Can BREACH be thwarted by simply adding a sort of "salt" into the page being compressed?

Question: Django supplies GZip middleware, but the docs issue a stark warning about BREACH, which I had, hitherto, forgotten about. The first thought I had was that I should be able modify the middleware to first add a random amount of random characters into the request body before GZipping it, and render BREACH useless. Is this correct, or am I completely misunderstanding BREACH?
Answer: If you have always the same amount of random data the attack will probably still work in the same way. If you instead add a random amount of random data the original attack might not work any longer but I think it can be modified by simply trying again and again in the hope that the same amount of random data is added often enough. Which means it will slow down the attack only and how much depends on the amount of random data you add. For example if you add between 1 and 10 random characters at the top of the file it will probably only slow down the attack by a factor of 10. If you add 1..100 characters the slowdown will be factor 100.
A better approach would be to create some random value and modify the CSRF token so that it will consist of this random value and an XOR or with this value with the original CSRF token. This way the CSRF token will change all the time inside the HTML so that BREACH will no longer work.  And the application can easily get the original CSRF token back.
Answer:
add a random amount of random characters into the request body before GZipping it

This will slow down the attack, but not prevent it.
The idea behind BREACH (and compression oracles in general) is that the plaintext (and therefore the ciphertext) will be slightly shorter if the attacker guesses the next byte of the secret token correctly.  If there is some randomness in the plaintext length, the attacker can make the same guess many times and "average out" the results.  Requests made using the correct guess will be slightly shorter on average.
(A real attacker would use a more sophisticated statistical technique to reduce the number of requests required, but you get the idea.)


How do you repair a grounded aircraft at a remote location?

Question: If a large passenger airliner needs repair at a remote airport without services, are there "flying repairshops" that can swoop in and do the work on site?
Comment: ...[like this!](https://www.youtube.com/watch?v=9ruArctYYbM) :) (OK, maybe the DC-3 isn't "large" by today's standards...)
Answer: If you can, you'll fly in maintance yourself to get it fixed, perhaps by chartering a cargo plane on the way. If you can land the aircraft, you can probably get any spare part in through the same runway, with varying degrees of difficulty.
If it's bad, Boeing has an Aircraft On Ground team who should be able to fix pretty serious damage. Often, it may just be patched up until it's good enough to fly to get it to a place with better facilities.
Comment: @jwenting: #3 engine is accessible reasonably. It's under right wing. Engines are numbered left to right, so the hard to access centre one is #2.
Comment: It seem airlines also share technical resources including their own 24/7 AOG rapid response teams. There are also independent companies offering this sort of service. [Example 1](http://www.ba-mro.com/baemro/aogMaterialServices.shtml), [example 2](http://www.monarchaircraftengineering.com/Services/AOGSupport)
Comment: yup. KLM used to have (maybe still does) a regular shuttle flying maintenance rigs, spare parts, and crews to stranded MD-11s. Maintenance facilities for them are few and far between and they were/are rather prone to trouble in the #3 engine (of course, always the hard to reach bits fail first).
Answer: If the problem is a bad engine and the aircraft is a 747, the most cost effect solution is usually a 3-engine ferry to a repair station. From memory, the protocol to be observed includes:

The Captain must do the flying.
Only the cockpit crew can be aboard.
The fan must be tied down to prevent it's rotation in flight.
A specific takeoff procedure is to be followed.

At the two 747 carriers I worked for, we usually did a 3-engine takeoff in the sim on our 6-month checks. I did two of them for real, and it's no big deal.
Answer: No airport capable of handling widebodies will have no services at all, but of course many places lack a full maintenance shop.
As mentioned elsewhere, Boeing has a go team that can get most repairable damage brought back to flyable condition. "Flyable" does not necessarily mean "normal, scheduled service" - it means able to leave the remote field and limp back to a facility able to do the proper work. This could easily be Seattle or Hamburg from Johannesburg. If you want to reposition an aircraft for maintenance you can have a LOT of stuff broken - basically if the engineers say they'll ride in it, and you avoid overflying large cities, you're good.
Several large aircraft have an option to carry an extra engine on the wing to a remote grounded aircraft - expensive, but cheaper than cargo (it can be on a scheduled revenue flight). Put a couple of mechanics on board and the job's done in a day or two. The bad engine comes back the same way.
Comment: @jwenting Things are hopefully better now than they were in the 1990s when I was captaining 747s, but we constantly faced the problem of having no repair capability (or even working huffers if the APU was inoperative) in third world countries, and especially in Africa. Right or wrong, we often didn't write up grounding problems in such places when there was no real compromise in safety.
Comment: Actually, quite a few do lack the facilities because no such aircraft are stationed there, they just land, drop off passengers, and leave again with hopefully more passengers. Think charter destinations. Or what facilities they have are for light aircraft and maybe business jets only, incapable of handling anything airliner size. That's in fact very common.
Comment: @Terry I doubt much has changed in most of Africa except for the worse in the last 20 years or so...
Comment: There are also issues that arise when an airplane has to divert sure to an emergency, and maintenance may not be available where they land.
Comment: @nitpickers: when I said "no services at all" I meant it. Nothing. No FBO, no terminal, diesel is delivered by truck only on request. You don't land a widebody there unless you're desperate.
Answer: That would depend on the damage that needs to be repaired.
Some repairs (like replacing an engine) can be done on site.
For more serious damage that can't be fixed on site they will make a quick patch and then fly the aircraft to a facility capable of properly doing the repairs while taking care to not aggravate the existing problem.


WPF 508 Compliance

Question: Are there any good guides on making WPF applications 508 compliant? For example, does every text input box need to be associated with a label via some tag type like with webpages?
Answer: There is this white paper at WindowsClient.net that appears to contain guidelines.
There is also this pdf about accesibility in Infragistics' WPF controls. It may contain some clues, but it is in the context of custom controls rather than entire UIs.
Answer: I find Engineering Software for Accessibility portal page very useful, as it covers all aspects of the accessible application development - with a handful of links to MSDN pages.

WPF 508 Compliance


Mean value property satisfied of continuous functions.

Question: If $u$ is only continuous and satisfies Mean value property , is it true that $u$ is harmonic in $\Omega \subset \mathbb{R}^n$ .
$\Omega$ is bounded and open.
What basically here should I know to prove it .
Hints are appreciated .
Thanks
Comment: What is $\Omega$? What exactly do you intend by mean value property?
Comment: @DavideGiraudo : i've edited.
Comment: Roughly speaking, you need to approximate $u$ by mollifiers, and then use the Mean Value Property to show that $u$ is harmonic. I learned this many years ago, and I have no reference right now.
Answer: You can find a (sketch) of the proof on Wikipedia, based on approximation by convolutions. A different approach, based on the solvability of the Dirichlet problem for the laplacian, can be read in Axler's book here.
Answer: One approach is to convolve with a radially symmetric mollifier and show that the convolution actually agrees with $u$ (it's better than an approximation). Intuitively, radial symmetry means that integrating over spheres gives you $u(x)$, and the for the radial integration use that the mollifier has weight one. This shows that $u$ is in fact smooth.
To show it is harmonic, an interesting approach is to use second-order Taylor approximation to show that for $C^2$ functions $u$, we have
$$\Delta u(x) = \lim_{r \rightarrow 0} \frac{2}{r^2}\left(\frac{1}{|\partial B_r|}\int_{\partial B_r(x)} u - u(x)\right).$$
Applying the mean value property gives harmonicity.
Answer: There are authors who define Harmonic functions as continuous functions satisfying the mean value equality, like Doob in his 'Classical Potential Theory and it's Probabilistic Counter Part'. This admits interesting generalizations, if, e.g, the Lebesgue Measure is replaced by other measures.


How can the impact of cold boot attacks be minimized?

Question: Short of powering down and maintaining physical security for sufficient time, what are effective strategies for keeping keys from being disclosed by cold-boot attacks, and can anything be done without hardware designed to prevent such attacks?
http://citp.princeton.edu/memory/
Comment: There's essentially two scenarios for this attack: Either the system is already running, but password protected, or the system has just been shut down. It's also generally aimed at hard disk encryption, since if properly implemented, the only practical way to defeat it is some kind of side-channel, key interception, or social engineering attack - there's seemingly better angles of attack if hard drive encryption isn't used.
Comment: Wow, that is really interesting!!! Perhaps you could somehow run a program that overwrites the at risk RAM several times before completely shutting down, if possible? I know that is what the government does when they dispose of harddrives....
Answer: One approach is to use highly non-redundant key schedules. For instances, if you can recover any large piece of AES's expanded key schedule in memory you can run the expansion in reverse and recover the original input (which then allows you to generate all of the round keys, even if you were only able to recover the values from a single round, or even portions of different rounds). To some extent, this is intrinsic to AES's design, so it's hard to avoid, but for contrast consider Blowfish's sbox expansion design - it seems quite difficult to recover the master key or the entire sbox if you manage to recover, say, half of the sbox values.
Another approach, one that works with AES, is to simply ensure the key never hits RAM by keeping the entire state in SSE registers or debug registers. This approach is only usable by an OS kernel, but often the kernel is going to be doing the crypto that is most prone to a cold boot attacks, like disk encryption, so this could be a promising approach.
Answer: I just found this article, http://www.blackhat.com/presentations/bh-usa-08/McGregor/BH_US_08_McGregor_Cold_Boot_Attacks.pdf while looking for something else, and it outlines several software-only solutions that seem viable (a couple of which have been touched on here already), although not foolproof.

