not the end of the story. The main reason physicists talk about information being 'on' the event horizon is that the Bekenstein-Hawking formula for the entropy of a black hole says it is proportional to the event horizon area. Laplace's idea tells us that all the information passes through the horizon, but gives no reason to expect it to do so uniformly. It is not clear why the information should be proportional to area. And of course we can consider concentric spheres inside the event horizon which also have the property that every worldline must pass through them, and must therefore also contain all the information, but which have a smaller surface area. What is special about the outermost of them; the event horizon? (There are special properties known, e.g. it's a null surface, but not reasons why that specialness should lead to this result.) I have not seen anyone give a clear and convincing answer, that doesn't amount to numerology (i.e. speculative deductions of deeper causes made from observing numerical coincidences). This is one of the current mysteries in physics.
There is a nice, fairly basic discussion of Cauchy surfaces and the implications for black holes and the information paradox in philosopher Tim Maudlin's (Information) Paradox Lost. Because he deliberately takes a provocative and controversial stance (which is good for science, of course), it's appropriate to point to replies, like (Information) Paradox Regained?
A Brief Comment on Maudlin on Black Hole Information Loss.
In summary - all the information about the interior of a black hole is deducible (by Laplace's 'Clockwork Universe' principle) from the state of the universe on the event horizon. The information is certainly there. But when physicists say this, they're talking about something different, which is that in a lot of calculations the information inside a closed region like a black hole or an entire universe is proportional to the area of the boundary. So far as I know, the reason for this is not currently understood.
Comment: @nielsnielsen "*welcome to the SE!*" - Don't get confused, this user is not new. Apparently he's been active on PSE for 7 years and posted a number of answers under the same unregistered name. This may be his way to avoid a negative feedback, but who knows. Your comments don't reach him, as this user account does not exist.
Comment: If the amount of information is proportional to the boundary area then perhaps that is because all the particles that created the black hole are on the boundary and there is nothing inside the boundary.
Comment: Perhaps the event horizon is important as a surface because events can happen simultaneously just above it in space so 2 or more bits of information can be changed simultaneously by hawking radiation just as two or more bits of information can be changed simultaneously in a gas in normal space by radiation.
Comment: For a spherical surface with a larger area it may be the case that bits of information don't have enough force holding them together as they are too far apart if for example the bits are  particles.
Comment: @Nullius in Verba, this is an entirely satisfying explanation, thanks for posting it here and welcome to the SE! I look forward to your next answers.-NN
Comment: @DaveTheWave "*What is the fundamental reason why information is on the event horizon of a black hole?*" - "*because all the particles that created the black hole are on the boundary and there is nothing inside the boundary*" - You've answered your own question in a comment.
Comment: "*This is one of the current mysteries in physics.*" - Too often people confuse their own lack of understanding with "mysteries in physics".
Comment: "*And of course we can consider concentric spheres inside the event horizon which also have the property that every worldline must pass through them*" - This is incorrect. Schwarzschild timeslices inside the horizon are not concentric spheres, but spherinders - 3D hyper-surfaces of infinitely long 4D hyper-cylinders of the spherical type.
Comment: @safesphere If the particles are all on the boundary is there mass at the singularity?
Comment: @safesphere- huh. OMG! I wonder if it's RON MAIMAN!!!
Comment: @safesphere, i read legends about him here... but it also was apparent that he was hard to take as an individual and had a mean streak. such a shame. -NN
Comment: @safesphere So,if I have this right, the black hole vacuum solution means the black hole just remembers the collapsing stars gravitational field  and twists it a bit at short distances.On an event horizon a photon can pass from one event to another (both events on the horizon) just in time. Or does the photon get pulled away from the  horizon?


