Change of intended date of arrival

Question: I was recently issued with a U.K. Standard visitor's visa. It's valid for 6 months and multi-entry. When I applied, my intended travel dates were in December. However with new family developments I am unable to travel in December. My purpose for traveling to the U.K. hasn't changed, nor has the duration, the only thing that has changed is the date. Would I have problems entering the U.K on a changed date that is still within my visa validity?
Answer: Since you journey is still within your visa validity period, another entry date will not be an issue. You can enter and leave the country between two dates printed on your visa - "valid from" and "valid until".
And entry date is not printed on the visa anyway so no one would even know that your planned arrival date had changed.
Comment: Thank you. I was worried that it might affect me when o get to the IO window. I makes sense though.

Change of intended date of arrival


Is there an idiom for letting an element scroll with the page only until it hits the top of the browser?

Question: Basically I am looking to accomplish something similar to the affect that yelp uses to have the map follow you down the page on their search results page but I'd rather have it be instantaneous rather than have it catch up a few seconds after you scroll.  I also don't want to just set the position to "fixed" because I want it to scroll until it would scroll off the page, and only then become "fixed" (and likewise, if the user later scrolls back up, I want the map to become "unstuck" and scroll back down the page).
I imagine that this could be accomplished with some convoluted javascript (using "watch", etc) that detects when the map is near the top of the page and changes the CSS values appropriately, but I was hoping for a combination of relative/absolute/fixed positioned divs that obviated the need for Javascript at all.
And of course what I'd really like is a drop-in library (or jQuery plugin, etc) that does just what I am looking for if such a thing exists.  If no off-the-shelf scripts exist to do this, I'd love to hear from people with first-hand experience doing or trying to do the same thing.
Answer: Ok I gave up and did it the yelp way.
Comment: What library did you go with I am looking into adding the same thing to my website but am having a little trouble on where to start looking


pollution degree and creepage distance

Question: I read in some books that pollution degree can influence the creepage distance in electronic  circuits, is there any relation (mathematical) between the pollution degree and the creepage distance.
Answer: Pollution degree is an empirical characteristic and not a real physical quantity, so no, there will be no strict analytical formula. The next best thing is to take a table with creepage distance values for different pollution degrees and approximate these values with some kind of function (usually a polynom).
Comment: If your designing for a standard it will usually have a requirement based on expected transients (Rated impulse withstand voltage) the device is to be able to withstand. Industrial IEC 60947 uses 5.5mm as a default but it can be reduced to 0.1mm based on very good transient control.

pollution degree and creepage distance


Make cutting in the SketchUp component

Question: I have got a component which is a wall.
I want to add different cutting to different walls.
However, I do not want to make walls unique, since I want to be able to resize walls in the future.
Comment: Hi. Welcome to GDSE. Your question isn't very clear. Could you please take a screenshot showing what you have done, and possibly also an image showing what you are trying to achieve. Thanks.
Answer: Windows and doors use cutting areas to cut into wall elements, and those windows and doors are saved as components. They cut into live geometry, not other components. If you declare a given component "unique", you can then make edits to it which do not reflect on other similar but not the same components, and if you then copy that component, changes you make to one will impact all others of that type.
You CAN cut components which are solid (watertight) using the SketchUp solid tools (Booleans) and THAT would work fine - but might be a bit... labour intensive - but I think it's your best bet given the constraints you've mentioned.

Make cutting in the SketchUp component


Tosafot Peirush

Question: I am looking for a sefer that offers an in depth explanation of tosafos (and preferably rashi) to aid me in studying in depth on my own. The ideal would be to have it in english, but a hebrew explanation would suffice as well. My question is what are my options for such a sefer, and which option is the most comprehensive and clear.
Comment: Though not close to complete, http://www.tosfosinenglish.com/ does cover some of the more common sugyos from [mesechtos that are learned in Yeshivos](http://judaism.stackexchange.com/questions/11135/883). There's a physical edition, though I don't think many stores carry it. See also this question: http://judaism.stackexchange.com/questions/40323/what-are-some-good-resources-for-deciphering-a-tosfos-comment-that-i-cant-figur
Comment: Iyun hadaf  has tosfos for many mashectot
Comment: @HodofHod Is that not a dupe?
Answer: The Daf Yomi Advancement Forum - dafyomi.co.il - has an English point-by-point translation of every Tosfos in the Maseches, in its entirety.
They cover the entire Shas until the end of Avoda Zara - and I'm sure they are working on the last few Mesachtot.
Each Tosfos starts with a summary, and then each phrase is brought in the original and then explained in English.
Comment: This seems to be an excellent resource, but it is not true that every masechta before avoda zarah is covered. Sadly the one I need is not.
Comment: There's also the מי מנוחות on many מסכתות
Comment: It is missing maseches kidushin
Comment: They emailed me informing me that: _Meanwhile you can access the Tosfos here
http://www.dafyomi.co.il/kidushin/tosqaindex.htm

We are working on a more permanent solution still._
Answer: Mesivta is a running Hebrew commentary with footnotes which includes a full explanation of Rashi Tosfos and the major issues and how the Rishonim and Achronim deal with it
It has about 120 volumes covering all of Shas.
You can view some of its features here http://www.oz-vehadar.com/en/department/16/editions-of-the-metivta
Answer: There is a set of Shaarei Tosafos which explains each Tosafos, in Hebrew, really well.
Answer: Mei Menuchos, by Rabbi Nachman Kahana, is a clear and systematic explanation of Tosfos (in Hebrew), and is available free online for several mesechtos:
http://nachmankahana.com/mei-menuchos/
Answer: Artscroll has many but not all tosfos explained in footnotes.

Tosafot Peirush


it's necessary to learn Magento 1 for starting with Magento 2?

Question: Is it necessary to learn Magento 1 for starting with Magento 2? and what are resources can i learn magento from it?
Comment: person having m1 knowlegde..definetly has a way too less learnin curve for m2..but if you are just starting out..no need to go back to older ways..the only advantage is..if online some blogs, docs are missing for m2, you canread up m1 and co-relate it
Comment: Possible duplicate of [Is it necessary to learn Magento 1 for starting with Magento 2?](http://magento.stackexchange.com/questions/116381/is-it-necessary-to-learn-magento-1-for-starting-with-magento-2)
Answer: It is not necessary to learn Magento 1 as most things have changed between 1 and 2.  Right now one of the best places is to go through the lessons on Magento U.  Some other good blog posts are those from Alan Storm http://alanstorm.com/, Vinai Kopp http://vinaikopp.com/blog/list/.  You could also look at the Devdocs.magento.com to help get started.
Answer: It is not necessary, but it helps. In addiction to the blog recommended by Brett (very good resources indeed) i suggest the Inchoo blog http://inchoo.net/category/magento-2/ (pretty simple) and the Alan kent blog https://alankent.me/category/magento/ for insights. Personally, I learn a lot by reading this forum. Good luck!


Book for "linear algebra & multivariable calculus"?

Question: is there a book that covers linear algebra and multivariable calculus, preferably computationally but with hard (maybe even proof based) exercises. I tried Apostol's Calculus volumes but they seem to adopt a rigorous approach. I am mainly trying to prepare for math 25 which I'm planning to take next year (deferred entry) at harvard.
Comment: I know of no book that does both at the same time.  Is there any reason you would want a book that does so?
Comment: @Omnomnomnom: Not really but I figured that since textbooks usually have a size limit of around 500 pages, then if I could find a book that did both I would be better off than reading a 500 page treatment of multivariable calculus followed by a 500 page treatment of linear algebra. I am fine with the 500 page treatment of both together (if one exists) being more concise, since I am mathematically mature enough for rigor but at the same time need to learn these topics quickly at a computational level. If you know of any brief treatments of either of these topics I would highly appreciate it.
Answer: In the late 1960s through the mid 1970s there was a slight tendency (in the U.S.) to combine calculus and linear algebra, at least for honors level courses. Two books that I'm fairly familiar with (because I worked through parts of each while in high school) are [1] and [2] below. While looking up the amazon web pages for these two books I also came across [3], but I don't know anything about this book.
[1] Calculus of Vector Functions by Richard E. Williamson and Richard H. Crowell and Hale F. Trotter
[2] Calculus and Linear Algebra: An Integrated Approach by Mary R. Embry and Joseph F. Schell and J. Pelham Thomas
[3] Multivariable Mathematics: Linear Algebra, Multivariable Calculus, and Manifolds by Theodore Shifrin
Answer: The two I know of are Ted Shifrin's Multivariable Mathematics and Hubbard and Hubbard's Vector Calculus, Linear Algebra, and Differential Forms.  I've heard good things of both, but I haven't actually read either so I can only recommend them as texts I know others have enjoyed.
Or if you just want short, geometrically motivated treatments of linear algebra and multivariable calculus, try Alan Macdonald's Linear and Geometric Algebra and Vector and Geometric Calculus.  Each one is only about $200$ pages so reading them back to back is entirely an option.  I have read both of these and they are excellent.  Neither one goes quite as far into the material as I might like, but what they do cover, they do very well.  Then again because you're just trying to prepare for future courses, you don't need books that cover everything.
A note though, Macdonald's book do take a little bit different approach than the normal curriculum.  He does everything from a geometric (Clifford) algebra perspective.  I really loved that, and it's certainly not going to hurt to learn some geometric algebra, but it will require a little mental adjustment when you learn it the standard (matrix-based) way at university.
Comment: I can vouch for Shifrin's book, it's quite nice!
Comment: @pjs36 but it's very expensive though
Answer: Since this question was re-asked, it'sworth mentioning there are good prospects among the AMS Open Math Notes.

Peter Philip, Analysis II: Topology and Differential Calculus of Several variables
Frank Faris, Extremely Advanced Calculus: Multivariable Analysis, Vectors, Forms, Metrics
Oliver Knill, Linear Algebra and Vector Calculus


What's the difference between Amazon's EC2 and S3 services?

