In predicate logic, is the "environment" only needed when free variables are present?

Question: Suppose there exist no free variables in a given predicate logic formula. Is then a model alone sufficient to fully interpret the formula and make inferences? Don't we need an environment or variable lookup table (as often mentioned)?
Answer: It is not so that the model alone allows you to make inferences and that you don't need an environment at all; it is rather that, if no free variables occur, then you may as well start with an empty environment. Quantifiers modify the current environment, so you do need one. However, if there are no free variables around, then any of the assignments in the initial environment will not be used, and all variable values will come from the quantifiers anyway. Therefore, by trimming unnecessary assignments in the initial environment, you get an empty one.
Comment: How do "quantifiers modify the current environment"?
Comment: Quantifiers bind variables to values.
Answer: The wikipedia article First-order logic calls the "environment" a variable assignment. A formula with no free variables is called a sentence. A model alone is sufficient to fully interpret a sentence. However, all deduction systems for first order logic that I know need formulas with free variables as intermediary deductions, and hence need the concept of a variable assignment for an interpretation of their inferences.
In general, this seems to be necessary. Some specific first-order theories (like Presburger arithmetic) allow quantifier elimination, but all theories that I know which allow quantifier elimination are decidable. So my guess would be that any deduction system for an undecidable first-order theory will need to work with free variables (or some equivalent concept) and hence will need the concept of a variable assignment (or some equivalent concept) for an interpretation of their inferences.
To see that the issue is related to the deductions mostly and not to interpretation, one can look at alternative semantics like game semantics.


When should I spread wildflower seeds in tall grass with a broadcast spreader?

Question: I've been trying to get some wildflower seeds to grow on the edge of my yard for a while, and am wondering when the best time to spread the seed kit is, and how I should do it. Think about broadcast spreading in a pasture.
Answer: For seeds to germinate they need firm contact with the soil and sunlight and frequent regular shallow watering until established.  As well as low nitrogen fertilizer.
I would cut that grass with a line trimmer first.  Grasses love to be mowed or eaten down.  Not too far but regularly.  Tall grass is wonderful once everything is growing vigorously.  You have to chop that grass down first.  Rake up the clippings, they do not have any fertilizer you should be interested in...allow sun to reach patches of your soil among the grasses where you are able to germinate wildflower seed.
Lightly rake bare soil patches.  Sow your seed by hand, tamp the soil down with your hands to firm over the seed.  Water those patches by hand for the next 2 weeks.  By the time your wildflowers start growing vigorously, your tall grasses will not be so tall as to hide the flowers nor shade the flowering plants.
By the end of the season you should be able to mow, weed wack the entire patch down, grasses and wildflowers.  This time, do not bag your clippings because there will be seeds you want to have back into your soil.  Your patches of wildflowers.
In the spring, make sure you fertilize with a formula of even numbers.  Remember to lightly water those patches to get the seeds germinated and established once again.
Patches of wild flowers look far more natural and breath taking done this way.  If you spread wildflower seed now in your tall grass you are asking too much of the wild flower seed.
Have fun, let us know how it turns out...please send pictures of before and later.
Comment: so under about 6 inches to 1 foot of snow should be fine for desired results?
Comment: In the UK, a common reason why wild flowers die out is because growing conditions have been improved so that are too good, not too bad. They tend to do best when they are on the edge of survival but competitor plants are in an even worse state. The best *natural* flower displays in my part of the UK tend to be on 30 degree slopes cooked by full sun, with about two inches of cruddy soil on top of solid limestone hundreds of feet thick, not well fertilized and carefully tended grassland.
Comment: Yeah, well I think that someone wasn't paying attention to what it was that helped the wildflowers...it most certainly wasn't planting them in the worst conditions.  That is a sad bit of propaganda Aleph!  Plants, as I say an awful lot are too good at surviving to be teaching us humans plant husbandry!  Natural versus artificial displays,?  Grins.  Every time we humans decide to throw some seed around imaging flower display without our intervention is just ...well, lazy. Gee, gag me I know, but I've created flower displays, gorgeous pot gardens of flowers and Wild flowers are just like any
Comment: ...other plant planted by seed.  To imagine brutalizing their environment and expecting even better display is seriously going to kill me!  Natural flower displays are NOT meant to impress humans.  They just happen because the plants want to procreate.  I've never seen a natural flower display that could awe a crowd like my installation and maintenance flower beds.  Especially when they see me cutting all the flowers off !


Why didn't my forsythia cuttings root?

Question: In May, I took some greenwood forsythia cuttings, dipped them in a hormone rooting powder and planted them in rooting mix. The mix was kept moist all year. I had thought they had rooted because they began to grow rapidly and the new shoots hardened off. In October the plants dropped their leaves, and by November they were dead. I dug them up and they had not rooted. There wasn't even a trace of root. What caused it to do this and how do I prevent it?
Comment: Oh - I have some Forsythia cuttings growing this year. I potted them last Autumn and overwintered in a cold frame. They appear to be growing well, but now I'm doubting! ;-) How do I check for root growth without disturbing them?
Comment: @MarkCooper How'd they turn out?
Comment: They budded, and started to grow, but when I tried to pot them on - no roots either. They didn't last after that.
Answer: I don't think the problem is forsythia specific - I've had the same experience with peach cuttings. The first time I tried it I took the cuttings at the wrong time of year (ie. a different unrelated problem), but the second time I took them at the right time and I had one success out of about half a dozen cuttings.
All of the failures in the second batch showed some leaves. Many were either too small (possible a problem specific to peaches where a cutting should be a minimum size); or with the larger cuttings when I dug them up, any roots had rotted away and the stem was beginning to rot. I believe this is your problem. The rot (or mold - rot is after all usually a kind of mold) is due to too much moisture. In your case you describe the mix as being moist all year - so it was probably too moist.
In my case I probably had more variability (outside, Texas, hand watering) - so it was probably too moist at a critical time.

Why didn't my forsythia cuttings root?


What is the correct pronunciation of the word "route"?