Magnetic field re-routing

Question: Can a magnetic field be directed in a particular direction, that is re-route it, such that the magnetic field comes out in only one direction?
Comment: Google mu metal.
Comment: Pretty sure they do something similar in disk electromagnets.
Answer: Not entirely, but a disk electromagnet works by routing a magnetic field though a ferromagnetic core. It's not a 100% efficient process and you do have a portion that is not directed through the core. The escaped portion is known as flux leakage, and is usually a fairly small percentage of the total field.
From Wikipedia:

The green lines represent magnetic field lines. The closer the lines are to eachother the higher the field density. You can see that most of them pass out the way the designers intended, through the ferromagnetic object, and back into the core C. The white arrows represent the direction of current flow in the coil, and the green arrows represent the polarity of the magnetic field. The lines marked $Bl$ are the flux leakage.
However, I know of no such method or device able to direct magnetic fields with 100% efficiency. There are certain superconductors and metamaterials that show promise, but nothing I know of as yet.
There is a hypothetical elementary particle in particle physics known as a "Magnetic Monopole" (imagine a bar magnet that only has a north pole, or a south pole.) However, as yet, there has been no conclusive experimental evidence that magnetic monopoles exist.
May be of interest:
Georgia State University's science website on Electricity and Magnetism.
Google Scholar search on controlling magnetic fields
Can magnetic fields be redirected and focused at one point?  (Physics.stackexchange)
Mu metal
Comment: @Kurtovic If you had to extrapolate, better just to edit it out. :)
Comment: I agree, except for the topographical map analogy. You have to extrapolate quite a bit to get to your sentence. Nice answer.

Magnetic field re-routing


In regression analysis what's the difference between data-generation process and model?

Question: In regression analysis what's the difference between 'data-generation process' and 'model'?
Comment: The answers to this question will vary, as they should, because both "data-generation process" and "model" are used in varied ways by various authors.  @Weijie, do you have a particular reference in mind?
Comment: Data-generation process is never known, we choose the model in hope that we approximate the data-generation process sufficiently. That is one of the possible answers, it would help if you provided more context, so it is clearer what kind of answer you are looking for. Check out the chat, currently ongoing journal club discusses article where this issue is raised.
Answer: We all have a good sense of what "model" might mean, although its technical definition will vary among disciplines.  To compare this to DGP, I began by looking at the top five hits (counting two hits with the same author as one) in Googling "data generation process".

A paper on how the US Air Force actually creates data in logistics support.
Abstract of a paper published in Environment and Planning A concerning how "synthetic micropopulations" are created via computer "simulation models."
A Web page on "synthetic data generation"; that is, simulation "to explore the effects of certain data characteristics on ... models."
Abstract of a conference paper in data mining, asserting that "data in databases is the outcome of an underlying data generation process (dgp)."
A book chapter that characterizes the data of interest as "arising from some transformation $W_t$ of an underlying [stochastic] process $V_t$ ... some or all [of which] may be unobserved..."

These links exhibit three slightly different but closely related uses of the term "data generation process."  The commonest is in a context of statistical simulation.  The others refer to the actual means by which data are created in an ongoing situation (logistics) and to a probability model for an ongoing data creation procedure, intended not to be analyzed directly.  In the last case the text is differentiating an unobservable stochastic process, which nevertheless is modeled mathematically, from the actual numbers that will be analyzed.
These suggest two slightly different answers are tenable:

In the context of simulation or creating "synthetic" data for analysis, the "data generation process" is a way to make data for subsequent study, usually by means of a computer's pseudo random number generator.  The analysis will implicitly adopt some model that describes the mathematical properties of this DGP.
In the context of statistical analysis, we may want to distinguish a real-world phenomenon (the DGP) from the observations that will be analyzed.  We have models for both the phenomenon and the observations as well as a model for how the two are connected.