Question: I heard Amazon is going to offer a free trail of their hosting services soon with their EC2 tier. I'm a new web developer and I'm still learning a lot about hosting and server configuration so I'm looking at a lot of hosts.
What are some main differences between EC2 and S3 services offered by Amazon?
Comment: EC2-we can install whatever software we want
S3-for data storage
Answer: EC2 = Elastic Cloud Computing
S3 = Simple Storage Service
One is for application hosting, one is for data storage.
Answer: Amazon S3 offer storage service. where you can read, write and access your data via REST/SOAP from Cloud.
EC2 offers renting of virtual computers(like how we use our vmware,xenservers), you can create/delete/modify your instances.


What is the tag [derivations] for?

Question: Since the tag derivations has no usage guidance at the moment, I do not know when it should be used, which is unfortunate.
Comment: I could also see it getting used for questions about logical derivations, i.e. formal proofs in the sense of natural deduction or similar systems.
Comment: My old algebra memory says:   a **derivation** on a ring $R$ is a map $D : R \to R$ satisfying $D(a+b) = D(a) + D(b)$ and $D(ab) = aD(b)+D(a)b$.  Perhaps that is what the tag is for.
Comment: @GeraldEdgar My concern is that some people might use it for questions on derivatives, i.e., as a synonym for differentiation
Comment: Some stats related to the tag ([tag:derivations]) can be found in the [MO editors' lounge](https://chat.stackexchange.com/rooms/10243/conversation/the-tag-derivations). Tangentially, this tag now has [a tag-excerpt](https://mathoverflow.net/posts/448693/revisions).
Answer: I think that indeed:

in analytic questions about specific derivatives/ differentiation, the tag should be removed (it is then not very relevant: if it were, it would be relevant for thousands of questions). When necessary, it could be replaced by better tags (e.g. real-analysis, ca.classical-analysis-and-odes, dg.differential-geometry, differential-operators... maybe also differential-calculus although the latter doesn't sound like a research-level tag).

tag info should be added, indicating that this concerns derivations in a (kind of) algebraic setting.
Comment: Actually, I just learned from [Wikipedia](https://en.wikipedia.org/wiki/Differential_algebra) that [tag:differential-algebra] might be both more specific and comprehensive for the algebraic uses of the term.
Comment: @FrançoisG.Dorais Differential field is a field endowed with a derivation. This does not cover "derivation" in several respects: a derivation can be defined on a ring $R$ (not necessarily associative), and can also be defined from $R$ into an $R$-module. Also, the set $\mathrm{Der}(R)$ of derivations on $R$ forms a Lie algebra and can be considered as a whole without singling out a single derivation. In important cases this is the Lie algebra of the group $\mathrm{Aut}(R)$, and occurs quite naturally. Quite many of the questions now with the tag would not be tagged "differential-algebra".
Comment: @FrançoisG.Dorais in any case, I agree that the tag [tag:differential-algebra] is better defined and should be used more often. It has no usage guidance. A draft: "Fields or rings endowed with one of several derivations. Differential Galois theory, algebraic study of ODEs/PDEs, etc."
Comment: Perhaps [tag:differential-field] would work for most algebraic uses? (I'm no expert but these are essentially the only algebraic derivations I personally have worked with. I understand that the concept is more general but we don't need a tag for every concept.)
Comment: Re: [tag:differential-calculus] I agree that it's not really "research-level" but isn't that a little disparaging towards Newton, Leibniz, Gauss, Euler, Bernoulli, Bernoulli, Fourier, Bernoulli (is there a fourth Bernoulli?), Jacobi, Riemann, etc.?
Comment: Yes, I am aware of the existence of such generalizations. I also believe that they are useful in contexts that I am not familiar with (or perhaps even some familiar contexts that can't remember at the moment). But this is mathematics as a whole, not actual MO usage. For tagging purposes, the tag name should ideally be both useful (in the MO usage sense) and unambiguous (when possible). The tag [tag:derivations] is neither!
Answer: Another possibility is removing the tag altogether. Let us consider the pros and cons:

I can easily see people repeating the same mistake and misusing the tag, even with better usage guidance. So keeping the tag tidy will require further effort.

There seem to be only few questions that would keep the tag after the retagging YCor proposes, and it seems to me that they all contain the word "derivation" in the title. So it will be simple to search for questions on derivations even without the tag.
Comment: This may be a case where it is better to have a small amount of narrower tags than a tag for the overarching concept.

What is the tag [derivations] for?


Can the name of a D&D character be changed without having to make a new character?

Question: I used a nickname of mine for a recently created D&D character, but it doesn't really suit her. Can I change her name and keep her EXP? I don't play in any organized format.
Comment: Which edition of the game are you playing?
Comment: No need to edit "thanks" into your question. Checking the "accept answer" box and upvoting serves as thanks
Answer: Ask your DM.
D&D doesn't have any rules about naming your character, other than that they should probably have a name, even if it's just The Man with No Name. Many groups don't even take in-fiction names very seriously, and just use players' names to refer to their characters.
Your DM will probably shrug and say, "OK, whatever" unless they've got a lot of documentation and/or play logs already written up with the old name in it, and since you're only level 2 that's not likely to be a big problem.
Comment: Not relevant for this question (as it specifically states it's not an organised format), but for anyone reading this answer - in Adventurers League you can change almost everything *except* the character name and story origin before it hits level 5 (and then you can't change anything) - see http://dndadventurersleague.org/rebuilding-your-dd-adventurers-league-character/ for details.
Comment: Of course, in AL play, the name is what binds the character sheet to the logs. The player can't lie to the GM about the character's name, because it's part of the record keeping, but the character can lie to other characters in play.
Answer: Think of it like the real world:
You decide one day you don't like your name so you tell everyone you're changing it. What happens?

Anywhere your name is recorded you'll have to get it changed or they'll still refer to you by it. Say, the local government or trade organization.
Your friends and people who know you might forget and call you your old name or they may do their best to honor your wishes.
Any new people will only know of you by the name they are told.
Does your world deal with true names, such as through magic? Maybe one or the other names is actually your true name or maybe it's something entirely different. That could be interesting if it ever comes up.

Of course, all this is an in-universe change. If you and your GM agree that the change is out-of-universe then the name change can be considered to be retroactive and have always existed. Then you can simply treat the new name exactly as you did your old name.
Personally, I think the in-universe name change is more interesting from a role-playing perspective.
In short, decide if you want the change to be in-universe or out-of-universe and discuss it with your GM. If the GM agrees then go with either version of the name change.
Comment: Side-note: regarding true names and nicknames, there was a truly *brilliant* scene in The Lies of Locke Lamora that played with that trope. :)
Answer: Yes.
There are no rules about names staying with a character. In a situation like a name change, talk to the DM to either have it changed, or role play the character's decision to change her name.
As a reference, you can use Elf Names, 5th edition PHB, which states that Elves are called by child names until they select an adult name some time after their hundredth birthday. So, in-universe, characters taking new names is not unheard of.
Answer: Go for it.
In character
If it's an in-character name change, then you'll need to role-play that change with respect to the other players and any NPCs who know you. There are plenty of reasons from history that people changed their name.
Historically, many people change their given names to a "formal" name upon rising to the title of pope or king. The bible shows that several people changed their names when they became believers. Many famous people change their names to pseudonyms.
So you can easily come up with an in-character reason and just role with it.
Out of character
Simply ask your GM. Explain that the concept doesn't fit the name, and you'd like to erase the name, but keep the character otherwise the same. Most GMs probably won't have a problem with this retcon, since it doesn't significantly change the details of your character.
Answer: It is up to the agreement you made with the DM.
As being a DM, I usually use such things for character-story/background improvement.
The current name you use could be a name;

given to you by your firends when you were a child.
picked by you when you are a teenager which do not suits you anymore.
given by somebody around you whom you do not like. But the others people used to it so do you.
taken by you back in your mercanary days when you do not want to give your real name since you swing your sword for money.

etc...
If you offer this with a such idea, then the DM may also like the idea.  Such things makes player background more relaistik IMO.
PS: There are many heroes/villains known by their fake names or nicknames. My favorite character in this context is Sparrowhawk (real name is Ged) from Earthsea, where knowing the real name of a person grants you powers over him/her.


'Resonant frequency' or 'resonance frequency'?

Question: I have seen both 'resonant frequency' and 'resonance frequency' used in scientific literature. But which one, actually, is correct: 'resonant frequency' or 'resonance frequency'?
Answer: As you noted yourself, both terms are in use (and actually yield similar amounts of Google hits).
However, in my eyes the term that should be used is 'resonance frequency', referring to the frequency at which resonance occurs.
'Resonant frequency', i.e. the frequency at which something is resonant, strikes me as a very unusual structure and I struggle to come up with examples following this concept.
See also: http://users.ece.gatech.edu/mleach/misc/resonance.html, an opinion pretty much mirroring my own.
As a native German speaker, I can add to that that in German the term is "Resonanzfrequenz" and not "resonante Frequenz", which, of course, boils down to the same discussion in the end.
Answer: Both are correct but have different meanings.

Resonant frequency means a frequency which is deep clear and continuing to sound or reverberate.

Here resonant is an adjective and modify the noun frequency.

Resonance frequency means the value of a frequency when the resonance occurs.

Here the resonance is a noun and has an attributive relation with frequency.
Comment: I don't understand the difference. Both of those things would have the same value in Hz wouldn't they?
Comment: @Dangph, As I said in my answer, resonant is an adjective which describes the frequency. You can also say a resonant sound, voice or wind but it doesn't mean you can measure it in db or km/h. It's as if you were trying to measure the clarity of water in litres or gallons.
Answer: Several answers have touched on this point, but I'll try to dive it home. The term resonant (adjective) frequency (noun) implies that it the frequency that is resonating. More likely, you mean to describe the frequency of resonance of some system. That's why "resonance frequency" would be the better choice.
Never the less, "resonant frequency" is widely enough used to qualify as an idiom. No one will think less of you for using it.
Answer: They both seem fine to me. One of the meanings of resonant is simply "pertaining to resonance", so I don't see there is a problem describing a frequency as being resonant.
Personally I like resonant frequency more than resonance frequency. It just sounds better to me, but that's just personal preference.


Definition of extra sample observation

Question: I have been seeing this term used in a few books in machine learning, however, the authors never care to explain what that actually means, they just use it. I hope to understand this concept as it seems fairly easy, but I don't want to have a wrong idea about it.
Answer: Please add some context in which you find this term, sometimes terms explain themselves out of context.
Without further context my understanding is, that machine learning models are trained on a sample of data and that extra sample or out-of-sample means, that this observation was not used in the training phase of the model.