Question: I have always used both "root" as in route 66 and "rooter" as in the networking device. The latter has gotten me funny looks often, however I could not bring myself to accept the inconsistency. Today I heard "rowt"  used for a path of movement by a radio presenter. Which is correct?
Comment: Comments are not for extended discussion; this conversation has been [moved to chat](https://chat.stackexchange.com/rooms/118882/discussion-on-question-by-jamesson-what-is-the-correct-pronunciation-of-the-word).
Answer: Both pronunciations are used in the US, but only root in the UK.
Comment: Be aware that *root* is /rʊt/ in much a America, so what you have there doesn't make much sense to such a person. You mean /ru:t/, which is entirely different from /rʊt/.
Answer: In my local dialect (Toronto, Canada), it is /ruːt/ for a roadway, and /raʊt/ (but that's very approximate; see Canadian raising ) for the act of specifying a path (and rowter for the computer networking device)
Comment: Exactly. As a verb, _root_ is to grow roots or to dig/search (as in to "root around" – what a pig does). _Rowt_ is the act of deciding a route (as in to "route around the enemy"), and so therefore a _rowter_ is a device which routes. I learned to speak in Northern Virginia, incidentally.
Comment: I probably should have said "route around the obstacle" or something, so as to avoid confusion with "rout", which of course is also pronounced _rowt_. Too late to edit...
Comment: Note I was referring to the networking device "router" above. Also, I would argue the woodworking tool is a _rowter_ for the same reason: it cuts channels, grooves, etc, which are aptly "routes".
Comment: Editing in IPA here doesn't work very well with how I originally wrote the comment. It should show the actual Canadian-raised dipthong and the corresponding pronunciation for "router", but I don't know how to write these. But yes, we do pronounce the networking device and the woodworking tool the same way here.
Answer: It's a question of dialects. In the UK, it is pronounced as a homonym to root, as already been addressed. In America, it seems that those that pronounce it as a homonym to root are more concentrated on the east coast, as can be seen in a map (link below) visualizing the results of the Harvard Dialect Survey.
In a study of American dialects (link below), Stephanie Nicole Hedges finds that the probability to pronounce "route" as rhyming with "out" is 0.5 in New England, New York, and the Mid-Atlantic States, while it is 0.8 elsewhere in the USA.
Sources

http://dialect.redlog.net/staticmaps/q_26.html
https://scholarsarchive.byu.edu/etd/6480/
Answer: US Midwest:  I pronounce the noun 'root' when it's used to name a road ("Route 66").  If I am asking what roads you will take to get to Chicago, however, I ask, "What 'rowt' will you be taking?"  And I might respond to a similar question by saying "The Triple-A 'rowted' me through La Crosse."
And the box that controls your internet traffic is a 'rowter'.
So 'root' (rhymes with "hoot") is used only as a "proper noun", and 'rowt' is used everywhere else.
(I suspect that the special treatment of named roads with "root" is due in large part to the TV show "Route 66" (1960-64), and the earlier (1946) song by the same name.  This pronunciation permeated US culture in the 50s and 60s.)
Comment: In my little part of America, your last paragraph is spot-on.  "Root" for the name of a road is *only* used for Route 66.  All other roads, e.g. Route 1 are pronounced "Rowt" 1.
Answer: In the UK, route is pronounced /ru:t/, rhyming with root. On the other hand, the pronunciation /raʊt/, rhyming with shout, is rout, meaning, among many other things, various kinds of gatherings of people (as a noun) and defeat (as a verb).
Comment: Note that *root* /rʊt/ rhymes with *foot* /fʊt/ in many places in America.
Answer: In my idiolect, a roadway is a "root," the communications device is a "rowter," and one "rowts" cables and things to where they need to go.
Seems like the pronunciation indicates whether you mean noun or verb.
Comment: Why would you say it is a *root*, when *root* is /rʊt/ for many of us?
Comment: This is what I don't understand. A network router is so called because it routes data packets between nodes on the network. If you have a "rowt" to work it is logical to refer to the network device as a "rowter". If you have a "root" to work I would have thought you would have a network "rooter" when you got there.
Comment: This is how it works in my head the noun rhymes with boot, the verb rhymes with out. @tchrist I've never heard someone pronounce root (like those of a tree or plant) in a way that rhymes with foot or put. I've only ever heard "root" in a way that rhymes with fruit, boot, shoot, or toot. What regions contain the "us" that pronounce it /rʊt/?
Comment: @McHobbes Oh it's quite common. My own idiolect originates in the rural (white) southern Wisconsin of more than half a century ago–it's also in Iowa—but in fact I noted it just yesterday on the news from a (black) speaker from the American South; I had a (white) dentist in Texas who had it even. I don't know a dialectology survey that includes this, unfortunately. It seems to be found in speakers who p**u**t /ʊ/ in all of *roof, hoof, foot, soot, root, look, book, hook, rook, wool, pull*. I have /u/ in all *shoot, boot, coot, coon, room, moon, loot, loon, loom, womb, tomb, lure, tour, pool*.
Answer: Today, the pronunciation of route as /raut/ (rhyming with out) is more prevalent in the USA.
But the two pronunciations are not equally distributed in the USA. In a study of American dialects, Stephanie Nicole Hedges found that the probability to pronounce route as rhyming with out is 0.5 in New England, New York, and the Mid-Atlantic States, while it is 0.8 elsewhere in the USA. This is reflected in the map from the Harvard Dialect Survey given in the answer by Holly Rayl.
If you consider other factors such as people living in urban or rural areas, age, sex, and education the distribution becomes even more complex: women, older people, people in urban areas, and people with higher education tend to prefer the pronunciation of route as /ru:t/ (rhyming with boot), while men, teenagers, people in rural areas, and people with lower education tend to prefer the pronunciation of route as /raut/ (rhyming with *out), at least in Canada.
This may correspond to the finding that in Britain the pronunciation of route as /raut/ (rhyming with out) is the one common among the British army.