store keys in memory regions that must be overwritten no matter what operating system is booted. (forces the attacker to use specialized hardware, not just a special OS)
store keys in MMX/SSE registers
spread keys across the address space to minimize the benefit of error correction to the attacker.
overwrite keys before reboot/shutdown
monitor chassis intrusion and temperature sensors for precursors to an attack (case opening or sudden drop in memory temperature) and begin overwriting keys on the spot if either event is detected. (Assumes that an attacker sophisticated enough to take the RAM out of the system for forensic analysis will also try to cool it in advance of system shutdown to maximize the effectiveness.)
Comment: Nice! The chassis intrusion sounds cool. It could protect against a cold-boot attack via a jumper reset!
Answer: Well if your system is successfully attacked by cold boot, it's not that your crypto system failed, but physical layer of IT security.
After cutting the power, data in RAM will fade away. If you lose one bit in decription key, key is not valid (well, with only one bit lost you can guess it, but I guess you got the point). All in all, shutting down and powering computer will corrupt any lefovers of key.
So, to preform cold-boot you need to freeze RAM modul. To do so, you need "freezer" and access to modules. Well, if intruder walks into room when someone is shutting down computer (and he, by some miracle, is not noticed), opens case and starts spraying freezer on RAM modules (while computer is still shutting down)...
Well, let's just say you have much bigger problems then failed crypto system.
EDIT: Suma summare, in real world this is pointless to discuss (due to ROI). The best strategy would be actualy from social angle - user must be next to computer until it shutdowns completely.
Comment: The paper I linked notes it's as much as "several minutes" before data is gone after powering down, and that freezing makes that considerably longer.
Answer: I think Jack Lloyd's advice is worth taking a look at.
Another frequently used trick is to XOR the keys on memory by themselves periodically... With short periods, of course.
An example of this is PGP. It used to flip bits in the memory once a second, since quite early versions. This shortens the life-span of key ghost images.
Of course, there's an extra bit used to store the current state of the keys, and the whole XOR process has to go within a single atomic transaction, with a locking mechanism as with the key read operations.
Answer: I can't think of many situations in which the memory but not the hard disk contains sensitive information and is accessible in the minutes after shutdown, but not while the computer is on... but if you, say, have government secrets on a laptop with the encryption key stored only in your memory and the computer's memory, and you leave that laptop on a train after turning it off, you could use secure-delete / smem to erase data in memory (the computer's memory, not yours) before shutting down. I would advise making this process automatic! (And also advise to erase the swap file, which will hang around a lot longer than data in RAM).
Answer: not sure if this is the "best answer" or if this is practical in your situation, but for the extremely paranoid, you can use both hardware-based disk encryption (wherein the cryptographic material never leaves drive enclosure) + s/w-based. The former mitigates cold boot (or any other memory-based attack, such as via firewire port) while the latter gets around the annoying issue w/ h/w-based FDE wherein a warm reboot does not (ever?) require re-authentication.
So, for eg, a Seagate Momentus FDE drive + BitLocker w/ TPM.
This is probably a bit extreme for most people, since most laptop thieves don't do the cold boot attack on their stolen (but screen locked) laptops--lest they damage the hardware & reduce the re-sale value of their warez.
alternately, you could implement s/w-based FDE (BitLocker+TPM+PIN, for eg) along w/ a secondary layer of encryption (EFS, or a non-system TrueCrypt volume, or whatever). While a screen-locked laptop might contain all cryptographic information in memory (if encrypted volume was mounted at the time of theft and robber did cold boot attack in time), it strikes me as unlikely that the laptop thief would have sufficient knowledge + resources to carry out the attack on both security "layers," or probably even the first layer.
The people who know how to do the cold boot attack are probably working somewhere & have too much to lose to enter a life of crime, so avoid constructing an "ideal" security model for data-at-rest while ignoring other threats (exploits when machine is in use, such as buffer overflow exploits).


How can I edit my own questions and answers?

Question: How can I edit my own questions and answers for example, give the symbols for pi, integration, and draw diagrams
Answer: You are looking for the Notation page. It provides a mini-tutorial on the use of inline equations and centered equations. For pi, you want \pi and for integrals, you want \int (I'd wrap those with $ to start MathJax, but it is not enabled on Meta).
Sadly, there is no method to draw figures via tikz or \includegraphics in the normal Latex methods. However, there is a button to include the graphics (circled below):

which can be used to pull an image from a website or from your computer. Also sadly, there is not an easy way to center the image.
Comment: For diagrams I find Google Draw to be good enough for most things. I draw the diagram in Google Draw then screen grab it and paste into Paint Shop Pro (other image editing apps are available). For graphs I use Excel, but Google Sheets is pretty good and free.


Content of AUTHORS file

Question: GNU recommends making an AUTHORS file to list the authors and contributors of a program. But how many "levels" of authors and contributors should the file contain? E.g. if I write a program foo that actively use some library, should I include the authors of that library in the AUTHORS file? It seems that the answer is "yes," because total code of foo contains code from the library. But if yes, that means I should also include the authors of all other libraries, including standard libraries of the compiler, authors of the compiler and other tools for producing final executable code, authors of the OS... When I should stop?
Answer: It seems reasonable to me to assume that the dependency project should have its own AUTHORS file and the whole process would reduce down recursively. You are responsible for identifying the authors for code you wrote. The authors of the other libs should already be mentioned in the AUTHORS file of each lib.
Answer: The authors of your libraries dependencies are not authors of your project. As you have pointed out, not assuming that can get out of control. Typically you just list people who have contributed something directly to the project, in particular code and documentation. If, however, a particular dependency or person has significantly influenced your project, then it might be appropriate to have a "special thanks" section identifying those projects, communities, or people.
Let me use GitHub as an example. Each project has an automatically generated "contributors" list. How do they decide who is on that list? I am not exactly sure, but it appears to be anyone who has had a commit pushed to the repository.

Content of AUTHORS file


Softwares to run old version of windows application/games on lastest version of windows 10

Question: Is there is any wine like software/emulator for windows to run windows XP or prior version of windows such as windows 95 application on latest version of windows 10?
Comment: You will probably have to use a virtual machine and an original OS image and license.
Answer: Yes, it is. You can download the Windows Virtual PC directly from Microsoft.
You can download Windows XP Mode directly from Microsoft. I don't believe that this actually has an expiration date.
You can also download Windows 7-10 virtual machines directly from Microsoft.
These machines are ostensibly provided to let you test browsers, but you can install other software on them. Read the procedure to snapshot them so that after they expire in 90 days.
Comment: Thanks for responding, the virtual machine required separate resources as hard-disk , memory etc which I am is not willing to install is there is any other better workaround?
Comment: It'll all run on your machine as is. The allocations are from existing resources. The memory is only allocated during use. The drive space allocation, however, is real and since you might only be installing a game or two, they will only need enough space to satisfy the game's requirements. Maybe a GB or two?


Why is Stephens in the cell with the Joker in The Dark Knight?

Question: After being captured, the Joker uses Stephens (the cop watching over him) as a hostage in order to escape from his cell to make his phone call, seen here. Why is there anyone in the cell at all, when they know the Joker is verbally manipulative? It seems purely to be an unnecessary risk.
Answer: First thing, I believe police was not much aware about the Joker being verbally manipulative till that point of the situation. They saw him as someone who can impress local goons and bring them as his gang members. But they underestimated him as someone who could manipulate a straight person like Stephens.
Till that point they thought that other local goons are following him as he is making huge money for them or as someone with more skills with them, which was irrelevant with Stephens.
And they need to have someone stand in the interrogation room with him for safety purposes, in case the Joker could attempt suicide or damage to himself, seeing his previous actions. As he didn't seem to care about his own life.
Comment: Good point, they might have been unaware of how dangerous he was. If they really thought he would harm himself, why not take away anything he could harm himself with, restrain him with hand cuffs, and stand guard outside?
Answer: The novelisation simply states that Stephens is standing guard. No further explanation is offered.

The Joker was certainly damaged — bloody, smeared — but he seemed perfectly content, sitting in the interrogation room, guarded by Gerard Stephens.
  "I want my phone call," he said.

The film script offers little extra by way of explanation, other than that Stephens is guarding the door

INT. INTERROGATION ROOM, MCU, GOTHAM CENTRAL – NIGHT
[The Joker sits, smiling, content. Stephens guards the door.]

Based on the fact that the Joker is a high profile prisoner, that would certainly merit someone keeping a very close eye on him, if only to dissuade other policemen from beating the snot out of him.
He is also in the process of being interrogated about an ongoing hostage/kidnapping situation (hence why he's in the interrogation room). They'd want to have someone in there with him, in case he says something that might be of use. Also, they'd want to prevent anyone not connected with the prisoner from entering the room in case they somehow prejudice his testimony.
Comment: Nice answer, I like the interrogation aspect. I can see why they'd want someone _inside_ the cell, as opposed to standing guard just outside, in case he provided any help. +1


Group-like Elements in a Coquasitriangular Bialgebra

Question: What do we know about group-like elements in coquasitriangular bialgebras?
In particular, we know that a commutative bialgebra in which all group-like elements are invertible is a Hopf algebra (see this question); does this result generalize to coquasitriangular bialgebras?
More precisely :
Let $(B, r)$ be a coquasitriangular $k$-bialgebra, i.e., a $k$-bialgebra with a $k$-linear form $r : B \otimes B \rightarrow k$ satisfying :

$r$ has a convolution inverse $\bar{r}$, i.e., $r \ast \bar{r} = \bar{r} \ast r = \varepsilon$,
$H$ is quasi-commutative, i.e., $\mu^{op} = r \ast \mu \ast \bar{r}$
$r (\mu \otimes \text{id}) = r_{13} \ast r_{23}$ and $r(\text{id} \otimes \mu) = r_{13} \ast r_{12}$, with $r_{12} = r \otimes \varepsilon$, $r_{23} = \varepsilon \otimes r$ and $r_{13} = (\varepsilon \otimes r)(\tau_{H, H} \otimes  \text{id})$,

and let $GLE$ = { $g \in B ~|~ g \neq 0, \Delta (g) = g \otimes g$ } is the set of group-like elements. Then if every element in $GLE$ is invertible, does $B$ have an antipode?
Answer: Take a look at [T. HAYASHI , Quantum groups and quantum determinants, J. Algebra 152 (1992), pp 146–165.] I'm not sure if he answer completely your question but for sure he discuss it in the context of CQT bialgebras. He construct families of (CQT) Hofp algebras by inverting all group-like elements.