Definition of extra sample observation


La Sardina - blank film

Question: I have recently bought La Sardina lomography camera and after shooting the film came out blank with some colour stains on it. The film was loaded and rewinded correctly and I don't really know what to do.
Comment: What's the rest of the film look like?
Comment: Please tell us how you shot the film and in what conditions, how you handled the film before, during and after you loaded into the camera. What film, what ASA?
Comment: Lomography is known for light leaks...  There might be a giant light "leak" in the film take-up compartment?
Comment: That camera has a **fixed aperture of F8 and a fixed shutter speed of to 1/100th of a second** ( plus bulb for unlimited time exposures )  **And no light meter**.  So your lighting need to fall within the range acceptable for  F8 at to 1/100th for the ASA of the film you are using,  **or adjust your development times** to compensate for more or less than proper light conditions. *You will need a light meter or a long history of judging lighting conditions for a given ASA of film*.
Answer: You film has been exposed to light from a source that did not reach the film via the camera's shutter. The darkened edges of the film would not be exposed by taking an image using the camera. Even if your exposure settings completely overexposed the film using the camera's shutter, the edges of the film would not receive any of that light as it is masked by the film gate directly behind the shutter mechanism.
Comment: Agreed.  There are edge markings, so the film was (more or less) correctly processed.  The camera is massively malfunctioning.  It does not look like the shutter is working correctly, and it also looks like the camera has significant light leaks.
Comment: @JimMacKenzie In the example in the OP, the film is lacking edge markings in the areas that were totally exposed. This may not have happened in the camera at all. It may have happened in the handling of the film outside the camera.
Comment: It could have happened outside the camera, but given that it's a Lomo, it was more likely in the camera.  Still, I can't be sure. The edge markings would be gone in the fogged area no matter how the fogging happened.

La Sardina - blank film


The importance of family

Question: Do the Sources anywhere specifically stress the importance of family in general?  Maintaining ties with family members, even though they may live far away, seeking long-lost cousins, having reunions, cultivating and prioritizing these relationships, and the like?
Comment: טוב שכן קרוב מאח רחוק, is that for close family ties or against?
Answer: There certainly was a precedent to maintain family connection in terms of the instructions Avraham gives Eleizer:
Breishit 24:38

אִם־לֹ֧א אֶל־בֵּית־אָבִ֛י תֵּלֵ֖ךְ וְאֶל־מִשְׁפַּחְתִּ֑י וְלָקַחְתָּ֥
  אִשָּׁ֖ה לִבְנִֽי׃
but you shall go to my father's house, to my kindred, and get a wife
  for my son.'

True that the main concern was that he didn't want his son to take a wife from the daughters of Cana'an. But, technically, there were others living in Padan Aram, right? Avraham could have instructed to find a non-Canaanite woman from Padan Aram or from any other place. It seems that he had faith that a good match would emanate from his own family, despite not having had contact with them for numerous years, most likely.
Similarly, Rivkah sends Ya'akov back to her father's home to find a wife from her own family. She too, probably had little or no contact.
Yosef insists that his father bring all the family - all the children and grandchildren to Egypt. Yes, of course, the motive was the famine. But, he does say, "You shall be close to me", and there's probably a sense that he wanted physical closeness of his family.
Comment: The use of "family" in this answer seems roughly analigous to "nation". I suspect the OP meant something narrower.
Comment: @DoubleAA I don't think so. Note the double usage אֶל־בֵּית־אָבִ֛י תֵּלֵ֖ךְ וְאֶל־מִשְׁפַּחְתִּ֑י . Maybe מִשְׁפַּחְתִּ֑י could mean "tribe" or "nation". But I think בֵּית־אָבִ֛י is quite narrow and means "house of my father". I don't think there's another definition used for that phrase.
Comment: It's hard to tell because at the time the Jewish nation (if I can call it that) wasn't much bigger than his close family.

The importance of family


Why do some Asian dragons have fur (or hair)?

Question: In the west, we tend to think outside dragons as purely reptilian in nature - in other words, all scales, no fur - but many Asian representations of dragons show them with substantial amounts of fur (or hair).

Scaly Western Dragon

Furry Japanese Dragons

Furry Vietnamese Dragon

Furry Chinese Dragon
This is even true in more recent depictions of dragons from the east - these two images were produced by the Japanese animation company Studio Ghibli:

Why is this the case with Asian dragons, and why the discrepancy between Asian and Western dragons?
Comment: By fur, do you refer to the hair on their heads? Canonically, Chinese dragons (from which Vietnamese/Japanese ones derive) have fish scales on their body, and your examples match this.
Comment: @Semaphore Heads and (in some cases) backs
Comment: @Semaphore - See my two most recent additions.  In any case, they are all 100% furrier than western dragons, which have no fur at all.
Comment: http://scifi.stackexchange.com/questions/34088/differences-between-dragon-drake-wyrm-and-wyvern
Comment: Not fur! Think pocupine quills. You only showed scales and spikes.
Comment: @WadCheber Is your question still not resolved by the answer?
Comment: It's possible that the Chinese based theirs off a creature that did have a mane-like feature. Despite their common depiction, many dinosaurs had feathers.
Comment: @Johnny humans have never seen dinosaurs alive.
Answer: It is important to note that East Asian dragons are in fact quite unrelated to Western dragons. While they may have influenced each other, the two traditions emerged separately. They share the same name essentially only because of translation choices; there is fundamentally no reason why Chinese dragons shouldn't be different to European ones. Hence, the hair disparity ought to be unsurprising since they are not the same creature in the first place.

Secondly, East Asian dragons do not have furs in the usual sense of the word, although they are often depicted with beards or manes. Canonically, the body of the Chinese dragon (from which Japanese, Korean and Vietnamese dragons originate) have fish like scales. While the exact appearance have varied over time, the fish scaled body has remained a constant feature of dragons in the Far Eastern tradition.
As early as the Han Dynasty, a scholar named Wang Fu introduced the concept that dragons resemble nine animals. The Ming Dynasty and scholar doctor Li Shih-chen writes in his magnum opus the Compendium of Materia Medica that:

龍者,鱗蟲之長也,王符言其形有九似:眼似兔,角似鹿,嘴似牛,頭似駝,身似蛇,腹似蜃,鱗似魚,爪似鷹,掌似虎 ... 口旁有鬚冉,頜下有明珠。
  Dragons are the first of the scaled beasts. Wang Fu says it resembles nine animals: eyes like rabbits, antlers like deers, mouth like cows, head like camels, body like snakes, belly like shellfish, scales like fish, claws like eagles, and paws like tigers ... its mouth is bearded, and there's a bright pearl below is jaw.

More relevant to visual depictions, a traditional mantra for dragon painting is:

一畫鹿角二蝦目,三畫狗鼻四牛嘴,五畫獅鬃六魚鱗,七畫蛇身八火炎,九畫雞胸。
  First, deer antlers. Second, shrimp eyes. Third, dog nose. Fourth, cow mouth. Fifth, lion's mane. Sixth, fish scales. Seventh, snake's body. Eighth, fire flames. Ninth, chicken's breast.

These traditional descriptions of dragons also gives us some insight into why the Chinese version has hair. Whereas the European dragon has a strong reptilian quality, their Asian counterparts are considered a composite of different beasts - many of whom has plenty of hair. Thus, despite a (probably) serpentine origin, the Chinese dragons acquired hair on the head and limbs. These were also the same body parts that did not come from fish or snakes.
Note that the protrusions on the back of the dragons are not hair, but rather backfins. This is an ancient feature of Chinese dragons, dating back over three thousand years to Shang Dynasty era depictions.

See below for a Song Dynasty conception of the dragon, painted by the artist Chen Rong. This was about 1000 years ago,

IMHO The question's anime dragon and 3D art examples do not really depict traditional dragons.


What methods should a model contain

Question: When I have a model Car where users can upload a single picture to, I can create a property ImageFile in Django to achieve that. Now I would also like to get the metadata from this file and save them to the database.
Now I'm very unsure where I should place this method a how I should design it. It would be obvious, to have a method like _set_exifdata() or _update_exifdata() that is called every time I set an Image to the model.
Or a method get_exifdata(imagefile) that returns a dict of exifdatas. But should this method be part of the Car model? Actually, I won't need it anywhere else, so it does not make sense to put it in a general helper class. But on the other hand, I would prefer to split methods in "retrieving data" and "setting data", so _update_exif() for example would do both of it in once, and maybe that's OK in design ways, but maybe its not and there is some rules of model design I should know and respect here.
I hope someone can help me with some guidelines for model design (especially for django) and what methods should be part of methods and which should not.
Answer: if you using only for the car then you can put that value in the model save method,
which will get call every time whenever you will update the data.

What methods should a model contain


Current analysis with 2 voltage sources

Question: I am not able to solve this question , I initially thought that in order to find the current of this series circuit it would be Vtotal / RTotal , however I am not getting the correct answer with this method. There must be another factor I'm not aware of when it comes to dealing with 2 voltage sources. Can anyone shed any light on why I'm not getting the correct answer ?
Comment: Please show your work. We won't do your homework for you, but if you show what you did we can help show you where you went wrong.
Comment: VTotal / Rtotal is the right approach. If you got the wrong answer it must be because of things that you haven't yet mentioned.
Comment: Remember that the voltage sources are not summing. Think Thevenin equivalent!
Comment: I=2.5A and V=17.5V
Answer: recognize that the two voltage sources are not acting in the same direction.so you need find the resultant voltage in that circuit which is 50v-12.5v=37.5V and use the formula you wrote above to find I=37.5v/Rtotal = 37.5/15 = 2.5A. The voltage across the 7 ohm resistor is gotten by applying ohms law: v=IR = (2.5)(7)= 17.5v

Current analysis with 2 voltage sources


What is the difference between "I can go swimming" and "I can swim"?

Question: What is the difference between "I can go swimming" and "I can swim"?
Are they the same?
Answer:
I can swim.

means you have the ability to swim.

I can go swimming.

means you are able to go somewhere to swim.  Someone may have given you permission to swim, or you are able to travel somewhere to swim.
Without further context the exact meaning can not be deduced.
Answer:
I can swim.