Historically, the pronunciation with a diphthong has been recorded since the second half of the 18th century and was at that time the preferred pronunciation of some, but not all of the commentators. It has disappeared from standard British English in the course of the 19th century.
Answer: Route, as in a way or course to be taken, is pronounced 'root' in non American English as it is in French from which it is taken. The verb, to send down a particular way, is the same. Hence the computing router which directs signals down particular ways, would be pronounced rooter. Except that the influence of Americanisms is so strong that I often hear 'rowter' here in NZ and on British TV programmes (not programs).
Rout (no 'e'), rhyming with about, means to force a disorderly retreat of beaten troops. It also means 'to force out as if by digging' which is where the power tool comes from. The words are NOT related and are pronounced differently.
Unfortunately this gives us one of the inconsistencies of English in that router (from route) would be pronounced 'rooter' and router (from rout) is 'rowter'. The Americans have put in a consistency in pronouncing everything rowt and rowter, non-Americans have maintained the original pronunciations and pronounce them differently with a spelling inconsistency.
Now, which is actually correct? Because I was brung up with Non-American English I would have to say root and rooter.
Comment: I hate to tell you this, but "Route 66" is pronounced as "root".
Comment: Yup I know and I stand corrected. However it possibly emphasises that root is the better (I hesitate to say correct) pronunciation. It also suggests that Americans may have changed as "rowt" seems to be fairly universal in American now.
Comment: @HotLicks Even though you may find this a [hut] — and I honestly don't mean to [pʰɑʊ̯t], nor [pʰʊɫ] the [wʊɫ] over your eyes with some unkind [kʰʌt] — but let me [pʰʊt] it to you this way: As a general [ɻuə̯ɫ] a rural [ɻɑʊ̯t] does **ɴᴏᴛ** [ɻɑɪ̯m] with [ɻʊt] where I come [fɻʷʌm].
Comment: @tchrist - Where I come from people don't generally speak Greek.
Answer: Here in Virginia, route can be pronounced root or rowt, but a rowter is for computers, whereas rooter would refer only to your pig.
Answer:
Route as in Route66 is pronounced root.

A Router puts packets on a route and so is pronounced the same as the road, ie rooter.

A rout is a disorderly retreat. And a router  does the same to wood chips, so is pronounced rowter
Comment: Living in many places in the US, and dealing with networking equipment in all of them, I've always, *always* heard *rowter* (rhymes with "shouter") for the networking equipment. If I said "rooter" they'd wonder what I was talking about and if the toilet was stopped up.
Comment: Martin does it that way, people in some other locations do it in another way.  This does not mean one is *right* and the other *wrong*.
Comment: In many North American dialects, including that of the [Inland North](http://en.wikipedia.org/wiki/Inland_Northern_American_English), *route* and *rout* [ɹaʊt] are homophones rhyming with *shout*; *router* [ˈɹaʊɾɚ] rhymes with *shouter*; the *root* [ɹʊt] in the ground rhymes with *foot* and *soot*, just as in *put* and *hood*; but *to root* [ɹuːt] for one's home team rhymes with *shoot*, which means that only a person who's doing that sort of cheering would be a *rooter* [ˈɹuːɾɚ]. And *roof* is [ɹʊf], rhyming with *hoof*, not with *proof* [pʰɹuf] or *prove* [pʰɹuːv]. And yet, *rut* is [ɹʌt].
Comment: It would be great if this answer were correct, because it at least presents some consistent rules.  It also explains why a cutting router is pronounced *router* instead of *rooter*.  But this answer doesn't give a hint as to why a road would be pronounced *root*, and unfortunately the electronic device is, in American English, always pronounced *router*, so the middle part is just wrong.
Comment: Both cases are acceptable English, it simply depends on what dialect you speak, as many of the other answers point out.
Comment: Martin's answer makes sense. Too bad nearly nobody cares.
Comment: @jamesson The question was about route, not router.
Answer: If you talking about a plant's roots or the roots of your hair and so on, it should be the only time the "root" pronunciation is root is used.
That's  because if you say "root" for route, it just confuses things and doesn't make any sense to me. A router is a router, said as it's spelled.
Comment: Router is spelled like ar-oh-yoo-tee-ee-ar, but you certainly don't pronounce it like that. The "root-" pronunciation isn't only for plants or hair roots. [Both](http://www.antimoon.com/forum/t4494.htm) pronunciations are used in [America](http://dialect.redlog.net/staticmaps/q_26.html), but only root in the UK, as you'll see from the other answers here, from people who learned English in different parts of the world.


Multiple flashes synced differently

Question: I was wandering if it would be possible to set up 2 flashes on triggers but synced differently. I.e one 1st curtain and one 2nd curtain. A lot of people will be thinking "why?" but this would be really helpful for some of the work I do.
Answer: The camera could trigger some external timer (StopShot for example), which could then trigger whatever sequence it wanted, whenever it wanted, with whatever timing you program into it.  But using only the camera, I don't see any way the camera could issue two triggers.
Comment: Thank you for answering. I've been racking my brain for ages about this I'm thinking about using 2 triggers. having 1 set up with my camera and the other set up to be triggered via the test flash button which I would have in my hand. Just asking to see if anybody would know of a solution which wouldn't involve a 2nd trigger. I will look into the stopshot idea. Would be very elaborate for event/rave photography. lol