Order book depth views preferences: order-by-order vs. total aggregated volume by price levels

Question: Which is best? Or is it irrelevant?
Hi! Considering Level 2 Data (Market Depth), I want to understand the advantages to have all the order book on an order-by-order basis (or tick-by-tick basis) versus only have a total aggregated volume by price levels?
Who are the principal users/traders of each method? Can you give me an example of a trading strategy using the order-by-order view?
Do stock exchanges and vendors should provide Level 2 Data on an order-by-order view?
Thank you very much in advance. Congrats for this very useful and interesting website!
Comment: Some exchanges disseminate the full order book; some exchanges disseminate only a price level-aggregated book. You'll rarely get a choice. As for an example of a trading strategy, that is not what this site is for at all.
Comment: Thank you very much AlgoQuant and chrisaycock for your answers and comments. Have a nice day.
Answer: As someone pointed out, most times you do not have a choice because how the order book is disseminated heavily depends on the exchange. In Asia there are exchanges that even show the name of the counter party (though only aggregate volume not each order). Other exchanges provide a full "view", again others price level aggregates.
Broadly speaking, some strategies benefit from being able to process each single order. One of such strategy would be a predatory VWAP algorithm (I have written some myself) which greatly benefit from information of each order. In combination with what I mentioned that some exchanges show counter party names/ids in aggregate volume from, such individual order information can be incredibly valuable. It is not one of the hardest exercises to quantitatively identify large VWAP orders that are "worked" in the market. Any individual order information is highly valuable in understanding whether they are aggressive, neutral, or defensive in their trading approach which in turn will give a lot of information about whether they most likely "chase" certain short-term momentum or wait to buy on expected pull-backs. Sorry but I am not able to pass on more detailed information but I think that should be detail enough to understand when order by order information can be put to use.


If God is good and knows the future, why does he knowingly create people who go to hell?

Question: If God knows the future, that means He knows some people will end up eternally suffering in hell. Isn't this a contradiction since God is good? Why doesn't God change these people, or make it so they are never born (if God is all possible he could do that).
Comment: I think this question is a good one, the answer is in the area of free will. God knows ahead of time, but if He only created the beings that would choose Him, it would be along the lines or making robots. Hell and the Lake were not made for humans, the Bible says. A human has to choose to go there themselves. This is free will.
Comment: I do not see this as a duplicate, as the referenced question "Why do evil, suffering exist?" does not touch on the topics of foreknowledge. Nor do I see any related questions on the right side bar that includes evil and foreknowledge, nor do my searches of "evil know future" reveal any. Please enlighten us.
Comment: The question you are asking is the question of theodicy, stated slightly differently: If God is all knowing (omniscient), all powerful (omnipotent) and all good (omnibenevolent), then how could evil exist/how could He send people to Hell/why did he create us capable of sinning. That question's answers address this. Omniscience is the foreknowledge portion of the question, and if you search for theodicy, omnipotence, omniscience, evil, or any combination of those, you'll find that the same question has been asked several times in slightly different ways.
Comment: This question could have been closed for at least three other reasons. I chose this one so that you could see how it was answered already because I thought it would be more helpful than closing it as opinion based, or as general philosophical question without a proper denominational scope, or as a Truth question.
Answer: You assume the apparent truism, that God can do anything, which is a logical fallacy. If God can do anything, then he can do the impossible, therefore nothing is impossible. Yet this would leave us with an absurd world. God could cease to exist, and yet also exist. He could violate the law of non-contradiction, he could make a rock so big he couldn't lift its, ....
Instead, take a reasonable, not absurd view of God. Perhaps it was impossible for God to create people who could freely reject him, and yet not have at least some people reject him. It's like offering a free lunch, yet being impossible to guarantee that some people will choose to go hungry.
Secondly, the general view is that though God knew some would reject him and go to hell, there is no evidence to suggest that He could have created a better world in which fewer people would reject him. This is Charles Finney's approach in his "Systematic Theology" book, or see section V point 4 in http://www.gospeltruth.net/1836SOIS/10sois_election.htm
Yet Finney, like most, bases his ideas on a belief in the unchangeableness of God, see the same article.(immutability)
Others, noting that "God changed his mind" dozens of time in the Bible, assert that the future is in fact full of possible actions, instead of inalterable events. Therefore God only knows who possibly will go to hell, because the future is only a possibility. You can read more about this from Greg Boyd and "God of the Possible."
Comment: Please check to see if a question is a duplicate before answering this.  This particular question has been asked in various guises a bunch of times.  Also, this question should have been closed because it's a general philosophical question, and isn't asking for a doctrinal perspective.  Another reason not to answer in the first place, but rather to vote to close.  You answered well based on reasoning, but this site isn't about that.  See [this post](http://meta.christianity.stackexchange.com/questions/4117/what-is-wrong-with-answering-general-questions-biblicaly) for an explanation of why.
Comment: To clarify, I agree with you, but the question itself is not fit for the site, and therefore answering it is not appropriate.
Comment: Thank you for your kind comments, I am still fairly new here. I thought the question dealt with foreknowledge and evil, which seems to me to be more theological than philosophical. As I commented above, I still do not see a relevant question that deals with both topics. Answering the problem of evil without reference to foreknowledge is insufficient, as well as asserting God's foreknowledge without reference to evil and God's character, also seems to be missing the point. Can you suggest a rewrite?


Can this be patched?

Question:
I was going this morning to run some errors and saw that my car PSI was at zero. Went to the gas station to pump some air and once I left the tires was slowly decreasing in pressure. Can this incision be patched up? I don't think its as bad as it looks. I'm going to the tire shop tomorrow but want to see what you guys think? Thanks.
Comment: Is that where the air is coming from?  Did you test with soapy water or is it obvious enough to feel?  I just ask because I've seen nicks like that on the sidewall; that never leaked air.
Answer: Expect to be told that damage that severe is not able to be safely repaired. It's not able to be unsafely repaired either. It's time for a replacement tire.
Sidewall damage, unless cosmetic, is always fatal. You may find a couple of posts here for small gouges in the heavier portion of the rubber. The key for those is "small" and "gouges," not rather large slices all the way through as in your tire.
Answer: If a shop says we can fix it - leave - that damage has affected the plies used in the construction of the sidewall and it will fail even if they can get it to hold air.
I had a sharp stone do this to a new winter tyre.... Replacement - ah well its only money - but the only thing to do.
Answer: That is not repairable damage; neither is a bald tire worth repairing.  If that were my tire, I'd be first asking who shoved a knife in it.
Comment: Now that you mention, it does look a lot like knife damage.
Comment: After I said that, I looked back at the picture.  It could be a lot of things, but that tire is in bad shape.  The sidewall is cracking, and you can tell by the tapered wear on the sidewall text that it's been run underinflated for a considerable distance.  It's entirely possible that they got something shoved into the sidewall by running it half-flat.
Answer: Even if could be repaired. Is it worth the effort? Your tire is bald to the point of actually almost having no tread at all. This could present a safety issue such hydroplaning during a rain storm causing you to lose control and destroying your more expensive vehicle and injuring both driver and passengers.
If you weigh all the vehicle repair costs,higher insurance premiums, highway code violations and the fines and demerit points against the cost of 4 brand new tires....The latter is subsequently much cheaper in the long run with added benefit of enhanced safety which is priceless.

Can this be patched?


Is "Heizölrückstoßabdämpfung" a real word?

Question: I saw "Heizölrückstoßabdämpfung" as an example of a word that is almost a pangramm without repetitition, but is it a real German word?
Comment: Are you kidding? It's one of the most important concepts in heating: http://www.stupidedia.org/stupi/Heiz%C3%B6lr%C3%BCcksto%C3%9Fabd%C3%A4mpfung ;)
Comment: Are people still using Fahrtrichtungsanzeigerkontrollglühlampen? I thought they would use Fahrtrichtungsanzeigerkontroll LEDs by now...
Comment: Hmm, with "real" I mean that it is actually used like Fahrtrichtungsanzeigerkontrollglühlampe or Einkommensteuerdurchführungsverordnung.
Comment: The marriage law in Austria is in 19th century German, so I guess that it will be quite some time until Glühlampen vanish from legal texts.
Comment: @bjoernz: Actually, Mercedes-Benz just replaced Fahrtrichtungsanzeiger with Blinker in manuals a few years ago, or so I've heard :)
Comment: In fact it's no panagram as it's missing several letters (jqvxy).
Comment: I'm amazed that nobody has mentioned the term 'zusammengesetzte Nomen'. That's what we're talking about here. You can create them in almost any language but the german language makes it easier than other languages and uses quite many of them naturally.
Answer: This word is made up only to be the longest "known" German isogram. So it is a phantasy term meaning really nothing. It doesn't make any sense, so I would not agree with the other answers stating that it is a "real" word.
Comment: I wouldn't say it has a "meaning". I cannot find an interpretation that makes sense to me. Where does *Heizöl* ever have a *Rückstoß* which needs to get *abgedämft*? It's in the same league as formally correct but meaningless sentences like "The colour of the smell was not very long."
Comment: Also see http://de.wikipedia.org/wiki/Isogramm
Comment: Depends on what you mean by *meaning nothing*. It does have a *meaning*, just not one that's actually used... +1 anyway for the correct answer (I guess).
Comment: This is true. It is not an existing term, it just shows the possibilities of German language. ;-)
Answer: It's a "real" word, in the sense that it doesn't contain any orthographical mistakes, but I seriously doubt that you'll find the word in technical literature.
My guess is that the word was composed for amusement's sake: getting the longest German isogram.
Comment: @arik-so It's not an isogram: http://en.wikipedia.org/wiki/Isogram.
Comment: What about "Oberweserdampfschifffahrtsgesellschaftskapitänsmütze"?
Answer: It's certainly a word that can be formed and understood in German. If it's actually used I couldn't say, as it seems very technical. I can at least imagine a "heating oil backlash suppressor" to exist though, even if I don't know what it is.
Comment: According to Wikipedia, it's fictitious: http://de.wikipedia.org/wiki/Isogramm
Comment: Gut zu wissen. :)
Answer: If a 'heating oil backlash suppressor' existed, it would be a real noun. If it does not exist, it is not real, but still valid.
It is a constructed noun with no orthographical mistakes, and it is constructed according to the rules of german grammar and syntax. Because you can construct new nouns in german by putting two existing nouns (even constructed nouns) together, german vocabulary is at the same time finite and unbounded. :-)
Answer: Fun fact about german: You can combine almost any noun with any other and still have a valid noun, at least from a grammatical point of view. That doesn't always mean that the word itself makes sense or even represent an actual "thing" in the real world.
So to answer the question: Yes, it is. In this case, the word even describes something that actually exists.
Words like this are often referred to as "snake words" (Schlangewörter), because you can go on and add more nouns to it without making it "wrong".
Example: "Heizölrückstoßabdämpfungstechnikerkabinenschlüsseldekorationsbeauftragtenmobil" - Still a valid german word.
Comment: Or my precious [Teetassenuntersetzermustermalpinselgebrauchsanweisungsübersetzer](http://area51.stackexchange.com/proposals/18413/german-language-usage/18421#18421)...
Comment: Dies ist ein Schlangenwortkonstruktionsmethodendemonstrationsbeispielwort! :-)
Answer: As indicated by markus, this is an example of compound word, a Kompositum. It is perfectly legal in German to create these from as many words as you like; the last word of the compound defines the main meaning, and the word or compound before that acts as refinement, not that different from english:

Kanne > Ölkanne > Heizölkanne (Can > oil can > heating oil can)

Whether the word still makes sense is another matter, of course. In theory, you could build an infinitely long word (and thus, if you do count compounds as "real words", German has infinite words).
So it depends on your definition whether Heizölrückstoßabdämpfung is a real word. Apparently, such an apparatus does not exist, but the meaning is clear, as Isotopp points out.
You could just invent such a thing, and you'll have invented a very real word as well :)
Answer: Once I've asked a heating contractor and he told me that he had never heard of this word before. So I guess it's just a clever combination of single words.
Answer: According to Wikipedia, Heizölrückstoßabdämpfung is fictitious.
Yet, there's the word Heizölrückstoßdämpfung meaning "heating oil rebound absorver". This word is both true in the sense it exists and it doesn't have grammatical mistakes.
Comment: Your answer to this (very old) question does not contain anything that is not yet covered by previous answers
Comment: Hi @VolkerLandgraf. That's not true, it includes this new word "Heizölrückstoßdämpfung" which is similar to the one asked about but not fictitious.
Answer: As already written, this word once won the competition in 1998 about a german isogram with each letter only once appearing (german wikipedia), thus it is fictional.
The usage itself I cannot link as I have no student's account any longer to access the information.
In my educational program it was widely used in programming courses to demonstrate how sorting algorithms and tree algorithms work.


2d bin packing problem, with opportunity to optimize the size of the bin

Question: I have been tasked with optimizing a manufacturing process. It is a non-trivial, NP hard problem. The problem is similar to the 2d bin packing problem, but we are trying to optimize the size of the bins.
The Problem:
Small rectangular shapes must be packed into larger rectangles in an optimal way. There are nesting algorithms to do this optimization where the large rectangle dimensions are inputs. However, we want to optimize the dimensions of the large rectangles.
The inputs are:
-A list of unique small rectangles. The list includes the dimensions of the small rectangles, a number that represents the rectangle's frequency of usage relative to the other small rectangles, and the orientation (whether or not it can be rotated 90 degrees)
-The maximum number of large rectangle dimensions. And the minimum and maximum dimensions of the large rectangles.
The output is an optimized list of large rectangle dimensions.
Proof One way to prove the validity of the result is by creating a nest of the small rectangles within the large rectangles. Here is a paper and an open source project that provides a solution to the 2d bin packing problem that we are currently using to nest. We are using the Maximal Rectangle algorithm and we vary the heuristics (typically bottom left or contact point heuristics).
http://clb.demon.fi/files/RectangleBinPack.pdf https://github.com/juj/RectangleBinPack
How do I solve this problem?
Comment: crosspost http://math.stackexchange.com/questions/352575/2d-bin-packing-problem-with-opportunity-to-optimize-the-size-of-the-bin
Answer: I solved this problem by getting a local optimal by using a greedy algorithm.


Debian Harddrive Fdisk - Same ID's and changing letters

Question: I am trying to create and install a debain NAS and ive been having a hard time because I am new to all of this. I used ntfs-3g in order to automount my 4 NTFS drives. I also have a partitioned harddrive which is for the OS.
When I was working on it and I ran this command I got this:
fdisk -l

/dev/sdae1

fdisk -l | grep NTFS

/dev/sda1
/dev/sdb1
/dev/sdc1
/dev/sdd1

The weird thing is, all of the NTFS harddrives listed had an ID of 7.
The next time I boot up my machine, I get an error about mounting /dev/sda1 and I run this command, and get the following results:
fdisk -l | grep NTFS

/dev/sdb1
/dev/sdc1
/dev/sdd1
/dev/sde1

I havent plugged in any drives, so whats going on? How to I make sure that my drives are mounted with the same sdXX name every time, and is the reason for this because they dont have unique ID numbers, if so, how do I solve this?
Answer: You should use UUIDs for each of the NTFS partitions, and put it in /etc/fstab.


Pinned Vs. Checkmate

Question: So say my queen is pinned in front of my king by the opponents rook. However if I move my queen to check their king it is certain checkmate, is that legal? I'm not sure because I'm intentionally endangering my king to be taken on the next move, but technically there won't be a next move since I checkmate my opponent.
Comment: No, it would be an illegal move to leave your king in check.
Comment: This question comes down to "can I claim an illegal move (the mating move), even if the game 'is' over (by checkmate)?" And to be honest: yes, you can. Otherwise you could just move any pieces to a checkmate position and declare the game as a win - pretty pointless. Although it's not unheard of in blitz games. :)
Comment: A related question (about the chess variant created by amending this rule): http://chess.stackexchange.com/questions/1515/bizarre-pin-rule-pinned-pieces-do-not-attack
Comment: I think OP's "move my queen" means "move my queen *off the pin line*". No chess piece can pin a queen; at most a rook can line-pin it, and that queen may move along the pin-line. For example white's line-pinned queen may play Qb6# here: wKb1, Qb2, Bc8, Pc5, bKa6, Rb7.
Answer: Think of it this way:
Check mate is where nothing the opponent can do will save them from having you take their king on the next turn, so even if by putting their king in checkmate, you expose your own king, they can take your king before you can take theirs.
There is NO circumstance in which your actions put your own king into check without an opponent making a move.
Comment: One could argue that this answer were slightly, technically incorrect but, in spirit, this is just the right answer. This is the way to think about it. However, once one has learned to think of it roughly this way, one should read the other answers to understand the techincalities, because those are important, too.
Answer: The rules are clear. A move that puts your king in check is an illegal move (I'll leave it to you to find the reference; you might find the other rules helpful to review once you get there).
You cannot checkmate your opponent's king (or accomplish anything else) with an illegal move. Once you make an illegal move on the board, the following things happen:

The position is restored to the position in effect before the illegal move is made.
You get penalized for an illegal move. For the 1st and 2nd offenses, this results in your opponent's being awarded an extra 2 minutes on his clock. For the 3rd offense, the normal penalty is forfeit of the game.

Nice try, though.
Comment: Currently the game is forfeited after the second offense, in rapid and blitz after first offense. Also in rapid and blitz games the position is not restored if the illegal move is not claimed immediately (although the players are allowed to agree to restore it). In the new laws for next year it is proposed that an illegal move is retracted only within the last 10 moves.
Comment: @IAPetrHarasimovic Thanks for clarifying about the FIDE Laws of Chess.  In the USCF Rules, the tournament director has discretion regarding the 10-move limit _if a king was in check and a move was made that left it in that state_. In this situation, each move that does not address the check is illegal, and the 10-move counter keeps getting reset as the score moves are reversed. So, if the illegal king-still-in-check condition occurred 20 moves ago, the TD can direct that the position be restored _either_ to the position when the king was _first_ in check, or any intermediate position.
Answer: Basically if you are confused about pin/checkmate rule, just play the game out until one king is captured.
You move you Q to threat taking his king in your NEXT move. Unfortunately it's now his move and he takes your K first.
On the other hand, if you use your other piece to attack so that his K has to move into your pinned Q's firing range, you can capture (not threat to capture) his K with your Q before he can capture your K. Or in standard chess rule, he is checkmated.
Comment: In USCF Rules, the king may be captured in blitz, but not in a standard game, when it would be an illegal move and penalized.  So, your comment should be interpreted to mean, "_imagine_ playing the game out until...".
Comment: An alternative way of looking at things would be to say that capturing an opponent's king would be a winning move *but* no player can be compelled to allow the king to be captured.  Except at blitz, a player whose opponent blunders so as to allow capture of the king would be required to let the opponent substitute a different move, and a player who puts his opponent into a position where there are no moves that don't put the king in check would be required to offer a draw (the opponent would have to either accept the draw or forfeit).
Answer: Official chess rules by FIDE
Rule 3.9 is as follows:

3.9 The king is said to be 'in check' if it is attacked by one or more of the opponent's pieces, even if such pieces are constrained from moving to that square because they would then leave or place their own king in check. No piece can be moved that will either expose the king of the same colour to check or leave that king in check.
Answer: In standard chess, no, you cannot move into, nor make a move that puts your king in, check.
I noticed this is tagged chess-variants, so I will add that the rules for speed/blitz chess may be slightly different, in terms of penalties. In addition, in casual blitz games, that may be acceptable.
Comment: Yes, in blitz games usually the player making an illegal move instantly loses the game (well, if claimed).

Pinned Vs. Checkmate


What happens if I buy Bitcoins and the price goes down?

Question: What happens if I buy Bitcoins and the price goes down. Will I lose a quantity of my money?
Consider:
Answer: If you bought a bar of gold at $1200 / ounce, then the price of the gold drops to $600 / ounce, you wouldn't actually lose any money at all unless you decided to sell your gold at that time.  However, if you wait until the price of gold rises to $1800 / ounce, then sell it, you will gain money.  The key is an ounce of gold is still an ounce of gold, regardless of the "price" in some other currency.
Same goes for bitcoin, or indeed any currency.  If you trade it at one price, then subsequently trade it at a different price, you'll either gain or lose money.
Tax law is a completely different area. Consult a tax attorney or accountant in you jurisdiction for details applicable to your scenario.
Answer: If you bought one bitcoin and the price goes down, you still have one bitcoin.  If the price goes up...still one bitcoin.
This is just like everything else, including groceries, gasoline, gold, stock certificates, etc...  Measurements of value in fiat (such as dollars) does not affect the amount something you own, only the price at which you will be able to sell that something.
Answer: tl;dr-  This is called a capital loss.  You're said to realize the capital loss if you sell the Bitcoins at the lower price, such that you lost money due to having bought/sold them.  However, note that this isn't legal advice and I'm unsure about what the current legal statutes are surrounding Bitcoin.