In regression, then, the DGP would normally describe how a set of data $(\mathbf{X}, Y)_i$ = $(X_{1i}, X_{2i}, \dots, X_{pi}, Y_{i})$, $i=1, 2, \ldots, n$ is assumed to be produced.  E.g., the $X_{ji}$ could be set by the experimenter or they could be observed in some way and then be presumed to cause or be related to the values of the $Y_i$.  The model would describe the possible ways in which these data could be mathematically related; e.g., we might say that each $Y_{i}$ is a random variable with expectation $\mathbf{X} \mathbf{\beta}$ and variance $\sigma^2$ for unknown parameters $\beta$ and $\sigma$.
Comment: You write the words "cause" or "related". I have a question about this. From your answer seems that DGP concept do not imply causation. However this "relation" is something more than correlation (or any type of association) or not? See also this my related question: https://stats.stackexchange.com/questions/399671/structural-equation-and-causal-model-in-economics
Comment: @markowitz "Correlation," strictly speaking, refers to a second moment of the bivariate random variable.  I use "related" in the wider sense of "not [statistically] independent."
Comment: I know, and exactly for this reason I stated "or any type of [only statistical] association". Can I repeat my question as: However is this "relationship" something more than the association or not? Starting from the concept of "true model", sometimes used as a synonym of DGP, it seems something more. If so, I don't understand exactly what it is. My previous link give an example.
Comment: @markowitz I'm afraid I don't understand what you're trying to ask.  That may be because I'm not sure what you mean precisely by "relationship" or "association."  I did look at your link, but the unusual English usage doesn't convey anything meaningful to me.
Comment: I'm sorry for my English. I tried to modify the linked question in clearer sense. I hope that it is understandable.
Answer: Whuber's answer is excellent, but it is worth adding emphasis to the fact that a statistical model need not resemble the data generating model in every respect to be an appropriate model for inferential exploration of data. Liu and Meng explain that point with great clarity in their recent arXived paper (http://arxiv.org/abs/1510.08539):

Misconception 1. A probability model must describe the generation of
  the data.
A more apt description of the model's job (in inference) is
  "Such and such probabilistic pattern produces data which resemble ours
  in important ways." To create replicas (i.e., controls) of the Mona
  Lisa, one does not need to bring da Vinci back to life — a camera and
  printer will suffice for most purposes. Of course, knowledge of da
  Vinci's painting style will improve the quality of our replicas, just
  as scientific knowledge of the true data generating process helps us
  design more meaningful controls. But for purposes of uncertainty
  quantification, our model's job is to specify a set of controls that
  resemble (D,$\theta$). Nowhere is this point clearer than in
  applications involving computer experiments where a probabilistic
  pattern is used to describe data following a known (but highly
  complicated) deterministic pattern (Kennedy and O'Hagan, 2001; Conti
  et al., 2009). We need a descriptive model, not necessarily a
  generative model. See Lehmann (1990), Breiman (2001) and Hansen and Yu
  (2001) for more on this point.
Comment: +1.  I especially like the distinction between *descriptive* and *generative* models of data.
Answer: The DGP is the true model.  The model is what we have tried to, using our best skills, to represent the true state of nature.  The DGP is influenced by "noise".  Noise can be of many kinds:

One time interventions
Level shifts
Trends
Changes in Seasonality
Changes in Model Parameters
Changes in Variance

If you don't control for these 6 items than your ability to identify the true DGP is reduced.
Answer: DGP is the virtual reality and a unique recipe for simulation. A model is a collection of DGP or possible ways that the data could have been generated.
Read the first page of this mini course by Russell Davidson:
http://russell-davidson.arts.mcgill.ca/Aarhus/bootstrap_course.pdf


Correct way to introduce acronyms

Question: What is the correct method of introducing the expanded form for Acronym in the bracket? is it expanded form first and then the acronym in bracket or Vica-Versa.
Also, what is the difference in introducing the acronyms to the American audience or to the British audience?
Comment: Acronyms are encouraged only to the audience that understands them.  Else the expanded forms are advised.  To help those who might need help, the expanded forms are given in brackets, but people use the other way round too. Expansion at the first time use is done such that in the succeeding part, the acronym could be used without expansion.
Comment: what about the audience?
Comment: You may refer to the different types of audience. Tech writing says, audience can be high-tech, low-tech or lay.  For the high-tech, acronyms need no expansion. Example, a coder can easily understand what is html.
Comment: I mean the British and American Audience.
Comment: I am not sure whether what I said will be different for the American or British audience. Possibly someone else could help you.
Comment: Purely a matter of style.
Comment: Yes, thanks to everyone for the responses :-)
Answer: There's no one correct way to introduce abbreviations. The U.S. Government Publishing Office Style Manual says (p. 221),