Multiple flashes synced differently


Special relativity: moving charge and twisting bar magnet

Question: ETA: Huh.  It's been more than three months since I posed this question.  Is it really possible that no one knows the answer?  I thought for sure someone would know.  Oh well.
You have a small bar magnet that is able to pivot about its middle (like a compass needle).  You fix the middle of the bar magnet to a point on the positive half of the y-axis.  Initially the bar magnet is pointing in some random direction.  Now a proton moves along the x-axis in the positive x-direction with speed v.  This moving charge generates a magnetic field, so the bar magnet will twist itself to align with that field, parallel to the z-axis.
The above description is in the rest-frame of the bar magnet.  But what about in the rest-frame of the proton?  Now the proton is at rest, so it generates no magnetic field.  Nevertheless, the bar magnet (now moving in the negative x-direction with speed v) must still pivot around to point in the z-direction -- if this happens in one reference frame, it must happen in every frame.  But what is the explanation for the twisting of the bar magnet in the proton's rest-frame?
I have come up with what may be a partial solution, but 1) I am not sure if it is correct, and 2) it only seems to work when the bar magnet is originally oriented along the y-axis.  If the bar magnet is originally pointing parallel to the x-axis, my explanation fails.
To start, let's replace the bar magnet with a circular loop of current-carrying wire, which has stationary positive ions and negative conduction electrons moving with drift speed v_d.  The ions and electrons have the same charge density, so the loop is electrically neutral.  This current loop is more or less equivalent to the bar magnet in terms of its own magnetic field.
Say that the bar magnet is originally aligned along the y-axis.  So the current loop we are replacing it with is originally perpendicular to the y-axis.  In the rest-frame of the proton, the current loop moves in the negative x-direction at speed v, and is therefore Lorentz-contracted into an ellipse, with its major axis in the z-direction and its minor axis in the x-direction.
Due to this contraction, the positive ions in the wire (all moving at speed v) will have a higher concentration at the ends of the loop's major axis.  At one of those two spots, the conduction electrons will have speed v + v_d, so they will experience more Lorentz-contraction than the ions and have an even higher concentration; thus the net charge there will be negative, and that part of the loop will be electrically attracted towards the stationary proton.  Conversely, at the other end of the loop's major axis, the conduction electrons will have speed v - v_d, so they will experience less Lorentz-contraction and have a lower concentration than the ions, resulting in a net positive charge that will be electrically repelled away from the proton.  Result: these two electrical forces torque the current loop until it is perpendicular to the z-direction -- which is to say, the original bar magnet will align itself with the z-direction, just like in the bar magnet's rest-frame.
However, this explanation won't work if the bar magnet is originally aligned parallel to the x-axis.  In that case, the current loop I'm replacing it with is originally perpendicular to the x-axis.  In the rest-frame of the proton, the circular current loop (moving along the x-axis) now remains a circle -- the Lorentz contraction doesn't alter its shape.  And all conduction electrons are traveling at the same speed in this frame, so I don't see how I would get parts of the loop to become positively and negatively charged, as I did previously.
And anyway, in order to twist the ring so as to be perpendicular to the z-direction, there must be a pair of forces pulling the ring in directions parallel to the x-axis.  These forces can't be electrical attractions or repulsions, as they don't point towards or away from the stationary proton.  And they can't be magnetic forces because the only magnetic field is the one from the current loop itself, and that can't cause the loop to twist (can it?).  So... what am I missing?
Much thanks for any help you can provide.
Comment: Probably relevant background information: [The moving magnet and conductor problem wiki](http://en.wikipedia.org/wiki/Moving_magnet_and_conductor_problem).
Comment: I'm familiar with that example, but I don't think it helps answer my question.
Answer: In special relativity, in the rest-frame of the proton, the moving magnet m appears as a magnet m' and an electric dipole p'. The electrostatic E field created by the proton makes rotate this electric dipole, actually the magnet. And the E' created by the electric dipole  is the responsible for the force the proton experiences. Remark that you will not found F=-F' but Fdt=-F'dt'. It is easy to found the expressions for writing p' and E' and make the corresponding calculations. I hope this will help you.


Unusual number of closures?

Question: Every so often, I check under "tools". I usually see that a few questions have been closed in the last day or two. I just went and saw over 30 questions closed in the last 24 hours, way more than I can remember ever having seen before. Are we suddenly getting a lot more cruddy questions? Or has someone organized a posse to hunt down old questions that don't measure up, with a view to getting them closed?
Comment: It might have something to do with the new design of [review page](http://math.stackexchange.com/review/), which was mentioned by Leonid [hre](http://meta.math.stackexchange.com/a/4900/)
Comment: I am *certain* it has to do with the new review page.
Comment: Now all that is left to do is for the firing squad to check the delete page as well...
Answer: I was about to accept LVK's answer, but I got here two minutes after the owner deleted it, so in order to have an answer of some sort, I'll echo the comments of Martin Sleziak and Asaf Karagila; it has to do with the redesign of the review page.
Comment: Haven't heard back from Bill and Shog9 about the outcome of their chat, so I'm accepting my own answer. It has the most votes, anyway.
Answer: To expand upon Gerry's summary of LVK's now-deleted answer: we recently introduced (in stunning beta-vision) a review queue for questions with pending close votes.
The way this works is, any question with at least one close vote (or a close "recommendation" in the form of a flag) ends up in the queue. Concerned Mathematics Stack Exchange members may walk through the queue, evaluating each question in turn, and recording their opinion as to what should be done with the question:

Close requires the selection of a close reason, and pushes the question further along the path to closure.
Do not close records their dissent. Enough "Do Not Close" responses immediately start the aging process for existing close votes, and remove the question from the queue.
Edit allows for correction of problems that might otherwise encourage closure.
Not sure skips evaluation of the question, leaving it for others to decide its fate.

This is important, as there are many (currently 287) questions that have received at least one close vote (or recommendation), but haven't yet been closed or had time (or sufficient views) for the votes to age away. Ideally, those of you with sufficient reputation will review these and either opt to leave them as they currently sit, or close them.
Because, after all, we all need closure.
Comment: And in the unlikely event that the two of you resolve your differences in chat, please be sure to come back here to report on the outcome.
Comment: http://en.wikipedia.org/wiki/Closure_(mathematics) would have been a more appropriate link, perhaps.
Comment: And I disagree, @Bill, that having few views or simply being a few months old makes these questions somehow *unimportant*. Why should someone working on a relatively obscure subject deserve the threat of closure perpetually hanging over them while awaiting an answer, simply because someone else didn't understand the question? Conversely, why would you want to keep poor questions around, simply because they'd been overlooked? It is the right of *all* members to participate in the moderation of their site to the extent that they are willing and able to do so.
Comment: Value is subjective, @Bill. For many people, spending time on an online Q&A site would seem a very poor use of their spare time, *regardless* of what they were doing here. As far as value to the site goes, I strongly believe that moderation is both necessary, and a constructive use of time for those able to do it effectively - given that diamond you volunteered for, I would rather hope you agree. We might disagree as to the exact forms moderation should take, but on *this site* closing and editing are core parts of the community moderation system - and *everyone* has some say in how it works.
Comment: @Bill: you're free to make your own decisions as to what forms of moderation you wish to engage in. So is everyone else. This fulfills several long-standing requests for better handling of close votes on low-activity questions, and for disputing close votes on *all* questions. I've heard from far too many people on far too many sites that the close-reopen system wasn't properly engaging and didn't reflect the actual views of the larger community to consider this *unimportant*. But as I said, you *personally* are free to ignore it if you wish.
Comment: Heh... I couldn't think of a bad pun for that one though.
Comment: @Shog9 You are disagreeing with things that I never said (nor implied). My primary point is that there are far more valuable ways that folks can spend their spare time, and that it is non-optimal design to deploy software that diverts attention from more valuable activities.
Comment: @Shog9 We simply disagree on the relative value of cleaning up delete votes on old questions vs. other activities that I think contribute much more value to the site.
Comment: -1  I  disagree with (explicitly?) actively encouraging folks to waste precious time on these matters when there are far more valuable ways where such precious time could spent. Random close votes that have not yet expired  are *very far* from the top of the queue of important matters MSE faces.  One of the most challenging aspects of UI design is to *naturally* draw attention where it is needed (and to divert attention from where it is not needed).
Comment: @Shog9 Again, it's not that I disagree that cleaning up close votes on old posts may be useful (see my comments to LVK's deleted answer). Rather, I simply think that there are more valuable things that we can allocate spare time to *on MSE.*
Comment: let us [continue this discussion in chat](http://chat.stackexchange.com/rooms/4572/discussion-between-shog9-and-bill-dubuque)

Unusual number of closures?