Bitcoin's a capital asset (at least conceptually; dunno about legally):

A capital asset is defined to include property of any kind held by an assessee, whether connected with their business or profession or not connected with their business or profession. It includes all kinds of property, movable or immovable, tangible or intangible, fixed or circulating. Thus, land and building, plant and machinery, motorcar, furniture, jewellery, route permits, goodwill, tenancy rights, patents, trademarks, shares, debentures, securities, units, mutual funds, zero-coupon bonds etc. are capital assets.
-"Capital asset", Wikipedia [links omitted]

When a capital asset appreciates in value, it's called a capital gain, and may be subject to capital gains tax.  And when a capital asset depreciates in value, it's called a capital loss (and sometimes results in a reduced tax burden).
Until you actually sell the Bitcoins, the loss is considered unrealized:

What is an 'Unrealized Loss'
An unrealized loss is a loss that results from holding onto an asset after it has decreased in price, rather than selling it and realizing the loss. An investor may prefer to let a loss go unrealized in the hope that the asset will eventually recover in price, thereby at least breaking even or posting a marginal profit. For tax purposes, a loss needs to be realized before it can be used to offset capital gains.
-"Unrealized Loss", Investopedia [links omitted]

Once you do sell the Bitcoins, then you know how much money you've gained/loss.  This is called realization:

DEFINITION of 'Realized Loss'
A loss is recognized when assets are sold for a price lower than the original purchase price. Realized loss occurs when an asset which was purchased at a level referred to as cost or book value is then disbursed for a value below its book value. Although the asset may have been held on the balance sheet at a fair value level below cost, the loss only becomes realized once the asset is off the books.
-"Realized Loss", Investopedia [links omitted]

Summary
If you buy Bitcoins and they depreciate in value, then:

You've suffered a capital loss.

Before you sell the Bitcoins, it's an unrealized capital loss.

After you sell the Bitcoins, it's a realized capital loss.
Answer: The answer is not unique to bitcoin. It would be the same if you're dealing with (non-crypto) foreign currency, stock, a stock derivative or commodity or commodity futures.
When you buy something like the above, you are giving up your "real money" (fiat currency) to take possession of said commodity/stock/bitcoin/etc (let's call these assets in general). You bought an asset at a particular market value, which can fluctuate over time. If it rises, it's worth more of the fiat currency. If it falls, it's worth less of the fiat currency.
While you're still holding on to said asset, what you're experiencing are called unrealised gains and unrealised losses. The valuation chart fluctuates, but you're not seeing your purse of fiat currency changing in any way after the initial outlay.
You won't actually feel the "pain" of a fall in value until you decide to sell the asset. At this point you will experience a realised loss. You will get back less of the fiat currency (real money) than you put in in the first place. Conversely, if the asset has risen in value, you'll get back more "real money" than you put in and you've made a realised gain.
(I simplified the analysis to omit things like trading overheads - brokerage fees and commissions, etc. that always apply (those things always eat at your gains and losses pretty much equally). And taxes are a complex and murky thing I won't touch. And all this assumes you paid "real money" you actually had in full for the asset. If you borrowed money to buy the asset, that's called trading on margin and it can be much, much riskier - you're losing money in interest all the time and your losses can be more than the amount you borrowed to begin with. But ignore all these complications and focus just on the paragraphs above to give you a head start in understanding. And please learn more (and try trading simulations) before you trade real money for any asset).
Answer: Yes, you lose a quantity of your money, at the time you gave it away in exchange for the bitcoin you received.  Subsequent changes in the exchange rate only vary the hypothetical value of what you would get if you wanted to trade back.
Answer: No; you lost the money when you used it to buy the Bitcoins. If the price goes down, you will get less money back if you sell them. If the price goes up, you will get more money back if you sell them.
Answer: Yes and no, depends on your perspective.
No: You lose money when you buy something, and you gain something else in return. In your case you lose money if you buy bitcoin, and you gain money when you sell bitcoin.
Yes: The monetary equivalent value of your assets/wealth decreases if your assets lose value, and increases if your assets gain value. If you own $1'000'000 in Bitcoin and Bitcoins lose half their value, you now hold $500'000 in bitcoin.
Notice that in the No case we count our Bitcoin in terms of # of coins, and in the yes case we thing of bitcoins in terms of $ of Bitcoins. If you were to count the value of all your assets in Bitcoins you'd gain value when the value of Bitcoins drops, because all your non-Bitcoin assets are now worth more Bitcoins, and the Bitcoin assets are still worth the same number of Bitcoins they used to be worth. It's more sensible, however, to value your assets in a currency that is stable and tied to most of your expenses.
Answer: I would like to add another aspect: minimize loss, in case you want to protect your principal (and commission paid when you bought Bitcoin). Please look into 'stop loss order' in case you want to get out of the position while minimizing the loss because you are not certain when the price will return enough to get your position back to 'gain'.
Answer: Many other answers explained the difference between realised and unrealised loss. I want to add some thoughts:

It seems you are rather new to trading. Please understand that there is no "magic money making" machine, and that trading profits are often viewed as the compensation for accepted risks. So be cautions with investing money whose loss you cannot afford.
Bitcoin is highly volatile, meaning that the price changes in rapid succession. It might be hard for a beginner to handle. You can, of course, but be extra cautious compared to trading, say, Dow Jones values.
Bitcoin has no intrinsic value. Gold can be used for making jewelry, Coca Cola can still produce more drinks, but for a Bitcoin you need someone else willing to pay for it.   Without spreading too much fear and doubt, it bears some reminiscence to once highly-priced collector cards.
Answer: The answer depends quite  a lot on the semantics of the word "money". If money is those rectangular printed papers or circular metal discs made by the Federal Reserve in the United States of America as well as many other institutions in other nations then the answer is NO. You gave away (traded) some of those items when you bought the bitcoins. But a lowering of bitcoin value after that trade does not further diminish the amount of papers and discs you have.
But I would rather use the definition of money used by wikipedia:

Money is any item or verifiable record that is generally accepted as
payment for goods and services and repayment of debts in a particular
country or socio-economic context. The main functions of money are distinguished as: a medium of exchange; a unit of account; a store of value; and, sometimes, a standard of deferred payment.Any item or verifiable record that fulfills these functions can be considered as money.

The papers and discs fit that definition so no money loss there. But, what else fits that definition?
BITCOINS
Before they got lowered in price you could go to Newegg and purchase a certain ammount of items. What happens if you go to Newegg after they got lowered in price?

If the answer is "I can buy the same" then no, you did not lose a quantity of your money.
If the answer is "I can buy less for the same ammount of bitcoins than I could before the lowering" then yes, you did lose a quantity of your money. They are diminished as a medium of exchange, now you can exchange those bitcoins for less than you could before.
Comment: I think there is an interesting point here, although perhaps not well expressed: the bitcoins might be exchanged for *something other than the original currency*. However, if it were possible to exchange Bitcoins valued at USD10 for USD20 worth of goods, this would be an anomaly that the market would correct for by adjusting one or both prices. Otherwise, it would be the economic equivalent of a perpetual motion machine: buy BTC for USD -> buy goods for BTC -> sell goods for twice the original USD -> buy twice as many BTC -> repeat.
Comment: @IMSoP - arbitrage is what happens when you trade a good or asset through multiple exchanges and make a profit.  i.e. if you sell BTC -> BCH -> USD -> BTC, and come out with more BTC at the end, that's arbitrage.  And yes, normally the "market" will sort that out extremely quickly, since stakeholders take advantage of the arbitrage forcing equilibrium.


How can I sync mail older than two weeks on my iOS 4.1 device?

Question: In the settings for my exchange account I only see options for syncing up to two weeks back, not more. Is there a way I could sync mail that is more than two weeks old?
Answer: The reason for this is that the Exchange Administrator has used the iPhone Configuration Utility on the server to adjust how many weeks of mail you are allowed to sync. It is not possible to override this value on the phone itself without jailbraking.
Answer: The limit is set by the Exchange administrator, it can be up to one month.


Simple voice recognition when whispering

Question: I'm trying to do simple voice to text mapping using pocketsphinx (. The grammar is very simple such as:
public <grammar> = (Matt, Anna, Tom, Christine)+ (One | Two | Three | Four | Five | Six | Seven | Eight | Nine | Zero)+ ;

e.g:
Tom Anna Three Three

yields
Tom Anna 33

I adapted the acoustic model (to take into account my foreign accent) and after that I received decent performance (~94% accuracy). I used training dataset of ~3minutes.
Right now I'm trying to do the same but by whispering to the microphone. The accuracy dropped significantly to ~50% w/o training. With training for accent
I got ~60%. I tried other thinks including denoising and boosting volume. I read the whole docs but was wondering if anyone could answer some questions so I can
better know in which direction should I got to improve performance.
1) in tutorial you are adapting hub4wsj_sc_8k acustic model. I guess "8k" is a sampling parameter. When using sphinx_fe you use "-samprate 16000". Was it used deliberately to train 8k model using data with 16k sampling rate? Why data with 8k sampling haven't been used? Does it have influence on performance?
2) in sphinx 4.1 (in comparison to pocketsphinx) there are differenct acoustic models e.g. WSJ_8gau_13dCep_16k_40mel_130Hz_6800Hz.jar. Can those models be used with pocketsphinx? Will acustic model with 16k sampling have typically better performance with data having 16k sampling rate?
3) when using data for training should I use those with normal speaking mode (to adapt only for my accent) or with whispering mode (to adapt to whisper and my accent)? I think I tried both scenarios and didn't notice any difference to draw any conclussion but I don't know pocketsphinx internals so I might be doing something wrong.
4) I used the following script to record adapting training and testing data from the tutorial:
for i in `seq 1 20`; do
       fn=`printf arctic_%04d $i`;
       read sent; echo $sent;
       rec -r 16000 -e signed-integer -b 16 -c 1 $fn.wav 2>/dev/null;
done < arctic20.txt