Implies that you're able to swim based on your own abilities, meaning that you're not inhibited from swimming because of something about yourself.

I can go swimming.

Implies that you are able to swim because of other factors not about yourself. This could be that there is a place available to swim in (a lake for example) or that you've been given permission to go swimming (perhaps by a parent).
It's easier to think about from the negative:

I can't swim. <- You don't actually know how to swim.
I can't go swimming. <- Your parent hasn't given you permission to go swimming.
Answer: They can both indicate the ability to swim, though "I can swim" is probably the better choice for this.
The second example could also be used to indicate permission, for example:

Mom said I can go swimming.

or location:

I can go swimming at the lake.


Do some US airlines now charge for ANY checked baggage?

Question: I've heard that some/most US airlines charge for any checked luggage, even the first piece.  Is that the case?  Are there any that don't?  What kind of charges would I expect per piece if that is the case?
Comment: Your question *title* was getting close to the type we're trying to avoid ([faq]) - asking for a list of all US Airlines.  I've rephrased it slightly, but have left your question itself intact.
Answer: Yes indeed, most US airlines charge for all check baggage.  There are exceptions if you have "status" with either the airline themselves or with any of their affiliated airlines (eg, Star Alliance, OneWorld, etc), or if you're flying in First Class, or if you're connecting from an international flight, or if you have one of their pay-for credit cards, but for most occasional domestic travellers you'll be charged to check any bags.
The fees vary from airline to airline, but are generally fairly similar.  International trips are higher, although most airlines still do not charge for (at least) the first bag for most international trips.
The fees are available on their airlines websites.  eg, for both United Airlines and American Airlines it's $25 for the first checked bag, and $35 for a second checked bag for domestic flights, and whilst the first bag is free for international flights the second will cost you up to $100.
Virgin America depends on the type of ticket you have purchased, but is generally $25 for each of the first two check bags.
The main exception to the above is Southwest Airlines, who do not charge anything to check your first 2 bags (with standard weight limits/etc).
Some airlines, such as Spirit, charge different rates depending on whether you pay for your checked bags in advance ($28/$35 for domestic up to 40 pound/18kg) or at the airport ($38/$45).  Mind you, Spirit also charge you between $20 and $45 (up to $100 starting in November!) for your carry-on bag, so avoiding them completely is generally the best option.
Comment: @MichaelMathews As mentioned in the answer, Southwest remains an exception to that. They still allow 2 free checked bags up to 50 lb per person. Given how heavily this factors into their marketing, I doubt it will change any time soon.
Comment: Effectively, almost all US airlines now charge for checked luggage (even the first bag). Some airlines waive the charges for elite passengers, full fare passengers, and/or premium cabin passengers. Some airlines treat a frequent flier ticket purchased with the maximum number of miles the same as a purchased full fare ticket.
Comment: Indeed, even still in 2023, Southwest offers free checked bags.


Can I level basement floor with cement board and self-leveling cement?

Question: I have about a 1" height difference on my concrete basement floor. My understanding this was for drainage in case of flooding. I have a wall in the way of where anything could potentially drain in the future so keeping the slope for drainage no longer makes sense. The 1" drop is over about 12' and there are a variety of reasons I would like to level the floor. I will have bypass doors for closets and there will be a funny looking gap under one side of the doorway if the floor is not level. The room will be an office and I don't want to sit at an angle or roll downhill on my chair. Desks and things would also be out of level. Baseboard trim would be slightly more difficult to install on a slope.
I do not want to raise the floor more than necessary because I want the doorway (on the highest corner of the room) to flow smoothly into the rest of the basement if possible. I plan to use Luxury Vinyl Tile/Plank on the floor.
I have a few ideas for how to level the floor without raising the height more than needed.

Pour self leveling cement. This would probably be the easiest and most straightforward but it would also be over $800 to level a 12'x16' area with a 1" slope over 12'
Sleepers with plywood subfloor for lowest area + self leveling cement on higher area. This would save a lot of money by filling the bulk of the area using sleepers and plywood and then for the smallest 4' or so of the slope use a small amount of self leveling cement that butts up to the plywood and sleepers
Cement board backfill for lowest area + self leveling cement on higher area. I would stack several sheets of cement board to back fill the areas I can then fill remaining space with self leveling cement (either going up to the cement board or overtop it and encasing it)

Are there any major problems with any of these ideas? #2 would be cheapest I think but also a bit more work and potentially more problematic for a variety of reasons. I'm leaning towards #3 but I'm wondering if there are any problems with that plan. I would just use thinset to attach it to the existing concrete floor, stair step the sheets of cement board, and then use self leveling cement over the entire room, including the cement board for a smooth finish.
Comment: No one can give you a proper answer without specific drawing, schematics and pictures.   #2 and #3 just seem ridiculous and I specialize in basement renos.   Also you mention a wall - guessing wood framing?   If so trust me water will still drain under it.  It sounds from your info that the floor slopes to some outer drain system...   You have a slope to it and tons of water gets in you can have it cleaned up in 30 mins and probably not have to pull the vinyl up.    If there is no slope you are pulling vinyl up every single time or you will have moldfest.
Answer: start with mortar or fine concrete screed where there's a lot to fill and then use self leveling cement to finish. where the fill needed is more than 1/4" self-levening cement is best used second.
Comment: And pouring concrete will be half the price of the sheets, plus save on thinset, and save headache trying to squeeze the sheets flat with thinset in between.
Comment: Screed is also known as deck mud, right?


User login cycles

Question: Just install 12.04-64bit and while I can login using the guest login, I cannot with my user login: it just cycles back to the login screen. I have performed the sequence of apt-get update/upgrade and install of the nvidia-current driver, but got back that it was already in use. The password appears to be recognized, as wrong one results in an error. Have also tried switching to the 2D Unity, without other results.
Answer: This can be the solution: http://www.icpep.org/solved-ubuntu-wont-login/
Comment: Whilst this may theoretically answer the question, [it would be preferable](http://meta.stackexchange.com/q/8259) to include the essential parts of the answer here, and provide the link for reference.
Answer: Initially from my clean install of 12.04 (64bit), the login screen would intermittently repeat, and the second time the login would be stuck with "Logging in..." under the user name.
Found a bug report about the "Logging in..." stating that this was caused by using a non-default wallpaper and an issue with lightdm. I have reverted to a plain blue background, and I now no longer get the "Logging in..." freeze issue - however, I consistently get two login screens!
I enter credentials correctly and am immediately presented with a second login screen (no errors). Authenticating against the second screen completes a normal login to the Ubuntu desktop.
Any ideas what is going on here? I don't see any obvious errors in the logs (maybe I am looking in the wrong logs).
Comment: This is potentially the same issue.  Has this been resolved by recent updates?
Comment: there are a lot of questions (and answers) regarding problems with the login screen. Please check them first (use the search box at top) and include which solutions didn't work for you. If it's a new issue that wasn't asked before, you should create a new question instead of adding it as an answer to an older question - thw current way, you're decreasing the probability that your question will be recognised by a larger count of users dramatically.

User login cycles


64GB CF Card exFat Format in D300?

Question: I just bought a Sandisk 64GB CF card to use in a Nikon D300. Display says card is full. I reformatted the card in my PC, no luck. I formatted the card in my camera, same thing. Card appears to format properly but camera still displays "Full" message. Is the exFat file system incompatible with a D300?
Comment: Just a FYI, I've been bitten similarly before.  Since then, I've *always* gone by what's officially supported (e.g.: https://www.nikonimgsupport.com/ni/NI_article?articleNo=000002637&configured=1⟨=en_US )
Answer: Some folks have reported similar issues with D700's and fixed by going into the shooting menu, and creating a new active folder, then formatting the card.
See 9th response in this forum thread
Answer: The Nikon D300 does not support 64GB cards this is because the exfat format required to address a volume of that size is not implemented in any firmware version for this body.
The D300s can support exfat with the appropriate firmware update but is a different camera, the firmware cannot be applied to the D300.
Comment: It's a firmware limitation, but you aren't going to see a firmware update, not even nikonhackers have one in the works.  Manufacturers rarely if ever add features through firmware updates and certainly won't be adding them to a dslr of that vintage.
Comment: exFAT is NOT required for 64GiB. FAT32 is fine for up to 2TiB with smallest sector size. CF protocol if implemented correctly does not limit the card size at all, many very old laptops with CF slots now accept 32GB+ cards without problems.
Comment: Does anyone know if D300 having some hardware limitation that constraint this or just firmware? If just firmware we can 'hope' that eventually a firmware update will come out to allow 64gb CF? (Or Nikon just think D300 development is not worthwhile anymore seeing that it is 'ancient'...)
Comment: Actually it is, because the use of nonstandard sizes have implementation issues that can mean data loss, plus standards-wise the limit was raised above 32gb with the SDHC to SDXC cards which require the use of exfat so support would be necessary.  Certainly D300 firmware would have physical limits addressing such a volume and are likely to have problems.
Comment: By force of habit, having been bitten before, I *always* check the tech specs before buying.  8GB is reported as the highest supported (YMMV beyond that?):
https://www.nikonimgsupport.com/ni/NI_article?articleNo=000002637&configured=1⟨=en_US
Answer: The D300s 1.01 supposedly had a software update that allowed it to use cards up to 64GB. I'm not sure whether or not the update was also available for the D300.
Many users have reported problems with using larger size cards. A good way to solve the problem is generally to format the card in the camera. Or perform a Low-Level format which will reset the card architecture.
It seems generally as a rule of thumb though that you don't use large cards in the D300/D300s.
Have a look at this forum post: http://www.dpreview.com/forums/thread/3237997
It Iooks like according to SanDisk that the D300 is incompatible with 64GB cards: http://kb.sandisk.com/app/answers/detail/a_id/3684/~/nikon-d300s,-d700,-d3,-d3x-firmware-updates-for-64gb-compactflash
Answer: So I have been able to get my D300 to "work" with a 128G CF disk by creating a 32G primary partition on the drive and formatting that with fat32. I did try formatting the entire drive in fat32 at 128G and 64G but the camera reads that it is full so camera is only able to read 32G FAT32 max.
I then made a NFTS partition on the rest of the disk to backup pictures manually on a computer with a CF card reader.
Of note however is do not try and format the disk with the camera. It will format the FAT32 partition up to max size, not the allocated size thus erasing the secondary NFTS partition on the disk and making the primary partition too large for the camera to use.
To make a 32G partition use disk management built into windows. Delete any volumes on the drive. Create a new primary partition and make it max 32,768mb which is binary for 32G. Format the drive in FAT32, not NTFS or exFAT. Then on the rest of drive you can create another partition with the rest of the drive and format it however you like
Comment: I apologize. I have a D300 not the 300S. The S I read with latest firmware can read exFAT.
Answer: I can confirm Joseph's answer works. I bought a 64gb card for my D300 right before a trip, not realizing that 32gb was the max. In Windows, you may have to logon as admin to get access to Disk Management. On Windows 10/11, right-click Start to find Disk Management. I didn't make a partition; instead, I used right-click on the card and Shrink Volume to drop it down to 32gb. I went a little less at 31.5 GB. Format as FAT32 and good to go!
Comment: That answer has already been given.