Is it "burning at/on the stakes", "burning at/on the stake", "burning at/on stake" or "burning at/on stakes"?

Question: Is it "burning at/on the stakes", "burning at/on the stake", "burning at/on stake" or "burning at/on stakes"? There's an entry for "burn at the stake" on the dictionary, so I am guessing it's correct, but I also see "burn at the stakes" and "burn on the stakes" on Google, so I am wondering if these other versions are also idiomatic, or they're just errors. Strangely, a lot of these unidiomatic expressions are found in books and articles.
Answer: The fixed phrase for the execution of e.g. witches, etc, is "burning at the stake". "Stake" is singular; there is only one; it is a wooden post to which the victim is tied. "Stakes" (plural) is an error. It is a good idea to use dictionaries to verify meaning and usages, since Google will find wrong as well as correct usages.

burn somebody at the stake
  to kill someone by tying the person to a
  post and burning them

Burn (Cambridge Dictionary)
Answer: The idiom is "(the witch was) burnt at the stake". It is not uncommon to drop "the", and if there are several stakes then the plural would be used.
Burning "on" the stake is less idiomatic, but still sees plenty of usage.
However if you are looking for the "correct" expression then it is "burnt at the stake".
Comment: My ear suggests the opposite: the expression I've always heard is "the stake", as metonymy, always singular, always with "the".  "Two witches were burnt at the stake".  Also "put to the stake".  Can I ask where you've heard plural or without "the"?
Comment: google.  As I've said "burnt at the stake" is the idiomatic expression, but there is clear usage without "the":  "Some of the methods of torture included: • Burning on stake • Tearing out fingernails..."   "John Huss who was burnt on stake..." This is not the usage that I recommend. The correct expression is "burnt at the stake".


How to get dynamically changing xrefs with IDAPython

Question: There is a situation where the number of xrefs for a function is dynamically increasing due to the dynamic creation of memory with execution rights attached by VirtualAlloc. I want to get the xrefs of the function, and in this situation, if I check manually in the UI, I get 8 references, but if I use IDAPython to get CodeRefsTo/XrefsTo, I only get 3. This is probably due to the fact that IDAPython does not allow xrefs to be applied to code regions dynamically allocated by VirtualAlloc, or because the cache is not updated. What is the best way to get dynamically changing xrefs with IDAPython ? Can you give me some ideas?
Answer: Unfortunately i can not write a comment thus i have to write an answer if i want to reply. Maybe you could write a script to scan the memory for that functions address or even scan for the bytes of call functionaddr using idc.find_binary. Ofc that would only work if there are no offsets used.
Comment: Thank you for your comment. I already know the address of that function(function A), so I want to get dynamically not the function(function A) but the many other caller's address that reference(xrefs) that the function(function A) address with IDApython.
Comment: yes but idapython has the module idc. So knowing the address of the function that you want the xrefs to you could search for the bytesequence of "call functionaddress". That should atleast yield you some of the calls to it. I dont know how to perform the ida-internal x-ref scan though in idapython.
Comment: Thank you for your comment. After that, I could solved the problem by using "plan_and_wait()". Your idea will be great too. Thank you.
Comment: @rouqwers please add an answer if you found a solution so others can benefit from it
Comment: Hi Igor, I've included an answer.
Answer: My question was resolved by doing the following.
This is an answer.
If you want to update xrefs, at any given time, do the following.
<code>plan_and_wait(minaddress, maxaddress)
</code>
It was effective to narrow down the scope of the project to a certain extent, because doing it in all areas would have been time consuming.


Garbage collection behavior with isolated cyclic references?

Question: If I have two objects on the heap referring to each other but they are not linking to any reference variable then are those objects eligible for garbage collection?
Answer: Yes, they are. Basically the GC walks from "known roots" (static variables, local variables from all stack frames in alll threads) to find objects which can't be garbage collected. If there's no way of getting to an object from a root, it's eligible for collection.
EDIT: Tom pointed this out, which I thought was worth lifting into the answer itself:

Technically, static variables are not
  roots - they are referenced by classes
  which are referenced by class loaders
  which are referenced by classes which
  are referenced by object which are
  referenced by root references.

The difference is likely to be irrelevant most of the time, but it's good to know :)
Comment: Technically, static variables are not roots - they are referenced by classes which are referenced by class loaders which are referenced by classes which are referenced by object which are referenced by root references.
Answer: Check this out: How does Java Garbage Collector Handle Self References.
You may want to check java.lang.ref.WeakReference
Answer: Skeet's on the money, as usual.  I would only add that the situation you describe is the reason that reference counting (a standard strategy with early C++ smart pointers) isn't used.


How do Autonomous Systems work?

Question:
Assuming I have BGP forwarding from my ISP, a BGP capable router and an ASN. What stops me from advertising IP ranges not belonging to me? Do routers check if an IP address belongs to the correct ASN from RIR databases ?
Do router routing tables contain the ASN? Or just IP ranges, metrics and interface names?
Does every router connected to the internet with more than one upstream provider know the complete internet routing table?
Comment: You might find this book useful: http://www.amazon.co.uk/Internet-Routing-Architectures-Cisco-Press/dp/157870233X/ It's a bit old but most of the theory still applies. Back in 2000 when I was dropped into the deep end to set up and run an internet exchange point I found it invaluable.
Answer:
The networks you connect to should set up filters to accept routes from you only if they are "yours" or those of your customer(s).  Some ISPs build filters from various databases (RIR or IRR).  Not all networks do this.
BGP tables (RIB) do contain ASNs.  Forwarding tables (FIB) do not.
Not necessarily.  It depends on your needs.  You could also accept just a default route from each provider, or a subset of the routes your upstreams know, or a complete table form one upstream and partial routes from another.
Answer:
You would find no one willing to peer with you. The Internet connections between ASs is cooperative, and, if you don't cooperate, you don't get in.
As far as Cisco routers go, the routing table contains the best route for a given prefix from all the routing protocol tables. In other words, "No." Each routing protocol maintains its own table, and the routing table chooses its content from those tables.
Not necessarily. It is the universal networking answer: "It depends..." You really need to know what you are trying to accomplish in what way to answer that question for any particular design.