I noticed that each time I hit Control-C this keypress is distinct in the recorded audio that leaded to errors. Trimming audio somtimes helped to correct to or lead to
other error instead. Is there any requirement that each recording has some few seconds of quite before and after speaking?
5) When accumulating observation counts is there any settings I can tinker with to improve performance?
6) What's the difference between semi-continuous and continuous model? Can pocketsphinx use continuous model?
7) I noticed that 'mixture_weights' file from sphinx4 is much smaller comparing to the one you got in pocketsphinx-extra. Does it make any difference?
8) I tried different combination of removing white noise (using 'sox' toolkit e.g. sox noisy.wav filtered.wav noisered profile.nfo 0.1). Depending on the last parameter
sometimes it improved a little bit (~3%) and sometimes it makes worse. Is it good to remove noise or it's something pocketsphinx doing as well? My environment is quite
is there is only white noise that I guess can have more inpack when audio recorded whispering.
9) I noticed that boosting volume (gain) alone most of the time only maked the performance a little bit worse even though for humans it was easier to distinguish words. Should I avoid it?
10) Overall I tried different combination and the best results I got is ~65% when only removing noise, so only slight (5%) improvement. Below are some stats:
//ORIGNAL UNPROCESSED TESTING FILES
TOTAL Words: 111 Correct: 72 Errors: 43
TOTAL Percent correct = 64.86% Error = 38.74% Accuracy = 61.26%
TOTAL Insertions: 4 Deletions: 13 Substitutions: 26

//DENOISED + VOLUME UP
TOTAL Words: 111 Correct: 76 Errors: 42
TOTAL Percent correct = 68.47% Error = 37.84% Accuracy = 62.16%
TOTAL Insertions: 7 Deletions: 4 Substitutions: 31

//VOLUME UP
TOTAL Words: 111 Correct: 69 Errors: 47
TOTAL Percent correct = 62.16% Error = 42.34% Accuracy = 57.66%
TOTAL Insertions: 5 Deletions: 12 Substitutions: 30

//DENOISE, threshold 0.1
TOTAL Words: 111 Correct: 77 Errors: 41
TOTAL Percent correct = 69.37% Error = 36.94% Accuracy = 63.06%
TOTAL Insertions: 7 Deletions: 3 Substitutions: 31

//DENOISE, threshold 0.21
TOTAL Words: 111 Correct: 80 Errors: 38
TOTAL Percent correct = 72.07% Error = 34.23% Accuracy = 65.77%
TOTAL Insertions: 7 Deletions: 3 Substitutions: 28

Those processing I was doing only for testing data. Should the training data be processed in the same way? I think I tried that but there was barely any difference.
11) In all those testing I used ARPA language model. When using JGSF results where usually much worse (I have the latest pocketsphinx branch). Why is that?
12) Because is each sentence the maximum number would be '999' and no more than 3 names, I modified the JSGF and replaced repetition sign '+' by repeating content in the parentheses manually. This time the result where much closer to ARPA. Is there any way in grammar to tell maximum number of repetition like in regular expression?
13) When using ARPA model I generated it by using all possible combinations (since dictionary is fixed and really small: ~15 words) but then testing I was still receiving somtimes illegal results e.g. Tom Anna (without any required number). Is there any way to enforce some structure using ARPA model?
14) Should the dictionary be limited only to those ~15 words or just full dictionary will only affect speed but not performance?
15) Is modifying dictionary (phonemes) the way to go to improve recognition when whispering? (I'm not an expert but when we whisper I guess some words might sounds different?)
16) Any other tips how to improve accuracy would be really helpful!
Answer: Regarding whispering: when you do so, the sound waves don't have meaningful aperiodic parts (vibrations that result from your vocal cords resonating normally, but not when whispering). You can try this by putting your finger to your throat while loudly speaking 'aaaaaa', and then just whispering it.
AFAIR acoustic modeling relies a lot on taking the frequency spectrum of the sound to detect peaks (formants) and relate them to phones (like vowels).
Educated guess: when whispering, the spectrum is mostly white-noise, slightly shaped by the oral position (tongue, openness of mouth, etc), which is enough for humans, but far not enough to make the peeks distinguishable by a computer.


Where can I read the Inazuma Eleven manga online?

Question: So far, I have only found chapter 1 of the Inazuma Eleven manga online. Is there any site where I can read all of the chapters online and legally?
Comment: just a suggestion. For a sports type, anime is always better than its manga
Comment: Refer to this : http://anime.stackexchange.com/questions/4696/what-databases-and-listing-sites-exist-for-anime-manga-etc . According to mangaupdates, only 3 chapters were translated.
Answer: Unfortunately, there are no official English-language localizations of the Inazuma Eleven manga in existence, in print or online.
You can easily verify this by:

checking the Wikipedia page for Inazuma Eleven (manga), which does not list any English-language localizations.
looking at the AnimeNewsNetwork encyclopedia page for Inazuma Eleven and seeing that this page also does not list any English-language localizations.
searching Amazon for "inazuma eleven manga" and seeing that you get no hits that are actually manga.


Why did most European countries adopt the proportional voting system?

Question: Is it because they are "young democracies/countries" (compared to the US and the UK) and at the time of their establishment there were more people with different political views and therefore the proportional voting system was simply more suitable for them?
Comment: One reasonable criterion would be "The legislature, the executive, and the judiciary are all either elected be a significant portion of the residents, or appointed by those so elected." Democracy need not equate to a fully-free society, nor one where every resident is a voter.  I believe that both Germany and Austria still had direct monarchical rule in 1850. I'd need to do some checking into the state of Italy and other countries
Comment: Welcome to Politics SE!  As it is, this question is far too broad, and a bit confusing as different European countries use different systems.  You are more likely to get an answer by asking about a specific country.
Comment: @Abigail  When a given country became a democracy depends on  how one is defining "democracy" but many European countries did not by any reasonable criterion become democracies until after 1850. Both the US and the UK I would place rather earlier than that. France also. The assumption in the question tht 'young democracies" are more likely to use a "plurality system": does not make sense to me.
Comment: This question confuses me: isn't USA using plurality for presidential election, at least at the state level (the candidate with relative majority in a given state gets all electoral votes of that state)?
Comment: It is not clear what the question means by "plurality voting" Wikipedia https://en.wikipedia.org/wiki/Plurality_voting uses the term for the first-past-the-post system uses in the US, and not used min most of Eurpoe. As written this question is unclear, and I would vote to clsoe on that basis if I had enough rep n this site
Comment: I meant proportional voting system.. Sorry
Comment: @DavidSiegel: German unification happened in 1871. Austria-Hungary indeed was a monarchy.
Answer: First, I think that the question is not very well posed.

Is it because they are "young democracies/countries" (compared to the US and the UK)

What is a "young democracy"? The Athenian democracy in antiquity, the Republic of Venice or Switzerland are much older than the US or democracy in the UK. The UK has a long democratic tradition with a Magna Charta and a Parliament, but it was still the king who ruled. Occasionally he had to summon the Commons, when he didn't have enough money for war.
By the way, the idea that the king had unlimited power and the populace had no say in medieval Europe is completely wrong. Royal power was limited by statutes and the king often had to convoke some kind of assembly to gather support. In Spain, these assemblies were called Cortes, in Germany Reichstag or Landtag and in France Estates General. England wasn't exceptional in having commoners as representatives either, there were commoners in the 13th century Cortes and the Estates General.

and at the time of their establishment there were more people with different political views and therefore the proportional voting system was simply more suitable for them?

The US had a population of more than 3 million people, when it became independent. That's more than quite a few democracies  with proportional voting in Europe and the world have today. And I don't see why proportional voting should be unsuitable for smaller countries - unless a polity is so small that a parliament becomes unnecessary or too burdensome.
Still, I agree that age plays a role. Most constitutions, which are currently in vigor in Europe, were written in the 20th century. When the US constitution was written, time wasn't ripe yet. There was no existing country that could serve as a model, no mass newspapers, important travel times and probably many other obstacles.
However, the idea that the people should be represented proportionally existed:

It should be in miniature, an exact portrait of the people at large. It should think, feel, reason, and act like them. That it may be the interest of this Assembly to do strict justice at all times, it should be an equal representation, or in other words equal interest among the people should have equal interest in it. (John Adams, Thoughts on Government, 1776)
A representative body is to the nation what a chart is for the physical configuration of its soil: in all its parts, and as a whole, the representative body should at all times present a reduced picture of the people, their opinions, aspirations, and wishes, and that presentation should bear the relative proportion to the original precisely. (Mirabeau in a speech at the Assembly of Provence, 1789)

Some European countries:

UK: No proportional representation. Much of the political system has a very long tradition.
France: No proportional representation. Actually, proportional representation was used during the Fourth Republic (after the Second World War), but it was abandoned in 1958.
Germany: Partially proportional representation with constituency candidates and a party list. The constitution dates from 1948 - designed under important American and British influence.
Spain: Proportional representation. The constitution of 1978 entered in force after the death of dictator Franco.
Netherlands: Proportional representation. The current constitution derives from a document written in 1815. Proportional representation was introduced in 1917.
Poland: Proportional representation. The current constitution entered in force in 1997. Unfortunately, I don't know about the voting system under the first post-Communist constitution and during Communism. (My guess is that it had already been proportional, even if the elections were far from free.)