64GB CF Card exFat Format in D300?


Should I use schema.org/CreativeWork to publish my list of talks/papers/publications?

Question: What's the best practice on publishing your list of talks/papers/publications on the web?
Is there any benefit with annotating it with http://schema.org/CreativeWork properties?
Are there any samples/examples of any famous people doing just that?
Is there any immediate benefit in having any such markup?
Answer: Using machine-readable vocabulary like schema.org is generally a good way to allow people and non-humans to re-use the information you supply on the web. For example, this will make it easier to process your publication data and get a formatted, database-like normalized list out of it.
One might think that search engines use schema.org vocabulary to analyze your website and hence make it better visible on the web. However, what search engines do is somewhat a black box (in the sense that something is happening in there, generating output from user-driven input, but the algorithm inside is not known in detail).
If you would like to reach a better visibility on the web for your publications, you might also like to consider using ORCID which is based on linked-data principles, widely known and used in library and information services - and some research funding institutions are starting to require complete ORCID profiles to be submitted with proposals anyway.
Comment: thanks for a pointer to ORCID; interestingly, they themselves don't actually annotate the list of publications of their authors with schema.org; so much for a standards body!


Why didn't Gremmy Thoumeaux just imagine Kenpachi dead?

Question: If Gremmy Thoumeaux can imagine anything and make it really happen, and if he's really that mad, why didn't he just imagine Kenpachi dying without any talk and fights?
Comment: Largely because by that point in the series, the writing had gone hit rock bottom and nothing anyone does or doesn't do makes any sense.
Comment: essentially as we see, his power relies on his own strength of will verses his opponents strength of will, kenpachi single focus in life is to kill, so he obviously wins because hes kenpachi :D/
Comment: No, gremmy defeated because he cant handle the power he imagined he has, which is the power that is stronger than kenpachi. Thats why hes dead. And also, because maybe hes just too stupid not to just imagine kenpachi dead straight away. Well. If the reason is that hes too stupid, its kinda hard to accept
Comment: I read this few days back and if i remember correctly there is one scene in which kenpachi tells greemy that he imagined him as a monster and that monster killed him.
Answer: Gremmy lost because:

 He imagined that he was as strong as Kenpachi but he failed to imagine that his body was strong enough to contain Kenpachi's power and thus the power tore Gremmy's body apart. Furthermore, Kenpachi also caused Gremmy to doubt himself and imagine his own defeat and death. So Kenny won because his imagination was greater than Gremmy's! Irony!  And this is speculation on my part but Gremmy doesn't seem to have the power to will someone dead. Certainly not someone as powerful as Kenpachi.  Gremmy was always creating physical objects to destroy Kenny (meteorite, guns, missiles etc.). The exception was Yachiro, but Gremmy touched her to turn her bones into cookies; I'll have to review the battle to see if Gremmy ever touched Kenny.

And the out of universe answer is that it would be a pretty lame fight if Gremmy could just imagine his enemies to death. So there has to be some challenge, drama and limitations in play.
Answer: Because Gremmy would have to physically touch Kenpachi first to do that. This is heavily implied given the fact that he only made Yachiru into cookies RIGHT AFTER he touched her hand.
Answer: Gremmy wants to have a toe to toe fight with Kenpachi because he wants Kenpachi to challenge him. That's why he didn't lift any of his fingers.
Comment: Do you have any evidence you could [edit] in that this is indeed the reason why he wanted to fight him?


Can I Pair an Apple Wireless Keyboard with multiple iOS devices (not simultaneously) without 'forgetting'?

Question: I just want to be able to switch my Apple Wireless Keyboard (latest) between my iPhoneX and iPad Pro without having to forget the last used device.
Both iOS devices are running the latest iOS. Not sure if inherently possible, or by app or hack.
Comment: I can tell you that 2 of my macs would both follow the apple remote - had to remove it from one...
Answer: The keyboard does support been paired with multiple devices at the same time. However, there's no "switch" to toggle between devices - neither a physical switch nor a software switch in an app or similar.
The only way to make sure the device you want to use is the one currently communicating with the keyboard is to ensure that it is the only one within Bluetooth range.
If your phone stays in your pocket, it is not that practical to get that device out of the range of the keyboard. A "hack" is then to simply disable Bluetooth on the device that you do not want to use with the keyboard with currently. When you want to switch devices, disable Bluetooth on the current device, and enable it on the device that you want to switch to. The keyboard usually switches over in the matter of seconds.
If you wanted to use Bluetooth for something else on either device, the above hack won't work.
There's no other solutions through third party apps or similar.
Comment: @Wowfunhappy That's exactly what I wrote in the answer, so I don't know what you want to add to that.
Comment: Apologies, I was reading too quickly.

Can I Pair an Apple Wireless Keyboard with multiple iOS devices (not simultaneously) without ‘forgetting’?


Hello is reading novel with a little bit of romace alright (between non mehram) or is it impermissible

Question: Hello I want to know is it alright to read fiction novel that has some aspect of romance in it between non mehrams,the main theme of the novel is not romance,it contain thing like dating each other and interacting with each other and also description of the non Muslim cloth that is contrary to sharia and also maybe kiss one or two time only but nothing beyond that,well basically you could say it is Japanese school genre light novel. The thing is we are only reading it and not acting upon it or thinking of commiting a sin so will it be sin to read it. Also give me a definite answer plz since according to quran and hadith that I know it is acting upon that sin not reading them that is sinful so plz answer what the scholar say about it
Answer: Assalamu Walaikum
Muslims are free to read whatever genres of books s/he likes; however, they must avoid anything that arouses unlawful sexual desire.
In Islam, prose and poetry are considered an expression of feelings and emotions that reflect tenderness and sensitivity. Islam does not suppress feelings and emotions. Prose and poetry can be, in many cases, an influential means toward psychological and social reform. It is worthy to mention that the means takes the same rulings as their objectives.
1- If romantic novels or poems call for social reform, express noble feelings and call for love among people and the like, it is then permissible to read such novels.
2- It is not permissible to read romantic novels if they incite the reader to immorality or contain anything that contradicts the Islamic creed.
Almighty Allah knows best.
Source: Dar al-Ifta al-Misriyyah
Comment: I understand that I should not read it if it incite me to do something immoral and I know that I can control myself,in that case would It be permissible to read


What do I do in Technic/Tekkit?

Question: As you may have noticed, I have always had a fascination with Technic and Tekkit. I started reading up on it, but got lost very fast since there are many mods to read up on, and many things to cross-reference and so on. Therefore, I have to ask: is there a tutorial for technic/tekkit or like some kind of beginners' guide?
Comment: Hope this is more of what you were looking for, sorry I didnt add this in the beginning didn't realize you wanted the extra fluff
Answer: Of course, mods like this would be near impossible to use without some sort of guide.
For starters here is a Youtube channel that has some pretty good videos all about the mod.
Technical Minecraft At Its Best W/ Odedex
In Odedex's Technical Minecraft playlist he starts of with a brand new game using the mod. He touches everything in the mod in a fun to watch series in an easy to understand pace.
If that doesn't do it for you here is a Wiki page all about getting started with Tekkit/Technic
Wiki Starting Guide
The Getting Started wiki guide does a really good job helping you get started with incredibly important aspects of the mod such as:

New Ores to mine for like Copper and tin, which will be the foundation of a large quantity of the new recipes.
An important tree to locate, known as the Rubber Tree and how to gather Rubber from it.
Basics of making Electricity w/ Generators and Batteries, which will help you get started in making more advanced machinery.
Some of the Basic Machines, like the Extractor that allows you to double the output of certain items like Sticky Resin and the Macerator, which will increase the output of the ores you mine by double.
Very importantly is how it touches renewable natural energy from Windmills and Solar Panels so you can use all your machines without burning through gathered resources.

Also here is a Wiki page that has a link to every guide available on the Wiki.
Tutorials
I'd like to point out a couple of them like:
Engine Cooling
Diamond Maker
This Mod is so vast and amazing, you'll never wanna go back to Vanilla. I recommend this to every avid Minecraft player as its a whole new game to anyone who enjoys Minecraft as much as I have.
Comment: It would be nice if you state what is in the links in case the website ever goes down, but this is good too. Thanks!
Comment: Ill add more no worries
Answer: I have recently started playing Tekkit.
I figure starting off with your average vanilla Minecraft helps a lot but as soon as you get to mining these awkward ores it gets pretty tricky but what I do is make sure I have NEI (not enough items) on and as soon as I get an ore I search it up in there but it can always helps to make a creative world first just to see what everything does.

What do I do in Technic/Tekkit?


Is there any physical reason behind the choice of Lie group in a Yang-Mills theory?