Abbreviations not generally known should be followed in the text by
  the spelled-out forms in parentheses the first time they occur.

However, it also gives examples (p. 196) of putting the abbreviations in parentheses. For acronyms that are well known, the Associated Press Style Book and Libel Manual, Sixth Trade Edition (Reading, Mass.: Addison-Wesley Publishing, 1996), gave the example of NASA, the National Aeronautics and Space Administration (p. 136): "NASA is acceptable on first reference [that is, without explanation].… mention the full name later." But as the comments point out, make sure that using the abbreviation actually helps communication.
Comment: I'll have to look up the page in my AP manual. I don't have it with me, and I'm not sure the AP manual is online; I think you have to buy it (I bought my paper copy). I wasn't looking at GPO online. Those are the page numbers in my copy of the manual.
Comment: Please give attributed links to quotes, and quotes rather than paraphrases.
Comment: Agreement. There is some degree of variation in style, and often an office or university department or newspaper editorial office, etc, will have a style guide indicating the local preferred use. Probably as long as you have a particular basically sensible style, and use it consistently, it will be acceptable.
Answer: This is a question of style, rather than a grammatical rule.  Either of the following are valid usage.

Many manufacturers now use Solid-state Disk (SSD) storage.
Many manufacturers now use SSD (Solid-state Disk) storage.

I prefer 1. but I'd have no problem to 2.  I wouldn't even object to text that mixed 1. and 2., but I would prefer a writer who was consistent.
The more important thing is that your first use of the abbreviation has the expanded form beside it.

Correct way to introduce acronyms


Does "laviniaque" from Vergil's Aeneid point to Romance palatalization?

Question: The second i in "laviniaque" from the 2nd line of Aeneid is supposed to be consonantal to fit the hexameter; therefore the pronunciation should be something like: /la'wi.nja.qʷe/.
My question is focused more on if that is a precursor to palatalization phoenomena seen from Romance languages, so this: /nja/ > /nʲa/ > /ɲa/; and if possible, are there any other examples of this sort of thing from the Aeneid or other CLassical Latin hexameter poems?
Comment: This is synizesis, and it's a common phenomenon in dactylic hexameter, even in Greek; the opening line of the Iliad famously has an instance of it.
Comment: Worth noting that there's an alternate reading *Lavinaque* which avoids the problem: https://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:1999.02.0050:book=1:commline=2
Comment: @TKR Lectio difficilior potior.
Comment: @fdb True, but I'm not sure what the lectio difficilior is in this case, since the adjective for "of Lavinium" would naturally be *Lavinius*, not *Lavinus*.
Answer: This is a phenomenon called synizesis (συνίζησις), and it happens in both Greek and Latin poetry. For example, at the beginning of the Iliad:

μῆνιν ἄειδε θεὰ Πηληϊάδεω Ἀχιλῆος

To fit in a hexameter, that εω needs to be a single syllable, maybe something like /jɔː/. Vergil uses it in various places; I particularly remember an instance of arjēs for ariēs "(battering) ram" during the fall of Troy.
Was this a foreshadowing of sound changes in Romance? Well, the standard pronunciation in both Greek and Latin poetry was still hiatus, with multiple vowels forming distinct syllables. Synizesis was an option used for poetic purposes, but distinctly not the default.
Since synizesis used in both Greek and Latin, and Greek never (re)developed a /j/ or /w/ like Romance did, I suspect the Roman authors were just imitating Greek practice rather than any pronunciation that was common in their era. The later changes in Romance were probably unrelated. (But this is just a guess, not something known for sure. It might be interesting to count the instances of synizesis by era and see if they become more common as Vulgar Latin develops.)
Comment: For *aries* you might be remembering 7.175 *hae sacris sedes epulis, hic ariete caeso*. Greek influence does seem possible but it should be noted that synizesis where the first vowel is ι is rare (https://dcc.dickinson.edu/grammar/monro/elision-crasis-synizesis).
Comment: @TKR That's the one!


why does adding a non volatile solute not decrease the boiling point?