Ultimately, let's have a look at a world map (Source: Wikipedia article on "Electoral System)":

If you look at Africa and Asia, you see that "younger" countries do not automatically adopt proportional voting, even if much of South America has done so. At least in some cases, colonial tradition may play a role (i.e., former British colonies like India, Nigeria, Kenya or Botswana).
Map Legend

Single-member constituencies:

(Red) First past the post (FPTP) (44 entries)
(Maroon) Two-round system (TRS) (23 entries)
(Puce) Instant-runoff voting (IRV) (2 entries)

Multi-member constituencies, majoritarian:

(Dark blue) Majority bonus system (MBS) (2 entries)
(Orange) Block voting (BV) or mixed FPTP and BV (14 entries)
(Brown) Party block voting (PBV) or mixed FPTP and PBV (3 entries)
(Yellow) Single non-transferable vote (SNTV) or mixed FPTP and SNTV  (3 entries)
(Citron) Modified cumulative voting (1 entry)
(Lime) Modified Borda count (1 entry)

Multi-member constituencies, proportional:

(Light blue) Party-list proportional representation (party-list PR) (80 entries)
(Mint) Single transferable vote (2 entries)

Mixed majoritarian and proportional:

(Purple) Mixed-member proportional representation (party-list PR and FPTP) (6 entries)
(Lavender) Mixed-member proportional representation (party-list PR and TRS)  (1 entry)
(Dark pink) Parallel voting (party-list PR and FPTP) (23 entries)
(Persian rose) Parallel voting (party-list PR and TRS)  (1 entry)
(Fuchsia) Parallel voting (party-list PR and BV or PBV) (2 entries)
(Light deep pink) Parallel voting (party-list PR and SNTV)  (1 entry)

No relevant electoral system information:

(Dark grey) No direct elections (9 entries)
(Light grey) No information (4 entries)
Comment: Please edit to provide the map caption or an indication of what the colors mean, and the map source. Then I will upvote.
Comment: @DavidSiegel: You're right. I wasn't aware that StackExchange doesn't show the real source, but only a meaningless *imgur.com* link.
Answer: While it is the case today that most European countries have a form of proportional representation, that apparently was notthe case when they first introduced democracy/elections, as described in a paper by Jonathan Rodden, which says

During the period from around 1890 to 1920, most European countries dramatically expanded the franchise to include the working class, abolished plural voting for the wealthy, and reduced the power of the landed gentry in undemocratic upper legislative chambers.  During the same period, the vast majority of these countries replaced electoral systems featuring a large number of small, winner-take-all districts with some version of proportional representation.

In other words, the question could/should be asked as: why did most of Europe reform their systems but UK, US, (or France) did not?
The last paper I quoted discusses various theories why a proportional representation system reform swept through most of Europe. Among the chief hypotheses:

Braunias (1932) proposed that in most European countries after WWI this was a strategy of the elite to prevent the socialists from taking power after voting rights could no longer be denied  to the majority of the population (who were poor).
More recent views such as that of Alesina and Glaeser (2004) is of a interesting coincidence of views: both the socialists and the old elite believed their interests were best served by a proportional representation in the aftermath of WWI in a number of countries, including Belgium, Sweden, and Germany. The socialists wanted proportional representation because the concentration of their supporters in urban areas translated poorly into seats under the old system(s). There's something more to said here about Liberal parties: in countries such as the UK or New Zealand where the Liberals (the "old left") did not support a proportional system at the time betting/thinking they could beat the emerging socialists/Labour in their urban constituency, they [the Liberals] were soon marginalized, leaving on the right/Conservatives and the socialists/Labor as heavyweight political parties. (The Canadian Liberals are the only exception to this having survived the socialist challenge without a proportional system, but having substantially nudged their platform further to the left.)
Answer: You should know that this is an active area of research in Political Science. Proportional representation is quite widespread in Europe. I would recommend the paper The Adoption of Proportional Representation (Leemann & Mares, 2014) which in the very first page lists the major comparative works made on the subject, and the two most common explanatory families:

Like any change in electoral rules, the adoption of proportional
  representation poses an immediate puzzle: why do politicians engage in
  risk-taking behavior and decide to replace the institutional rules on
  the basis of which they have been elected? Existing explanations for
the reform of electoral institutions and the adoption of proportional
representation cluster in two broad explanatory families, stressing
  either economic transformations or changes in the broader political
  environment as the source of political anxiety among right-wing
  politicians. Let us consider the larger political changes first. Both
hypotheses—stressing either the rise of Social Democracy or
disproportionality in the allocation of votes to seats—as determinants
  for demand for changes in electoral institutions can be traced back to
  the work of Braunias and Rokkan (Braunias 1932; see also Rokkan 1970,
  157–58).

The major works listed are:

Ahmed, Amel. 2012. Democracy and the Politics of Electoral System Choice: Engineering Electoral Dominance. Cambridge: Cambridge
  University Press.
Benoit, Kenneth. 2004. ''Models of Electoral System Change.'' Electoral Studies 23 (3): 363–89.
Blais, Andre, Agnieszka Dobrzynska, and Indridi Indridason. 2005. ''To Adopt or Not to Adopt Proportional Representation: The Politics
  of Institutional Choice.'' British Journal of Political Science 35
  (1): 182–90.
Boix, Carles. 1999. ''Setting the Rules of the Game: The Choice of Electoral Systems in Advanced Democracies.'' The American Political
  Science Review 93 (3): 609–24.
Calvo, Ernesto. 2009. ''The Competitive Road to Proportional Representation. Partisan Biases and Electoral Regime Change under
  Increasing Party Competition.'' World Politics 61 (2): 254–95.
Colomer, Josep M. 2005. ''It's Parties That Choose Electoral Systems (or, Duverger's Laws Upside Down).'' Political Studies 53 (1): 1–21.
Cusack, Thomas, Torben Iversen, and David Soskice. 2007. ''Economic Interests and the Origins of Electoral Systems.'' The American
  Political Science Review 101 (3): 373–91.
Kreuzer, Marcus. 2010. ''Historical Knowledge and Quantitative Analysis: The Case of the Origins of Proportional Representation.''
  American Political Science Review 104 (2): 369–92.

Further this is not a static context. For example:

Macron (in France) is advocating for a model of proportional representation (link1, link2, link3).
There is some talks of advocating the same for the UK. Particularly because parties such as the new Independent Group (former Labour MPs) might need these reforms to survive. Moreover proportional representation was implemented for European Elections in the UK (see Trends chapter at the link).
Proportional representation is not new in certain regions of the US. There's also considerable activism into changing the current system.
Answer: Yes, I think older democracies (the US, the UK, France) and countries that explicitly modeled their systems on those (Australia, Canada) tend to have First Past the Post, or double ballot majority rules (districtal election of representative bodies). Countries that have established their democratic constitutions more recently, and particularly after WWII, tend to have proportional ballot.  Why that, I am not sure; perhaps proportional ballot is a reaction to the perceived problems of FPTP; perhaps it involves technical difficulties that were more easily solved more recently; or perhaps the older democracies were established at a time when modern political parties still did not exist, so proportional vote would not make much sense.
There is however a more recent tendency to revert to majoritary vote, or otherwise abandon pure proportionality. Minimal thresholds, mixed proportional-districtal vote, non-party lists, individual candidacies, bonuses for most voted parties, etc., seem to be on the rise for the past quarter century.
Comment: The german and dutch democracy are way older than the UK democracy. And I belive the swiss have had a democracy "forever".
Comment: The Swiss have something that can be called "a democracy" since about the time they have a country called Switzerland. But they have proportional representation only from 1918. So most of their democratic history unfolded under a majoritary vote system. See https://www.swissinfo.ch/eng/war-and-revolution_the-road-to-proportional-representation-in-switzerland/44461330
Comment: This seems to be the case of Germany, too, as the Social Democrats clearly won the popular vote in 1907 but their caucus in the Reichstag was only the fourth in size. And in the Netherlands, proportional representation was introduced in 1917, through the political pact known as "Pacification". See https://static1.squarespace.com/static/56330ad3e4b0733dcc0c8495/t/56b86c99a3360cac2760b18a/1454927019274/GLJ_Vol_08_No_12_Leenknegt.pdf


If I UDP multicast to 10 listeners in a multicast group, will my upload bandwidth usage increase by 10 fold?

Question: My internet has an upload limit of 1 Mb/s. If I am in a UDP multicast group with 10 people and I send something to the group at a rate of .1 Mb/s, will my upload rate be 0.1 Mb/s or 1.0 Mb/s? Assume all people are on the public internet.
Comment: You can't multicast on the public Internet. Also, the number of receivers doesn't affect how much bandwidth multicast uses.
Comment: No, you can't. Your ISP will drop packets with a multicast destination address since it has no place to send them. Multicast packets are not routed the same way unicast packets are. The networks in between the sender and receiver need to be multicast capable, aware, and have multicast routing enabled. The Internet is not set up this way.
Comment: *In general*, ISP networks are not setup to handle user multicast streams. (a: it's a mess. b: nobody needs it. and c: script kiddies.) Uverse has a multicast video distribution network, but it's *one way* / *to the user*
Comment: You can't? Not even if your router supports multicast and the other person's router also supports multicast?
Comment: Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
Answer: Multicast is a special case of broadcast. It is a one-to-many mechanism. The sender sends a single packet and the network copies it everywhere it needs to go.  To a switch or WiFi access-point, it's handled just like broadcast traffic[1], 'tho multicast filtering may apply. To a router that isn't configured for multicast routing (which is basically every router, ever), it's random broadcast noise.
For this to work, the various routers between you and your targets would have to be configured for multicast routing. This is unlikely. While it is, in theory, possible within a single ISP network, across the internet would be unmanageable. (not to mention immediately abused)
[1] This is why Uverse uses a dedicated AP for their wireless set-top boxes. Those APs run special software that handles the multicast video stream(s) like unicast. Wifi broadcasts go out at a basic rate (a minimum every associated client can handle) that's just far too slow.
Answer: The sender will not duplicate the content so your rate will remain .1.  You will need to tunnel your multicast traffic across the Internet.  Specifically, you may want to tunnel multicast over a GRE tunnel.
Comment: This assumes that the receivers are all on the same LAN. If the ten receivers are all on different LANs, the OP may as well unicast to each, avoiding all the headaches of tunnels and multicasting, and it will still take multiples of bandwidth to send.
Comment: That depends on where the mrouter is. If he's the tunnel junction for 10 spokes, then he will, indeed, be sending 10 packets for every 1. (PC sends 1, router copies it into 10 tunnels)


What could Executioner's Calling be used for?