How do Autonomous Systems work?


Boggle variations?

Question: This summer we have company staying with us, and they greatly enjoy playing Boggle. I'm looking to spice things up a bit, though - any ideas for Boggle variations that we can try? There will be two to four of us playing at any time.
Answer: A few Boggle variants I've heard of:

Scrabble-style:  Lose the timer - take turns making words, end game when (n) words made (try about 20 to begin with).  You cannot repeat a word already made.

Collapsing Boggle:  Use a shorter timer.  After it runs out, remove 1 die at random and keep going.  Remove another die and have a third (and final) run through.

Selective Boggle:  After the dice are placed, each player may rotate one die to a different side.  (For a milder version, have one player rotate one die, changing players each round.)

Generous Boggle: Allow each die to be used multiple times in one word.

Scatter Boggle: Only try this with a large size 5x5 Boggle set.  Instead of being adjacent, each die in the word must NOT be adjacent horizontally or vertically.  (Don't count diagonals with this variant; there won't be enough options.)
I've just invented one more while typing, so this is completely untested, and may not even make sense:

Apples to Boggles.  Get an Apples to Apples set and play it, except that instead of picking a noun from your hand to match the topic for the round, you have to pick a word from the Boggle board to match, and convince people it's the best candidate as usual.  First person to call a word gets it as her "apple" for the round.  (Definitely use the "generous" rule above for this one.)
Answer: 5 letter Boggle. Minimum word length is 5 letters. You can try this with 6 or 7 letters too.
Answer: Boggle Quiz: a description or question is posed to all participants. A boggle board is randomly generated containing the answer and distributed to all participants. The first person to find the correct answer (either fitting the description or answering the question) on the boggle board wins the round.

Boggle variations?


What caused a capacitor in the power supply start smelling?

Question: I was using my computer in the morning then later in the afternoon I noticed some bad smell coming from the power supply. As I checked in, I noticed that one of the big capacitors was leaking and dry. The black wraping on it had peeeled out and some black matter was pasted on the sides.My machine has been doing great since, but today it caused trouble. what could have caused this and how do I fix it?
Comment: *> what could have caused this?*   [Capacitor plague](http://en.wikipedia.org/wiki/Capacitor_plague)?     *> how do I fix it?*   New PSU?
Answer: That capacitor has blown, probably it's suffering from the dreaded capacitor plague.  What you're smelling and seeing is the electrolyte that's leaked out.
Depending on what the capacitor in question is actually doing, you may not see an immediate impact (as you noticed), but things will degrade, and what's worse is that the other capacitors are likely to start failing shortly.  You could solder in replacements, but it's probably just best to replace the power supply entirely.
Answer: Many things can cause a capacitor to fail.  Manufacturing defects, low quality components, and adverse environmental conditions can shorten the life of the capacitor.  Once you see it bulging or leaking, you should consider the device failed and stop using it or you risk damaging any equipment connected to it.
If the capacitor is connected to an accessible board and you're comfortable soldering, you can purchase a replacement capacitor of the same type and rating and very likely repair the device.  For a power supply, however, I would very strongly recommend against servicing it yourself unless you absolutely know what you're doing.  Power supplies use capacitors to transform voltages, and as a result can store very dangerous levels of electricity.  Personally, I would never service a power supply, as the potential risk is too high compared to the cost of a replacement power supply.


What is my user ID?

Question: I'm in the data explorer, and I need my user ID for a query.  Problem is, I don't know what it is, if it's not my username.
Comment: You are number 6.
Comment: The data explorer is updated with live data only once per month, usually the first of the month. Perhaps you have to wait a few days to see the desired data.
Comment: Note that it *is* different for different sites. A lot of people are suggesting your Meta.SO account, which is all fine and good, but if you want ELU or SO, you need to check *those* IDs on the user page for that particular site.
Comment: Your profile view, http://meta.stackoverflow.com/users/164012/drm65 - I would think your id is the "164012" segment (on meta).
Comment: I am specifically asking about my ELU account: [http://english.stackexchange.com/users/10041/drm65](http://english.stackexchange.com/users/10041/drm65)  I would think the same thing (10041), except the query doesn't come up with any results when I input 10041 into [here](http://data.stackexchange.com/english/s/229/test-what-is-my-reputations-percentile)
Comment: +1 @Rosinante. I'm surprised that nobody ever +1 that until now.  (But does that make you number 1, at least for the moment)?
Answer: You can find it in several ways.

The link from your flair on the question contains your id - https://meta.stackexchange.com/users/164012/daniel - in this case it's 164012 here on meta.
Go to your profile page and select the accounts tab. The list of sites links to your profile on that site. So for EL&U the link is https://english.stackexchange.com/users/10041/, so your user id there is 10041.

Your id is different on each site as it depends on when you joined the site and how many people had joined before you. This means that if you just change the site name in the profile url you won't get your account on that site, you'll almost certainly get someone else's. The only exception to this is this site for those users who existing on Stack Overflow when Meta was first created. In that instance Jeff just copied the user table so we have the same id.
There is also your network id - https://stackexchange.com/users/429575/daniel - in your case.
Comment: @drm65 - I don't know. That is your profile. How long have you been a member of EL&U? The data explorer data is always at least one month out of date. So if you're less than a month on the site you won't appear just yet.
Comment: But 10041 doesn't work in [the query](http://data.stackexchange.com/english/s/229/test-what-is-my-reputations-percentile).
Comment: @drm65 ChrisF is correct. That query works for other userIDs whose owner joined earlier than you.
Comment: @ChrisF: Ahhh... that must be it.  I've only been on SE for 29 days.
Comment: The 2nd way doesn't work anymore. Can you confirm and update your answer ?
Answer: You can find your user ID for a particular Stack Exchange site by performing the following steps:

First, visit your user profile on the site in question.