Question: If I add a solute that decreases the bond strength of the pure solvent then wouldn't the solvent molecules evaporate more easily as it has lesser force holding it down now?But in my textbook, it's written that BP always increases when we add a non-volatile solute.
Comment: At least some solvent interacts strongly with the solute, to use your picture.
Answer: Decrease of the solvent activity due its decreased molar fraction is bigger than the solvent activity increase due eventual solvent solute interaction.
So the pressure versus solute molar fraction curve may not be linear as by the Raoult law, but is monotonous.


Does the current gets reduced?

Question: I was studying magnetic effects of electric current . I made an apparatus in which I set a straight wire above a magnetic compass and connected the wire to two cells of 9 volts. When I completed the circuit , the compass showed deflection . But after some time the deflection reduced and kept reducing until the compass needle pointed in its actual north south direction. I could not understand what the reason of this reduction in deflection could be . Please help me .
Answer: Your two cells have run down which is not surprising as you had connected a low resistance, the wire, to them.
This had resulted in a very large current flow and hence a rapid discharge of the cells.
To confirm this disconnect the cells and allow them to partially recover.
If the cells have not totally run down then during the recovery process the chemical reactions within the cells may allow the cells to deliver some current again.
Reconnect the cells to the wire and you might again see a (smaller) decaying deflection of the compass needle?
It is best not to totally discharge the cells before disconnecting and then reconnecting the wire to cells.
You might have seen a related effect when your torch goes out.  You switch off the torch, wait for a minute or so, and then switch the torch on again.  It comes on again.
Comment: Thank you it really helped... could you also tell me how i can maintain a constant potential difference?
Comment: Use a high capacity rechargeable battery eg lead acid accumulator with a series (variable) resistor?

Does the current gets reduced?


Does a data flow diagram seem correct when one external entity is connected with too many arrows?

Question: I am trying to model a data flow diagram in accordance with a requirement document. The document calls for a system with many roles that users can play, where it needs constant information exchange between different roles.
Here is an example of its functions, where 'Role A' is a reviewer or judge of some elements, and 'Role B' is a manager of those elements:
1. Role A is shown the list of elements (that are stored in a database), and chooses the items that he is interested in;
2. Role B gets a summary of the preferences of different Role A's, and assign the elements to them;
3. Role A receives the elements assigned to him, views them and leaves his feedback on the elements;
4. Role B summarizes the feedback from all Role A's, and decides which elements to eliminate from the pool.
Based on what I have learned, I would consider Role A and Role B to be different external entities, and so the DFD comes out this way:

As can be seen, there are too many arrows linked to each external entity; while the processes are not connected. This is different from most of the DFD examples I have found.
So, is my work correct? If not, how should I modify it?
Comment: This looks more like a use-case diagram than a dataflow diagram. In a use-case diagram you would partition the diagram into 2, with the first and last bubble on their diagram. Since you don't indicate what data is flowing along the arrows, I am very suspicious of process bubbles like "Leave feedback".
Answer: My opinion: I think your model looks more like a business process diagram than a data flow diagram. I do understand that they are sometimes easily interchanged. Your model would have as well be done in such a way that the roles are depicted in stream lanes (bearing in mind that the data base runs beneath each action depicted in circles)
A data flow diagram would only show input of data into a system and output of data (information) out of a system.
Answer: When your data flow has cycles, you're going to have a diagram like this.  Data flow diagrams typically don't show ordering, so what you have is normal.
There are a few things you might do if you wanted to make it look more ordered:

You should label the arrows with the data content that is flowing between the elements.  Often when there is a cycle, showing the content that is being exchanged clears things up.  For example, when there are two roles, customer and retailer, and the customer places an order, and the retailer sends a receipt.  This does have a cycle; however, it can be inferred that the order is placed before the receipt is sent.
You can step outside the traditional data flow diagram elements and augment the data flow diagram with numbers, if your diagramming system allows that.
In your case, since you have so many responsibilities being done by both role A and role B, you might split the roles into sub-roles that are more functional, where each sub-role only has one input accepted and one output returned.  The result would be that the data flow has fewer or no cycles.  In your case the sub-roles might be Role A Chooser, Role B Assigner, Role A Feedback, Role B Finalizer.  This would remove all cycles except the large one that involves the database.
If you then wanted to show that these sub-roles are all the same role, that would be a different kind of relationship, which you might (1) leave implied by the naming of the sub-roles, (2) show on a different kind of diagram, or (3) add to this diagram using illustration that steps outside of traditional data flow diagramming (like a well-labeled dotted arrow).

And lastly, of course, you can combine the above.
Answer: First of all you need to decide if data flow is the right diagram type for your main purpose, is it:

A: show how data flows
B: show in which order the steps of the process are done
C: show who is doing what

You might find an UML activity diagram with roles/actors to be a good alternative, as you seem mostly concerned with purpose B & C.
The linked example borrows ideas from BPM where swimlanes show who is currently active in the process:
http://www.uml-diagrams.org/ticket-vending-machine-activity-diagram-example.html?context=activity-examples
General points:

Define unique names for pieces of information and always use the same name. In your example you use item and element interchangeably.
If the diagram you draw suits your purpose, I wouldn't care too much about rules. Of course if your diagram is to be read by (many) other people,
it gets more important that it mostly follows a standard which they already know.

If you stay with the data flow diagram, I'd suggest:

I'd like the arrows to have labels - which piece of information flows here? (Erik points this out as well)
Call the roles something meaningful, i.e. reviewer and element manager
Comment: Well, actually it is not me who decides which kind of diagram to use, but it is an assignment by my teacher to model a given requirement document with DFD... (. _. ) So were there a better solution to my case, with a better understanding of 'how data flows' rather than merely showing the process order or responsibilities, if I had to take the purpose A as you have mentioned above?


Converting gdb files to shapefiles for AutoCAD

Question: I am new to GIS world and been slowly teaching myself Civil 3d. I have been working with importing maps and raster dgn etc., but now I have hit a wall with the contour files being in .gdb format. Is there anyway to convert these file types to shape files for AutoCAD?
Comment: Welcome to GISSE.  Please take the [tour](https://gis.stackexchange.com/tour) to learn how our focused q&a format works and how to ask question to get the best answers.
Comment: You can use a number of software packages to convert the .gdb files.  ArcGIS, QGIS, GDAL, FME, etc.
Comment: I recommend downloading and installing QGIS.  It's a free open source GIS software package.  It has tools to convert files between several formats.  https://www.qgis.org/en/site/
Comment: What GIS software are you currently using, this will determine the necessary steps to convert the files.
Answer: If you choose to take the QGIS route, it's as easy as opening a new project and going to Layer > Add Layer > Vector Layer... and be sure to select TYPE = Directory and SOURCE TYPE = OpenFileGDB or ESRIFileGDB
from here you can select the GDB folder and it will load into the Layers pane. Right click the layer > Export > Save Feature As... and choose 'ESRI Shapefile'.
Keep in mind, shapefiles have multiple files that need to be linked with them so you should compress all exported files to the path you chose into a .zip. After that, you should be able to do pretty much anything with it, including bringing it into AutoCAD
-Sven
Answer: There is a free plugin from ESRI called "ArcGIS for AutoCAD". I never used it so I can't really talk about it but it seem designed to handle case like yours.
If you prefer going to the GBD to Shapefile path be avare that shapefile have limitation and you need to take them in account to prevent unwanted result.
From Esri on Shapefile :

[...] shapefiles are acceptable for storing simple feature geometry.
  However, shapefiles have serious problems with attributes. For
  example, they cannot store null values, they round up numbers, they
  have poor support for Unicode character strings, they do not allow
  field names longer than 10 characters, and they cannot store time in a
  date field. These are just the main issues [...]