Question: A Yang-Mills theory can be constructed for any Lie group that is compact and semisimple. The motivation behind this is discussed in this question. Is there any physical reason we choose $SU(3)$ or $U(1)$ for QCD and QED respectively? I am still learning about Yang-Mills theory, but so far I get the impression that these groups are chosen either because they're relatively simple compared to other groups and because the math just works out. Is there any physical reason for choosing these groups over others?
Comment: Behaviour of $\beta function$: Asymptotic freedom of QCD (nonabelian gauge group $\rm SU(3)_c$) vs asymptotic slavery of QED (abelian gauge group $\rm U(1)$). You find this in all good text books on particle physics.
Comment: @Hyperon Isn't the $\beta$ function behavior a consequence of the choice of Lie group and not the other way around?
Comment: The choice of a Lie group is related to local gauge invariance. For QED, it was already known in classical electrodynamics than gauge invariance was isomorphic to $U(1)$. For QCD I'll let others anwer, but one possible clue is the number of generators: $SU(N)$ has $N^2-1$ generators, which helps select an appropriate Lie group.
Comment: @CBBAM See my answer explaing the history of the physical motivation for using a nonabelian gauge theory for the description of strong interactions.
Answer: Well let me say first that the "physical reason" is that the predictions of QED and QCD match hundreds and hundreds of detailed experiments fantastically well. But of course just a link to the body of scientific output of the LHC, or SLAC, or the measurement of $(g-2)_e$ is not a particularly insightful answer. I assume what you are looking for is a simple empirical reason distinguishing QFTs based on these Lie groups from others.
For QED perhaps the most basic answer is that the photon does not self-interact, and so the Lie group must be Abelian. Then $U(1)$ is the only compact choice.
Of course historically $U(1)$ was 'chosen' first in classical electrodynamics. Then maybe your physical answer would be that electric charge was discovered to be one-dimensional, given by some integer multiple of $e$ at long distances.
Along these lines, perhaps the most basic data to pin down QCD from the point of view of Lie groups is the dimensions of the representations. For the quarks, I think historically this information could first be gleaned from measurements of the 'R-ratio',
$$
R \equiv \frac{\sigma(e^+ e^- \rightarrow \text{hadrons})}{\sigma(e^+ e^- \rightarrow \mu^+ \mu^-)},
$$
the ratio of the cross-sections for electron-positron annihilation to produce hadrons vs. a muon-antimuon pair.  Of course we can't see the quarks directly, only the colorless bound states they combine into (the hadrons). Physically, this is taking place through an $s$-channel photon exchange, a QED process. Then the color quantum number of the quarks is just a bystander here, and one must sum over all the colors of quarks. Finding that the data fit a factor of $N_c=3$ then tells us the dimension of the representation of the quarks. Here's a 1974 review from Burton Richter, but the experimental situation at that point was still confusing, because we have multiple light quarks and hadronic physics is messy.
For gluons, there must be a similar statement in terms of the production cross-section for three-jet events, in which gluons were originally experimentally discovered as recalled in this nice historical manuscript by Stella and Meyer. Certainly there you are also summing over the 8 possible internal orientations of the gluons, though to my knowledge there isn't quite as clean a quantity to point to in this case as the R-ratio.
So given these empirical determinations of the dimension of the adjoint (8) and of the matter fields (3), this may be enough information to point to $G=SU(3)$ Yang-Mills, at least if you assume $G$ should be a simple group.
Of course the real, historical answer is not Lie-group-theoretic, and it's not just these dimensions that particle physicists collectively measured in the 1970s but tons of differential cross-sections and decay rates and energy splittings etc. etc. that are all consistent with the quantum field theoretic predictions of QCD.
But I agree that it's an interesting question and point of view to take, and I regret not having a sharper answer for you, or knowing where one exists to point you to. It would be great to read a review from one of the early QCD theorists if they were thinking seriously about, for example, $Sp$ or $SO$ groups; if I find one I'll update this answer.
Comment: Thank you, this was exactly the kind of answer I was looking for!
Comment: Of course any number of copies of $U(1)$, i.e. $U(1)\times \dots \times U(1)$, is abelian and compact
Comment: You're right, the fact that we only have one photon is also needed there.
Answer: Experiments on deep inelastic scattering performed at the end of the 1960's and early 1970's suggested that quarks confined in hadrons were behaving essentially like free particles at small distances (i.e. large energies). This was a big challenge for a possible quantum field theoretic description of the strong interaction, as at that time all known QFTs had turned out having positive $\beta$ functions, meaning that the corresponding interaction becomes strong at small distances. In 1973, Frank Wilczek, David Gross and David Politzer computed the $\beta$ function of a nonabelian  gauge theory finding that it was negative. This came as big surprise at that time, although earlier remarks by Gerard 't Hooft in 1972 and a paper by Iosif Khriplovich in 1969 had already pointed in this direction. This surprising result was (among others) one of the main arguments for the introduction of QCD based on the nonabelian gauge group $\rm SU(3)_c$ as the theory of strong interactions and at the same time the "rehabilitaion" of quantum field theory.
Comment: Thanks, this partly answers my question but I'm still not clear on why SU$(3)$ was seen as a potential solution to the asymptotic freedom problem before any attempts to compute its $\beta$ function. Was it just a guess that turned out to be right or was there a deeper reason that led Wilczek, Gross, and Politzer to compute its $\beta$ function?
Comment: @CBBAM The task to compute of the $\beta$ function of a (pure) Yang Mills theory was given to Frank Wilczek as a thesis work by his supervisor David Gross and, at the same time, to David Politzer by Sidney Coleman. As the work of Khriplovich and 't Hoofts remark was appeaently unknown to them, this problem was supposed to complete the list of beta functions of all types of quantum field theories. The expectation was $\beta >0$, as it had been in all previous cases and the result $\beta <0$ come out as a big surprise.
Comment: @CBBAM This in turn resuscitated the attempt to find a quantum field theoretic description of the strong interaction that had already been given up for some time with (nowadays completly forgotten) alternate approaches advocated by prominent physicists of that time. The new result for the Yang-Mills beta function paved  the way for the formulation of QCD based on the gauge group SU(3) as a common effort by Heinrich Leutwyler, Harald Fritzsch and Murray Gell-Mann. Details can be found e.g. in the highly recommendes book "Particles, Fields, Quantum" by Gerhad Ecker (Springer Nature 2019).


javafx-2 ComboBox, undocumented feature: ComboBox maintains a value setted via setValue(T object) even if it is not in the items collection

Question: javafx-2 ComboBox seems to have an (yet) undocumented feature that suggest a whole new combo box concept behind the scenes.
Simply put, the value property is not constrainted to be one of the elements of the items collection.
While the user can only choose one of the items, the setValue method allows you to set a value even if it does not belong to the collection (provided that it is of the correct type, of course, but this is enforced by generics).
In the same way, if the valueProperty is bound to some other property, valueProperty will follow the value of the other property whether it is in the items property or not.
This may be useful in several circumstances, such as when a user chooses an item that is subsequently removed from the items collection.
Since this feature is not documented (or i couldn't been able to find documentation), i wonder if I can rely on it, or it has to be consider some sort of "side-effect" and in so it might be removed in future releases of the framework.
SSCE (just fix imports):
<code>public class T08 extends Application {
@Override public void start(Stage primaryStage) throws Exception {
    ComboBox<String> c = new ComboBox(FXCollections.observableArrayList("item-a", "item-b"));
    primaryStage.setScene(new Scene(c));
    c.setValue("outsider-item");
    primaryStage.show();
}
}
</code>
The output is as following. As you can see, the combo displays the "outsider-item" value, but then it is not proposed in the list.
The getValue() method is coherent with the value displayed by the component.
Answer: This behavior is known,
I've filed an issue http://javafx-jira.kenai.com/browse/RT-27543
to describe this behavior in javadoc.
It is a fixed and tested behavior, and not expected to change.


How Prototypal inheritance works in reverse order

Question: As you can see below is my code. I have created two objects halogen and balloon and given halogen property of sing and balloon property of read. I am calling halogen.read() it is getting read property but halogen is prototype for balloon but balloon is not prototype of halogen. How js is working under the hood??

const halogen = {
  sing: function() {
    console.log('I can sing');
  }
}

const balloon = new Object(halogen);

balloon.read = function() {
  console.log('I can read');
}

halogen.read();
Comment: The `new Object` call basically doesn't do anything, it's the same as `balloon = halogen`. There's no prototype involved.
Answer: When you call the Object constructor with an argument that isn't null or undefined, it doesn't create a new object (not even when you use new, surprisingly; spec link), it just converts the argument to object if it isn't already an object and returns it. Since halogen already refers to an object, there's no conversion required and you're basically doing balloon = halogen and they both end up referring to the same object.
You probably meant to use Object.create, which creates a new object and assigns the object argument you provide as the new object's prototype:

const halogen = {
    sing: function() {
        console.log("I can sing");
    },
};

const balloon = Object.create(halogen);

balloon.read = function() {
    console.log("I can read");
};

console.log(typeof halogen.read); // undefined
console.log(typeof balloon.read); // function
console.log(typeof balloon.sing); // function