Take a close look at the URL for your profile page:

You should spot a number right after the users/ and before the slug for your username. This is your user ID for that site.
Comment: You felt the other answers didn't adequately explain this?
Comment: @MichaelMrozek: Actually, to be honest, I'm writing a PHP library for the API and I needed a single answer somewhere that clearly described how to retrieve a user's ID. None of the other answers seemed to do that.
Comment: +1 this answer is indeed more clear than the others.
Answer: Your user ID is 164012. You can find it in the url of your user page: https://meta.stackoverflow.com/users/164012/drm65
Answer: Get all stackexchange user ids for a user
Find your user-ID of your Network Profile in your user section, then you can get all other IDs with this bash script:
ID=429575
USER=daniel
curl https://stackexchange.com/users/$ID/$USER?tab=accounts|grep account-container -A1|grep http|cut -d"/" -f3,5

This will give you a list of all SE-sites and user-ids.
If you need a list of all questions and answers:
#/bin/bash

SE_ID=429575
U=daniel

users=$(curl https://stackexchange.com/users/$SE_ID/$U?tab=accounts|grep account-container -A1|grep http|cut -d"/" -f3,5|xargs)
for i in $users; do
  site=$(echo $i|sed "s~/.*$~~g")
  id=$(echo $i|sed "s~^.*/~~g")
  for a in questions answers; do
    URL=http://$site/users/$id/$U?tab=$a
    echo $URL
  done
done

Comment: if you don't want to parse you can use SEDE : https://data.stackexchange.com/askubuntu/query/1259066?UserId=34298
Comment: Pretty nice solution, @rubo77! Simple and efficient
Answer: Your user ID is different on each SE site; in the list of user profiles listed, for example, in https://meta.stackoverflow.com/users/164012/drm65?tab=accounts, the user ID is always the number after users/.
The one you need to use depends on where you are executing the query: The one that you need to use in https://data.stackexchange.com/stackoverflow/queries is your Stack Overflow user ID, while in https://data.stackexchange.com/english/queries you should use your EL&U user ID.

What is my user ID?


Wheat crackers that are ha'adama

Question: Paskesz (a brand that makes lots of kosher Jewish food in the USA), makes wheat crackers. The crackers are 99% wheat (per the ingredients label). While most wheat crackers require the blessing of mezonot, these are specifically labelled as needing the bracha of ha'adama. What is Paskesz doing (or as I suspect is the case, not doing) to these crackers such that they're ha'adama?

Comment: the ingredients indicate no water. If one were to take a wheat stalk and chomp on it, what would the bracha be?
Comment: @rosends Ha'adamah
Comment: @JoelK then based on the ingredients, isn't that the right bracha for these cracker?
Comment: @rosends It's not clear to me how these crackers are manufactured, but yes, it could very well be
Comment: Water is very often not listed in the ingredients for baked goods, since it provides no nutrients (calories, vitamins, minerals, etc.) and is not an allergen. So lack of water in the ingredients listing does not mean much. As the answer shows, this is indeed a non-water process, but you can't tell from the ingredients. Matzah will list "wheat flour and water only" but that is for Halachic emphasis, not for the usual "ingredient" reasons.
Answer: Refer to this communication between Dayan Westheim zt"l and Rabbi Yehoshua Kaganoff here on p.64. The full letter is very enlightening and sheds light on their production/manufacture and the resulting halachic breakdown:
It is worth noting that the argument for them being ha'adamah is as follows:

Regarding the proper Brocho on the Paskesz product of Wheat Squares: Thank you for describing to me their production processing, i.e., complete whole wheat kernels including their bran are placed between two metal plates approximately 3/4 inch distant from each other and then they are heated until they pop. Simultaneous to their popping they become fused together to form the "pattycake"-like format as it appears when it is marketed.
I had posited that this would be equivalent to a cooking process "bishul" of whole wheat kernels until they completely disintegrated and adhered to each other of which the MB 208:3 (end) states that the Brocho is Borei Minei Mezonos. You countered that all of this discussion in the Shulchon Aruch and Acharonim revolved about only a liquid based processing but a dry process such as the puffing/popping of the grains as described above would not engender any discussion or dispute and would unanimously be Borei Pri Ho'Adomo.

Wheat crackers that are ha'adama


What is chaos damage in Path of Exile?

Question: What exactly is chaos damage in Path of Exile? Does it bypass all resistances? Is it poison damage?
Answer: Chaos damage is poison in some cases, but not always. Chaos damage ignores armour, elemental resistances and energy shield. The only way so far to have resistance (in this case immunity) to chaos damage is by getting Chaos Inoculation from passive skills tree. If you have high evasion you can dodge some chaos attacks (like the "Viper Strike").
With the release of Open Beta, several forms of chaos resistance now exist in addition to Chaos Inoculation, including several nodes, armor affixes, and chaos resistance flasks.
Sources: Official forum , PoE wiki
Answer: It bypasses energy shield, not resistances. If you have chaos resistance, you will take less damage from it, but I haven't seen a single chaos resistance item dropped yet, as of Act 3. However, you can get chaos resistance from a skill list.
Answer: It's another damage type, apart from elemental. Reduced by resistance to it, similar to elemental, going upto 75%
What's special about it:
a. It bypasses energy shield, and hits life direct! (hence chaos innoculation).
b. Chaos resistance is generally hard to come by. Much rarer on tree and gear. Currently no chaos resist auras like the elemental purities. This is true on monsters as well. Chaos resist is rare on monsters and no chaos resist map mods.
c. Arguably the most important - it cannot be reflected! There's elemental and phys reflect, but no chaos reflect.
It's not "poison" but Poison effects deal chaos damage.
Most meta builds today utilize poison scaling btw. Triple entente - chaos/gem scaling double dip, no reflect and very little monster resistance.

What is chaos damage in Path of Exile?


Possibilty to flash a TWRP/CWM on a smartphone that doesnt have custom recovery?