More on shapefile


Lifting a congruence $f(X) \equiv gh (\textrm{mod} \ p)$ to a higher power moduli

Question: I have the polynomial ring $R = \mathbb{Z}[X]/(X^N - 1)$ with $N = 11$ and $h \in R$ with
\begin{equation*}
h = 7 -11X - 7X^2 -12X^3 + 8X^4 - 11X^5 + -8X^6 + 11X^7 - 4X^8 + 2X^9 + 3X^{10}
\end{equation*}
Now I want to find the polynomial $u$ such that
\begin{equation*}
u\cdot h + v\cdot (X^N - 1) \equiv X - 1 \ (\textrm{mod} \ 32)
\end{equation*}
where we assume $\gcd(h, X^N - 1) = X - 1$ reducing coefficients $\textrm{mod} \  32$. If we also let $*$ denote the multiplication in $R$, I can easily find $u$ such that $u*h \equiv X - 1 \ (\textrm{mod} \ 2)$ using the extended Euclidean algorithm, and it's $u = 1+X+X^{4}+X^{5}+X^{9}$, but I'm having issues using this to construct the solution such that $u' * h \equiv X - 1 \ (\textrm{mod} \ 32)$. Can someone outline an algorithm or link me some reading material on how to do this?
Comment: Maybe this question should be moved to math SE, there is no crypto related question in here
Comment: Thank you. The reason I asked in crypto SE is because this is a part of an attack on the NTRU cryptosystem where an unpadded message block $m$ has been encrypted with different randomly generated $\phi_i \in L(3, 3)$, that is $\phi_i \in R$ where three of its coefficients are 1, and three of its coefficients are -1 with the rest being 0. The first step in this attack is to find the $u$ in my original question.
Answer: One way to raise solutions to higher powers of the modulus is Hensel's Lemma.

First, you need to express your element as a root of a function: $g(u)= hu - (X-1)$
Find the solution modulo 2, which you already did.
Use the Lemma to raise this solution to higher powers of 2. Maybe it is useful to do this in several steps. Find the solutions mod 4,8,16 and then 32.


Does LabVIEW Linx affect the communication speed with Arduino?

Question: I am using LabVIEW to connect to Arduino Mega. I control a logic gate to switch the current On/Off and at the same time there is an I2C connection with a sensor. It came to my attention that the sensor time resolution is very low and I cannot get enough data points from the sensor (I know that the sensor is capable of much better resolutions). My question, is this a problem inherent to LabVIEW and Linx module or my program in LabVIEW? Should I turn to coding with Arduino programming language or C?
note! My reason for using LabVIEW is that I need to sync timing of data aqcuisition with another device which is connected through another VISA port.
Comment: As far as I know, VIs are kind of compiled into c/c++ under the hood. It might help us if we could see your LabVIEW program and know which sensor it is. How do you connect to the arduino? I2c as well?
Answer: For specific timing requirements, my recommendation is to write your own Arduino code.
The Linx module deploys its own ino program to the Arduino and uses serial IO to send it commands and receive data from it. Open NI I/O Trace to observe that traffic.
Comment: Thanks for the answer, I exactly did that.
Linx communicate every command to the arduino and wait for the answer, the loop frequency with linx drops as the linx blocks increases in the loop. it droped to 10Hz for me. I am now using the arduino code and send the data back to labview using serial communication. Its much faster (Up to 300Hz loop frequency).