Writing a Sci-Fi novel

Question: I'm trying to write a Sci-Fi novel in one year and I'm wondering how many words I should write per week. I'm shooting for a total of ~90,000 words by the end of the year.
In terms of when I can write I'll say no Sundays, not till after 4pm on weekdays and all day from July through September.
This will be my first novel and longest written work.
Comment: Does this answer your question? [1000 words a day for a part-time writer](https://writing.stackexchange.com/questions/41041/1000-words-a-day-for-a-part-time-writer)
Comment: @Pureferret Yes, it answers my original question. Thank you.
Comment: Hi @PixelDud - welcome to Writing SE! AS you've stated the question linked by Pureferret answers your question I've gone ahead and marked this one as a duplicate. Glad you're finding the info you need and hope you stick around!
Comment: @motosubatsu Thanks for the welcome to the community! I plan to stick around for quite a while.
Answer: It is not that simple.  Do you want a very rough draft or a finished novel?
And it will depend on your experience and your process. So what is your writing background and knowledge of methods to do a novel?
Writing will not be able to be predicted in exact amounts as  'X' words/week.
It is much harder to write all day than for an hour or two at a time.
And more so  when days in some weeks will be 4-5x as long than others considering the actual time available to write.
Abe Lincoln said that if he had an hour to cut down a tree he would spend 50 minutes sharpening his axe.
So how much time will  you spend first on planning so you will not waste every so much very more time later with rewrites and detours down dead ends that you throw away?  Do you have the ability to NOT edit while you go,  but do that last?  If not then plan to write a lot more words that end up in the trashcan.
If this is your first novel, I would suggest that you first plan it carefully so you know where the story goes and how it ends.  Then be satisfied with finishing a decent novel at all whether you can do it in 6 months or two years.  There are many writers who never finish a novel at all.  There are many more  bottom drawers full of unfinished mss by pantsers as well as too many by planners who gave up.  Can you stick it out and not quit?
Anyway,  to answer the question in the OP you will need, in my estimation as a paid professional writer, to write as many as 7000 to 15,000 words a week, on the average to end up with a 90k novel in a year, if you do it like so many new writers attempt;  down to a low of 3500/wk if  you are experienced and actually do it in only 6 months, or you could do it in 12 months with doing less than 2000 words/week.
Context:  I easily write 1500 words , or more, at one sitting -- IF  I have an idea what I need to say.  As most of my published writing is relatively short, it does not need nearly as much planning or preparation as a long novel.
If I were to try a novel I would do sufficient planning so I know at least the way points on the journey from start to finish and then expect to write 3000-6000 words/week without any stress.  To finish in a year I would expect to spend about 8-9 months planning, and then write for 3-4 months ending up with a good clean first draft ready for editing.
Comment: dont know how long you have been planning but if it is just a week or so then you really need to do the all the development editing and be sure you have the roadmap laid out like @DWKraus implied so you wont waste time fixing things you had written down in detail but need to be discarded and redone.    if you are doing a series  then are you going to plan out a long arc or just have stand alone novels in a series. but when it comes to wordsmithing and SPAG you will save a lot of time by waiting to do that when you finished the rough draft.
Comment: I'm working on the plot and plan to finish it by the end of this week. My "drafts" are usually not far off from the final, so let's say it's a draft. I intend for this novel to be the first in a series though I'm not sure if a stand-alone novel would be better. This will be my worst novel. When I'm in the mood I can pump out a few thousand words, I do some slight edits here and there.
Comment: dont know how long you have been planning but if it is just a week or so then you really need to do the development editing and be sure you have the roadmap laid out like @DWKraus implied so you wont waste time fixing things you had written down in detail but need to be discarded and redone.if you are doing a series
Comment: I've been planning to write the book for a while, I have the main character and information about the fictional universe where it takes place. It is possible that my planning will extend past this week, I just hoped to finish then. Now, having received input from others, I will likely spend an extended time planning and then write for set amounts of time every day.
Answer: BEWARE THE PITFALLS:
Unless you need the deadline for motivation, I wouldn't be so rigid. Figure out how much time you can spare from your life, then cut that in half, and do that. Set writing time goals that are achievable and sustainable. Time is constant, but productivity (in the form of words) can be extremely variable depending on research, revisions, etc. You will resent your writing if you spend all your free time doing it. I personally was using writing as a kind-of therapy to work through the death of my parents (providing motivation), but I find it harder to give up free time now that I accomplished my goal.
Explain your goals to the important people in your life and get their buy-in. You won't get far if your (parent/husband/kids) are angry and resentful of your time demands. Most of those people love you and will understand, IF they get buy-in first. If I had followed my own advice, I would have had a happier family.
DON'T put off revisions (IMOHO). You will need to change the story as plot elements shift (especially if you don't have a perfectly fixed vision of the story). Those revisions affect the WHOLE story, and you will need to do ten times the amount of work rewriting ALL your story just because the events in chapter 3 changed and affect the rest of the novel. OR you will be unwilling to make the changes needed afterwards because of the amount of work involved to fix them.
Otherwise I think it's quite achievable. I did 127K words to beta-reader-ready state in about 13 months for my first novel while working full time.
Comment: Thanks for the response. This makes more sense for me, I'm a student and as such have limited time. I've already shared my goal with my family and they like it.
Comment: @PixelDud  Welcome to writing! Upvote answers you like (you can upvote more than one) and accept one answer as the best (but wait at least 24 hours to give a chance for people around the world to give their responses).

Writing a Sci-Fi novel


Regional pricing within a country

Question: I have a client that requires their website to have regional pricing, but within the same country.
They service their web orders from their network of physical stores (some franchise and some not) and, as each store can have a different price for an item, these prices need to be reflected on their website.
Ideally, when a customer first arrives they are shown the pricing for the physical store closest to them.
From my understanding though, the only way I can show different prices like this is by creating different websites under the Magento Website > Store > Store View hierarchy, which will not work as I would like to have only the one product catalog.
Is there any other way to accomplish this or is this a lost cause and I should just create the different websites?
Answer: Closest thing you can achieve with vanilla Magento is to use website price scope instead of global price scope. Yes, it requires you to create bunch of different website level stores, but this feature is available out of box.
Another possible approach is to use multi-warehouse extension (can't link it here as this is commercial stuff). As an additional bonus you get multi warehouse (doh!) support which considering your problem description might be something you need anyway.
Comment: Hadn't thought of the multi-warehouse, thanks I'll have a look into this
Answer: The websites can all link to the same products, that is not a problem, however you have a larger problem. Once you start getting to 10s of stores Magento slows to a crawl due to the way it works internally. Once you get to 100s of stores (very few have done this) you need very special hosting, 2GB+ PHP threads and such.
There is a company we know that can do this as realistic cost, for example the Nexcess clusters are $1,500/mth upwards. Specialist Magento hosts will not get very far, and anything else is a no-go.
There are other options but that would require going in to the business requirements as to whether they are suitable, this is not the forum for that and we still can't find where to point people to. The same people also developed special dynamic pricing which sounds like what you are trying to do, whether it would work for your situation is another matter.
Comment: Dynamic pricing is an option, I'll Google it and see what I can find

Regional pricing within a country


Disable user and change user password using SSH

Question: How do I disable and change the user password using SSH on a Linux
Answer: Login with the root account, and type: passwd otheraccount hit enter. replace otheraccount with the account name of the user you want to have the password changed.
Answer: You don't. You perform the actions on the underlying system user via passwd.
Comment: right, once I login as admin, how do I go about doing it?
Comment: With the `passwd` command. See the `passwd(1)` man page for details.
Comment: not for the user login but for an alternate user
Comment: As the man page says, you can specify an alternate user for `passwd` to use.
Answer: You have a few options, somewhat dependent on what you mean by "disable". For the purposes of this explanation I will use the string foo as a placeholder for the account name in question.
You cannot disable an account, per se, without deleting it wholesale, however, deactivating the account password passwd --inactive 0 foo will effect a similar functional change.
The closest to what I believe to be your objective would be to lock the account, passwd -l foo which changes the password to an unknown, encrypted value.
Another option, although not immediately apparent perhaps, is to delete the account. This will likely be performed with the userdel foo command, depending on your particular operating system. userdel will, by default, not delete any files owned by the user but will remove the account itself from the system account files. Be careful when exercising this option, however, as it can adversely affect cron execution and other automated processes if execution normally occurs as the deleted user.


Security certificate only displays with HTTPS and not HTTP

Question: I have a security certificate installed on my Ubuntu server, running Apache.  When I visit my site using the HTTPS protocol, the green certificate appears at the start of the URL.  However, when I use the regular HTTP protocol, the certificate does not appear.  This used to be working, but I must have changed something to break it and I don't know how to get it working again.  Any ideas?  Thanks
Answer: Er, that's the way it always works. If you are actually looking at a website via HTTP, you're very specifically not using HTTPS, and thus not using the certificate.
You might be thinking of HTTP->HTTPS redirection; you can configure a webserver to redirect all HTTP requests automatically to HTTPS. Even though you type in HTTP, when the page finishes loading, you will see that it is now actually using HTTPS.
So, figure out how to fix HTTPS redirect on your server. You can do this more than one way. A common way is with mod_rewrite, which might help you figure this out. If you were doing it some other way, you'll need to do some more research before we can help you.
Comment: Ah yes, this makes sense.  I seem to recall that it was working previously with mod_rewrite.  Can you point me in the right direction for where I need to look for this stuff?
Comment: I'd suggest kindly starting with reading the documentation, then reviewing your config files for relevant portions. Once you've found something, make a change and test it. If you are puzzled by the results, then edit that into your question so that you can get a technical answer. Right now, you're asking us to troubleshoot this totally blind.
Answer: I managed to solve my problem by adding the following lines to the .htaccess file:
RewriteEngine On
RewriteCond %{HTTPS} off
RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI}


System calls supported in running Kernel

Question: Is there a way to get the number of or list of system calls supported by currently running Linux Kernel? So I want to find a way to 'read' the syscall table of a running kernel.
Answer: The file /proc/kallsyms lists all the symbols of the running kernel. By convention, system calls have a name that begin with sys_. On a 64-bit system, system calls for 32-bit programs have a name that begin with sys32_. Strictly speaking, this lists internal kernel functions, not system call, but I think that the correspondence does work (every system call invokes an internal kernel function to do the job, and I think the name is always the name of the system call with sys_ prepended).
</proc/kallsyms sed -n 's/.* sys_//p'

This is usually not useful information, because system calls change very slowly. Optional components provide functionality in terms of existing system calls, using general features such as devices (with ioctl when read and write doesn't cut it), filesystems, sockets, etc. Determining the list of supported syscalls won't tell you anything about the features that the system supports. Other internal function names won't help either because these change very quickly: the name of the function that implements some feature on one kernel version may change on the next version.
Comment: @JohnWHSmith "Can be manipulated like any other file"… with the caveat that on systems with kernel ASLR, this file should be readable only by root.
Comment: +1. Now that's what I meant when I said *"I'll let someone with more experience than me answer you"*. Additionally, since `/proc/kallsyms` can be manipulated like any other file, it becomes quite easy to use it in a program.
Answer: TL;DR
I kept finding new alternatives when writing this answer, so I just wrote a bit of details about each of them, and made some stats. Basically, you can either:

Read Gilles' answer, which provides a clean and fast way to do it (relies on /proc).
Use the documentation resources.
Use your system's C header files.
Use the kernel source code itself.
Use the /sys directory.