Question: I know Executioner's Calling reduces HP regeneration and healing, but who would this actually counter except for Dr. Mundo? I've seen this item built once even when I play Dr. Mundo pretty often.
Can someone shed some light on this item?
Comment: Haha love this item - although it takes a lot more micro to notice its payoff
Answer: It is a good buy versus anyone who has ultimates or abilities that rely on health regeneration.
The active makes this item extremely effective against champions with healing abilities, as well as the use of health potions. It is also effective against champions and item builds that rely on spell vamp and/or life steal.
Examples of champions/abilities/items that it counters include:

Health Potion
Dr. Mundo's  Sadism
Fiddlesticks's  Drain
Soraka's  Astral Blessing and  Wish
Swain's  Ravenous Flock
Warwick's  Hungering Strike and  Infinite Duress
Sion during  Cannibalism
Tryndamere's  Bloodlust
Renekton's  Cull the Meek
Volibear's  Chosen of the Storm
Vladimir's Transfusion
The Bloodthirster /  Sanguine Blade
Summoner Heal

Source
So, if there is particularly abusive Swain, Warwick, or Sion then it can help stop them.
Comment: By the way, since team life regen during sion ult is based on how much sion regen his own life, you will nerf the whole team buff.
Answer: Executioner's Calling is indeed an interesting item. I personally find it to be underused, but there are only a few characters that I feel can make use of it. Gangplank is a good champion to use this on/against. Being able to cast this on Gangplank as you pressure him will prevent his heal from being as effective.
Also it is generally useful against any champ that relies on sustain in lane. Sadly I think it needs a buff, as there are very few characters that can really benefit from grabbing this.
As you've noted, also a good mundo counter if your team does not have ignite/Trist/Fizz/any other heal debuff
Answer: This is one of my favourite items in the games. It is similar to Wriggle's Lantern, in that it helps you out in-lane but is probably not something you will be keeping in late-game.
The stats on it are pretty good for the cost, I get this item on Gangplank if the opponent has any form of healing including pots. He loves the crits and extra sustain, and the passive 4dmg a second on top of his grog soaked blade makes his Parrley harass even more powerful. This item is super cheap, and is a real bargain on any champion that benefits from crits & lifesteal but also needs some way to shut down an opponent's sustain.
I should note that the damage over time will have you prioritised by towers, so you must be careful of this if you plan on diving. It also helps stop people getting into stealth, porting back home, and it can keep Malphite's passive down perpetually.
Answer: When your team has 1 or no ignites with them and they ahve a tryn/sion/mundo/yi on the other team. Or pretty much any champion who has high lifesteal and is able to take a ton of damage and just heal up easily. Its purpose is straight down to stop the healing affects and give your team the advantage to burst down the other enemy champion like fiddlesticks who heals so much from his w
Answer: Lifesteal and spellvamp champs, against healers (heal spell, sona, janna ult to lesser degree, trydamere, nidalee), people using bloodthirster, warmogs, ect.  It is, in my opinion, not the best choice but it could be useful against those.  Ezreal, graves, any ad carry with lifesteal for example can outheal in some 1v1 situations.  This would cripple them fairly effectively but the item is indeed very situational.
Answer: It's basically ignite: the item. :) Great against heal/regen-heavy champs. Try it in an on-hit build! :D
Answer: You could use it in case their carry is stacking lifesteal and that's somehow giving him too much survivability, you could use it against Mundo, Warwick, Sion, Fiddle, just about anyone with a heal, and it's very cost effective.


What is the difference between \over and \frac?

Question: What is the difference between
{some-expression}\over{another-expression}
and
\frac{some-expression}{another-expression}
?
Which is preferable?
Answer: The command \over is a so-called "primitive" command that's available in Plain TeX and in LaTeX. \frac is a LaTeX-only command that builds on the \over macro to provide something that's much less likely to throw unexpected errors if the user doesn't handle the syntax of the \over command just right.
Assuming you have the amsmath package loaded, the \frac command is defined as follows:
\DeclareRobustCommand{\frac}[2]{{\begingroup#1\endgroup\@@over#2}}

where \@@over is an amsmath-internal version of \over. Note that the first argument of \frac (the numerator) is carefully encased in a \begingroup ... \endgroup construct and that the entire command's "content" is surrounded by a double set of curly braces. The braces serve to remove potential ambiguities that might arise from a less than fully careful use of the \over command by itself.
If you're a user of LaTeX, there's no reason whatsoever for not using the \frac command -- and several good reasons for not using the \over command directly. In short, if you use LaTeX, use \frac.
Comment: I would go as far as to say that including `\over` (and friends `\atop` etc.) were a bad design decision by Knuth.

TeX uses different layout style for display mode, text mode, superscripts, and subscript. However, because of macros like `\over`, when TeX is building the math token list, the current math mode is not known. This causes all sorts of difficulties like having to use `\mathchoice` in simple macros like `\text`. If macros like `\over` are ruled out, the design of a lot of macros will simplify a lot.
Comment: I've discussed this with Knuth.  His rationale for using `\over` rather than `\frac` is that it's the natural vocabulary of a *mathematician*, and his model for TeX was that of a mathematician, not a computer scientist.  (One concludes that there are concepts which are for him more important than ease of programming.)
Comment: Could you give an example of the sort of problems that could occur with `\over`?
Comment: @BlueRaja-DannyPflughoeft - Did you see the example given in egreg's answer?
Answer: The latter is preferable. It corrects a behavior that in some situations can reveal itself in a bad way. The LaTeX definition is
\def\frac#1#2{{\begingroup #1\endgroup\over #2}}

In some cases an assignment made in the numerator can affect also the denominator. It's difficult to produce it with standard LaTeX commands, but you can compare the result of
\[
\fam0 a\over b
\]

with
\[
\frac{\fam0 a}{b}
\]

In the first case both "a" and "b" will be upright, which is probably not expected.
Another relevant argument against the \over syntax is that it's foreign to LaTeX, which always uses first the command and then its argument and never an "infix" syntax. Moreover the grouping around numerator\over denominator is automatically provided, avoiding mistakes such as
1\over 2 \over 3

Update
Starting from the 2019-10-01 release of LaTeX, the definition of \frac uses \DeclareRobustCommand, as part of a program to remove as many fragile commands as possible:
% latex.ltx, line 4705:
\DeclareRobustCommand\frac[2]{{\begingroup#1\endgroup\over#2}}

In olden times it was not possible to make all commands robust, because of memory restrictions. These considerations are no longer a problem.
The amsmath package has used \DeclareRobustCommand for its modified definition of \frac for many years.
Comment: @MikaelVejdemo-Johansson `\choose` is not documented in the LaTeX manual, as far as I know. :)
Comment: *never an "infix" syntax*
… except for with `\choose` …
Answer: In User's Guide for the amsmath Package p.14 it is written the following:

The primitive generalized fraction commands \over,
\overwithdelims, \atop, \atopwithdelims, \above,
\abovewithdelims produce warning messages if used with the amsmath
package, for reasons discussed in technote.tex.

And in Technical notes on the amsmath package p.2:

Not only is the unusual syntax of the TeX primitives rather out of
place in LaTeX, but furthermore that syntax seems to be responsible
for one of the most significant flaws in TeX's mathematical
typesetting capabilities: the fact that the current mathstyle at any
given point in a math formula cannot be determined until the end of
the formula, because of the possibility that a following generalized
fraction command will change the mathstyle of the preceding material.

(...)

There are additional bad consequences following from the syntax
of those generalized fraction commands that only become evident when
you do some writing of nontrivial macros for math use. For example, as
things currently stand you cannot measure the size of any object in
math without going through \mathchoice and leaving and reentering math
mode via \hbox{$ (which then introduces complications regarding
\everymath and \mathsurround). And it seems that uncertainty about the
current mathstyle is the only barrier to allowing the use of mu units
with \vrule, to make vertical struts in constructing compound symbols
or notation. And so on and so forth.
Comment: Links ave been repaired.
Comment: The hyperlinks you provided don't appear to work anymore. Can you provide updates?
Comment: To be fair, you avoiding \over will probably *not* make TeX change to fix that oddity because of the stability requirement. (by the way [LuaTeX has `\Ustack`](https://tex.stackexchange.com/questions/1223/is-there-a-test-for-the-different-styles-inside-maths-mode) to fix that issue already)
Answer: In some cases, I found \over doesn't need curly braces whereas \frac does.
\[ P(n,m) = {n! \over (n-m)!} \]
\[ n! \over (n-m)! \]
\[ P(n,m) = \frac{n!}{(n-m)!} \]
\[ \frac{n!}{(n-m)!} \]

Comment: Welcome to TeX.SE! Please add an screenshot of your result and explain the difference.


Which number should replace the question mark?

Question: There is a picture. Find out what number is needed to replace this question mark.
Answer: The answer is

 41

because

 Each number in the third row is the difference of the product and sum of the numbers in the same column of the first and second rows.
 i.e. $C = A\times B -(A+B)$

 $5\times7 - (5+7) = 35 - 12 = 23$
 $12\times9 - (12+9) = 108 - 21 = 87$

 $15\times4 - (15+4) = 60 - 19 = 41$
Answer: I found the same answer as @Apep's, but in a different way a different answer.

 $?=17$

For some $x$, let $|x|=\sqrt{x^2}$. This is the notation I will be using for the following pattern.

 The first column has $5$, $7$ and lastly $|23|$. $$23+1\times |5-7|=5^2\tag{$\times 1$ since it is the $1^\text{st}$ column}$$ $5$ is the lowest out of $5$ and $7$.   The second column has $12$, $9$ and lastly $|87|$. $$87-2\times |12-9|=9^2\tag{$\times 2$ since it is the $2^\text{nd}$ column}$$ $9$ is the lowest out of $12$ and $9$.   Note that since this column has a minus instead of a plus, then the overall pattern is therefore going to have alternating signs, implying that the following column will have a plus sign.

Therefore:

 In the third column, if we let the question mark be $\rm A$ for Answer, then it is going to look like this: $$\rm{A}+3\times|15-4|=4^2$$ Since $\rm A = -17$, then in the last number of the third column, the answer is going to be $|$$-17|$ which is $17$.

Having the inclusion of $|\cdot |$ allows there to be no negative numbers.