How to programmatically list all controllers in Rails

Question: I'm trying to build a RESTful app to actually manage many kind of configurable objects, so there are a large amount of "resource" types, and hence a lot of controllers. I'm still at the POC phase, so it will be nice if I can show all controllers in a first navigation page, so any easy way (programmable) to do that?
Answer: In Rails 3.1+:
Rails.application.routes

This will give you all the controllers, actions and their routes if you have their paths in your routes.rb file.
For example:
routes= Rails.application.routes.routes.map do |route|
  {alias: route.name, path: route.path, controller: route.defaults[:controller], action: route.defaults[:action]}
end

Update:
For Rails 3.2, Journal engine path changed, so the code becomes:
routes= Rails.application.routes.routes.map do |route|
  {alias: route.name, path: route.path.spec.to_s, controller: route.defaults[:controller], action: route.defaults[:action]}
end

Update:
Still working for Rails 4.2.7. To extract the list of controllers (per actual question), you can simply extract the controller and uniq
controllers = Rails.application.routes.routes.map do |route|
  route.defaults[:controller]
end.uniq

Answer: ApplicationController.subclasses

It'll get you started, but keep in mind that in development mode you won't see much, because it will only show you what's actually been loaded.  Fire it up in production mode, and you should see a list of all of your controllers.
Comment: You can require all files in "#{RAILS_ROOT}/app/controllers" to populate your list in development mode.
Comment: This is not a complete solution because It does not get all controllers, only direct subclasses of AplicationController. If you have two levels of nesting in your hierarchy you will not gat all of them. Use ApplicationController.descendants instead. And for getting all controllers at load time you can use Rails.application.eager_load! in your development or test environment initializers but be aware thar it will load all rails application an it can slow down boot and have any side effects.
Comment: It is important to note that this works in production mode but not in development mode, since the controllers are loaded lazily.  Fire up your app in rails console mode and look at the subclasses/descendants and you won't see any.  Type in the name of one of the controllers, and then the named class will appear as a subclass.  ActiveAdmin seems to do a good job at this... you might want to check out their code on GitHub.
Comment: Thanks for explaining this I was wondering why ApplicationController.subclasses wasn't showing much!
Answer: While @jdl's method will work in a number of situations it fails if you have controller subclasses (e.g. ContactPageController inherits from PageController which inherits from ApplicationController) .
Therefore a better method is to use:
::ApplicationController.descendants

Using routes may not work if you have a complex app where parts are abstracted as engines, these will be hidden.
Comment: subclasses didn't work for active admin but your works! thank you
Answer: I particularly liked @jdl solution
ApplicationController.subclasses

But in my case, where I really needed all the controller names in underscore format (I don't have subdirs eg: /admin) I used the following:
Dir[Rails.root.join('app/controllers/*_controller.rb')].map { |path| path.match(/(\w+)_controller.rb/); $1 }.compact

Answer: Google: 'rails list controllers'
First result.
http://snippets.dzone.com/posts/show/4792

After learning about the subclasses, I think the code from my link could be done simply as..
ApplicationController.subclasses.each do |c|
  puts "Controller: #{c}"
  puts "Actions: #{c.constantize.action_methods.collect{|a| a.to_s}.join(', ')}"
end

Comment: If you're going to list controllers and actions, why not just use rake routes? Sure the output is messy, but that's fixed by expanding your terminal window.
Comment: I just was putting down quick code that would be similar to what the link was doing.  Practicing a bit so I remember.
Answer: Another solution (if you're in Rails 5) and in Development and need to map controllers and actions:
Rails.application.routes.set.anchored_routes.map(&:defaults)

You can reject [:internal] if needed:
.reject { |route| route[:internal] }

Answer: Dir[Rails.root.join('app/controllers/*_controller.rb')].map { |path| (path.match(/(\w+)_controller.rb/); $1).camelize+"Controller" }

This gives us the exact name of the controller as it appears within the controller file.