After doing the math, I'd recommend (among my alternatives) using the /sys filesystem, since it seems to give the best result in terms of number of system calls. You may jump right to that section if you don't want to read about the other tricks.
Using the documentation resources
While you might miss some of them, you can use apropos to list all manpages belonging to section 2 (system calls) :
$ apropos -s2 . | awk '{print $1}' | column

Remove column if you don't want fancy columnised output.
I just found it out, but there is a Linux man page about system calls, and you'll be able to find most of them in it.
$ man syscalls

I also came across these two web sites which could be interesting:

System call table
Linux/i386 system calls

Using headers files
Edit : Now, when it comes to programatically (or at least, without relying on documented features) determining what system calls are available, I'm afraid the kernel does not keep a table of its system calls, at least not under the form of a list of strings (as you'd probably expect to manipulate them). At this level, we're talking more about functions addresses and pointers, rather than function names.
I just browsed my /usr/include directory and grep-ed a few things: you might find the following directories interesting. Some of them may be different on your machine, depending on your architecture and distribution, but I'm sure you'll be able to adapt them.

/usr/include/linux
/usr/include/x86_64-linux-gnu
/usr/include/sys
/usr/include/asm-generic

By looking for function definitions in this file, you'll come across many system calls, even though they will not be fully defined in there. I ran a few greps in these directories and I was able to find mentions of some system calls. Here's an example:
$ grep 'sys_exit' /usr/include -R
asm-generic/unistd.h:__SYSCALL(__NR_exit, sys_exit)

So, I'm guessing another way to find some of them would be:
$ egrep '^__SYSCALL' /usr/include -Rh | awk '{print $2}' | tr -d ')'

Using the kernel's source code and its syscall table
Another solution is to use the kernel source code itself (and not just the headers!), and find a way to search it efficiently. Since kernel commit 303395ac3bf3e2cb488435537d416bc840438fcb, you might find this a little easier than before. Here is an example for 3.13 (which is my kernel):
$ wget https://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/plain/arch/x86/syscalls/syscall_64.tbl?id=refs/tags/v3.13 -O syscall_64.tbl

Now that you got the actual syscalls table, just browse it:
$ while read line; do awk '! /#/ {print $3}'; done < syscall_64.tbl

You could find a way, using uname and arch, to download the tbl file straight from git.kernel.org, based on your running kernel version and architecture.
Using the /sys filesystem
Gilles' answer gave me a little bit of inspiration, and you may find those system calls inside /sys/kernel/debug/tracing/events/syscalls. This directory is used to monitor the use of each system call on the system. Each syscall has two directories in it:

sys_enter_[syscall]
sys_exit_[syscall]

Therefore, using ls, grep and cut...
$ ls /sys/kernel/debug/tracing/events/syscalls | grep 'sys_enter' | cut -d'_' -f3

Stats
On my system:

Using the man pages revealed 440 system calls.
grep-ing for __SYSCALL in the header files revealed 212 system calls.
Reading the syscalls table from the kernel sources revealed 346 system calls.
Using /sys revealed 290 system calls.

Now, if I bring everything together...
$ apropos -s2 . | awk '{print $1}' > system_calls.txt
$ egrep '^__SYSCALL' /usr/include -Rh | awk '{print $2}' | tr -d ')' >> system_calls.txt
$ while read line; do awk '! /#/ {print $3}'; done < syscall_64.tbl >> system_calls.txt
$ ls /sys/kernel/debug/tracing/events/syscalls | grep 'sys_enter' | cut -d'_' -f3 >> system_calls.txt

$ sort < system_calls.txt | uniq | wc -l
707

There we go, 707 system calls! Of course, this number reflects a very flexible definition of a "system call", since 3.13 is supposed to provide 274 system calls only (reading /sys seems to be the closest solution).
Comment: If you system call isn't found, then you did not implement it correctly. My answer tells you how to find Linux's system calls, but not how to debug your own (since it's not what you're asking for). If you're having trouble developing it, you should ask a question specifically about it, and avoid [the XY problem](http://meta.stackexchange.com/questions/66377/what-is-the-xy-problem).
Comment: I am looking for a way more in terms of 'reading' the system call table in some way rather than finding out which of the system calls are documented in man pages
Comment: I don't think the kernel keeps a list of its syscalls, at least not as a list of strings. I edited my answer. If there's an actual way to do this, I'll let someone with more experience than me answer you ;)
Comment: so I was wondering about this after I added a system call to the kernel and trying to use it was giving "function not implemented", and I wondered if there was some way to get the syscall table for current kernel. when I do '#make install', update grub and start a new kernel, at what step does the new kernel get relevant include files which contains the new system call?
Comment: @swair It is highly unusual to add functionality by adding a system call. We can't tell for sure what's wrong since you didn't provide any code (and if your question requires C code, it's off-topic here, but at home on [so]). I suspect that you implemented a system call (correctly or not) and that you're trying to use it from a C program, and that you're missing the step of writing a C function that makes the system call. A system call is not an ordinary function call.
Comment: @Gilles, I did figure out the issue with my implementation but that made me wonder if there was a way to read the syscall table of running kernel.
Answer: All answers are fine.
If you are looking for a specific System Call name :
$ cat /proc/kallsyms | grep <sys_call_name>

If you are looking for list of all System Calls :
$ cat /proc/kallsyms


If I give my wife an item I intend for her to use, how can I be sure she won't sell it?

Question: I married Lydia and since she's also a follower, I occasionally give her weapons and armor for use when we're adventuring. Does the game keep track of the difference between things I sell her (I.e. "What have you got for sale?" Option) vs things I place in her inventory? ("I need to trade some items with you.") When I put items in her inventory, I noticed some showed up for sale in her store, but when I sold her items, they simply disappeared.
Answer: From the UESP Marriage article.

You don't need to worry about losing items by placing them in your spouse's inventory because they sold them to someone. Like all merchants in the game, they will actually only sell things to you, and not other people.
Answer: Well, since you are the only actor in the game capable of trading with a merchant, the best way to make sure she doesn't sell the items you want her to keep is to not buy them.


UIScrollView Custom ViewController

Question: I have an odd situation. I want user to choose a category by clicking an button inside UIScrollView. Here's the relevant code of page view controller (the one containing UIScrollView made with IB):
<code>- (void)viewDidLoad
{
    [super viewDidLoad];

    uiScroll.contentSize = CGSizeMake([[SawData shared].categories count]*320,480);
    int i = 0;
    for (NSString* key in [SawData shared].categories) {
        Baton *b = [[Baton alloc] initWithNibName:@"Baton" bundle:nil];
        b.view.frame = CGRectMake(i++*320, 0, 320, 480);
        [uiScroll addSubview:b.view];
        [b release];
    }
}
</code>
Baton is new UIViewController subclass containing only one button with touchUp action that prints some log info using NSLog. With this code I get the Batons properly arranged side by side. Problem is that clicking the button gives EXC_BAD_ACCESS as Batons view controller is released.
If i remove the [b release]; line I have Batons overlayed one over other as i++ * 320 is giving same result (which i checked and its correct). In this case click on button is working.
Thank you for help!
iOS SDK 4.3
EDIT 1:
After much trial and error, I have managed to find out whats happening. Its 320px width of controls. If i changed them to 321 or 319 all works as it should. I dont need to release the controller to image fill the UIScrollView content AND clicking the button works. What the hell is this?
Answer: This might be the reason.
<code>Baton *b = [[Baton alloc] initWithNibName:@"Baton" bundle:nil] autorelease];
</code>
You should also make sure that the UIButton is initialized before you click on it. You can do this easily with the debugger.
Take a look here
Does UIView's addSubview really retain the view?
Comment: I am sorry but this didn't help.

UIScrollView Custom ViewController


Does a hot water dispenser heat water by itself?

Question: Does such a hot water dispenser heat water by itself? https://www.grainger.com/product/IN-SINK-ERATOR-0-7-gal-Hot-Water-Dispenser-23NU44. I saw one that looks like the one in the link was installed near a regular tap water faucet. I am not sure how it gets water, if it heats the water by itself, and whether it makes water more clean or safer to drink than tap water. Thanks.
Comment: It probably tees off one of the water lines and heats the hot water to very hot(boiling temps).  Does not seem to have a water filter.  Do to the high water temperatures it would be dangerous for kids/people not careful. It will burn skin, not good for washing hands.
Comment: Same tap-water, more electricity wasted keeping a relatively-less-well-insulated container (.vs standard water heater) scalding hot all the time. More efficient to put a mug of water in the microwave or use a kettle only when you actually need scalding hot water (for tea, etc.), but you might have to wait a whole minute or two...they fail regularly from mineral buildup (and have no practical means of cleaning that out) from what I've seen in places that installed them.
Comment: If you're in the first world and worried about water safety, that's usually about chlorides, chloramines and fluoride put in the water to kill bugs in it.  For that you need filters, which can be gotten that attach to a tap like this.
Comment: There are various installation configurations that can minimize or eliminate mineral contamination:  An ordinary inline filter with a filtering element that you'll have to change every couple of years, or an RO filter (we use it to eliminate the salt after our water goes thru the water softener).   Regarding energy efficiency that @Ecnerwal mentioned... the one I have is fairly well insulated and saves running the hot water tap until it gets hot, saving both water and the energy required to heat it.   ....continued below....posting this as an comment bc it's not really an answer. .....
Comment: I find the instant hot water dispenser EXTREMELY convenient and if it costs me a few dimes a month in power, I'm fine with that. Also, you have to consider the delta change between using a micro wave or kettle vs. the instant hot.  Those also take energy.   Next: When you are fixing breakfast for 3 kids and they all want hot chocolate and the wife wants her tea, the instant hot is great.  It would take quite a bit of time in the microwave to heat up 4 cups of water.   I'm willing to pay just a bit extra to make my life easier.
Answer: This type of device has a small storage tank. It heats water in the tank to around 200 F. Where a typical whole house water heater has 30 to 50 gallons, this has a 0.7 gallon tank under the sink. Because it is at point of use, it is "instant". But to keep installation costs low, it has limited capacity. Perfect for quick hot water for a beverage and that's the main use. Not a substitute for hot water to wash the dishes. Too hot to wash your hands.
Answer: The makers website has a photo that includes the under-bench component that heats the water.
https://insinkerator.emerson.com/en-us/shop/insinkerator/insinkerator-sku-44719

Usually when water needs to be boiled to make it safe you should actually boil it, this device only almost boils it.
A filter is probably a better way to make water safe.