Question: I have a Huawei Y7 Prime that doesn't have any released custom recoveries. Is it possible to flash or even just boot TWRP/CWM for another smartphone (eg. for Huawei P8) on my Y7 Prime? Is it possible to do this in general? and if no, is there any way to root my device without custom recovery just by using fastboot or...?!
Answer: Simple answer NO
It is not possible - recovery is device specific and unless it has been developed for your device, you are out of luck
You can try one-click apps that root, if it works for you (like Kingo root etc but be aware that they usually have their own set of problems)
Answer: Advanced answer, Yes:
If your device is Mediatek Oriented and you have it's stock ROM, the one to be flashed to the device using SP Flash tool,

You can install the SP Tool on computer
Extract the stock ROM
Locate the extracted ROM when choosing a scatter file from the tool
Don't panic here in this mode. SP Tool is by default at the Download Tab after loading the scatter.txt file since you might brick your phone
Switch the tab to Readback
Set the start adress as written at preloader.bin at the Download tab
Set the stop adress as written at data at the download tab and Start the readback
Switch your phone off and plug it on the computer(Note I'm assuming SP flash tool is well configured alongside its VCOM drivers installed but I don't want to talk about them so as to be simple. Otherwise it won't be detected)
Let the readback process complete
Locate the ROM dump in SP Flash Tool installation folder usually C:/Program Files
Download Readback extractor or alternatively (use MTK_Droid_tools exe) that will convert the dump into flashable images and locate from it the source dump.(Some extractors depend on this dump while some work independently but I have to include this because one might encounter an issue)
Start conversion.
It will prompt you when extracting the recovery.img whether to backup the stock recovery or to generate ClockWorkMod(CWM).img instead. Chose the later. It will be called cwm-automade.img or recovery.img in the final flashable ROM
Use this to have a more free recovery than the stock which is CWM recovery
Flash the recovery. img from fastboot or the SP Flash tool Download tab.

Note: Do not tick preloader.bin when flashing anything otherwise you'll brick your device to a state it won't ever get alive again
Comment: Appreciate the effort, but since OP already made clear what device he owns (not a MediaTek one, as can be simply looked up), this would all be pointless to him...


What is the effect of aliasing on the magnitude of the autocorrelation?

Question: I've a question about the effect of aliasing on the magnitude of autocorrelations. From a simulation in MATLAB, I don't see any effect of aliasing or any need to anti-alias filter when I take the magnitude of the autocorrelation. Which means I can undersample the data and then take the autocorrelation. There is a paper "Effects of Aliasing on Spectral Moment Estimates Derived from the Complete Autocorrelation Function" which says something like what I claim. Would anybody please let me know if I've made a mistake?
Answer: Decimating before calculating the autocorrelation, in the presence of noise, is inferior to calculating the autocorrelation using the full dataset. Assume that the signal of interest is embedded in white noise. The vector $x[n], n = 0, 1, ..., N-1$ consists of samples from a discrete random process. The autocorrelation function of the vector $x[n]$ is:
$$
A_x[k] = \frac{1}{N-k} \sum_{i=0}^{N-1-k} x[i]x[i+k]
$$
That is, $k$ is the lag used for the autocorrelation calculation. In your proposed scenario, you are decimating the autocorrelation function output by a factor $D$ (i.e. you are only calculating the function for lags $0, D, 2D, ...$) and comparing that result to the autocorrelation function of $x[n]$ decimated by the same factor $D$. Let $x_d[n]$ be the decimated sequence; its autocorrelation function is:
$$
A_{x_d}[k] = \frac{D}{N-k} \sum_{i=0}^{\frac{N-1-k}{D}} x[iD]x[(i+k)D]
$$
(for simplicity here, I have assumed that $D$ is a factor of $N$ in the above equation)
Your inquiry can be written as:
$$
A_x[kD] \stackrel{?}{\approx} A_{x_d}[k]
$$
$$
\frac{1}{N-kD} \sum_{i=0}^{N-1-kD} x[i]x[i+kD] \stackrel{?}{\approx} \frac{D}{N-k} \sum_{i=0}^{\frac{N-1-k}{D}} x[iD]x[(i+k)D]
$$
Looking at this qualitatively, the summation on the left hand side has more terms than its counterpart on the right side. If $x[n]$ is second-order stationary, then the expected value of each term in each sum is the same; the act of averaging multiple samples that have the same expected value increases the signal to noise ratio. Stated a little differently, you can think the terms in each sum as samples from a new random process:
$$
y[n] = x[n]x[n+kD]
$$
Since the noise present in $x[n]$ is white, the expected value of $y[n]$ is the true autocorrelation of the signal of interest at lag $kD$. Therefore, we would like to accurately estimate the expected value of $y[n]$. Our method for doing so is by calculating a sample average; it can easily be shown that the variance in the sample average estimator decreases given a larger sample size, converging to the actual expected value as the number of samples tends to $\infty$.
So, if there is white noise present in the signal (which is often the case), you're going to get a better estimate of the underlying signal's second-order statistics by using a larger sample size in the calculation (this might sound intuitively obvious). In the context of your two approaches, this is accomplished by using the full, non-decimated signal in the autocorrelation calculation and decimating afterward (i.e. only calculating the result for certain lag values).
Comment: Thank you very much. You are right but in case of my signal it is not the dominant problem. My problem is mostly the effect of aliasing. You explained that full non-decimate signal can be better but if we are going to decrease the effect of aliasing we should take even more samples like two(3) times of number of samples and it really increases the complexity.
Answer: Seems a little odd to me. The Matlab script below compares the "downsampled autocorrelation" to the "autocorrelation of the downsampled signals". For dual sine waves this actually comes pretty close (relative error of about -50dB)  but for white noise it's simply wrong (relative error > +6 dB). While there may be some computational advantage it's not clear to me how useful the downsampled autocorrelations is even in the dual sine wave case. The peaks in the spectrum still show up in the wrong place.
<code>% script to check whether autocorrelation is immune to aliasing
% create two sine waves at 18k and 21k (assuming sample rate of 444.1k)
n = 8192;
t = (0:n-1)'/44100;
x = sin(2*pi*t*21000)+sin(2*pi*t*18000);
% calculate autocorrelation of original signal and one that's downsampled
% by 4 and thus heavily aliased
y = xcorr(x,x);
y2 = xcorr(x(1:4:end),x(1:4:end));
d = y(4:4:end)-4*y2;
% calculate the error in dB
err = 10*log10(sum(d.^2)./sum(y2.^2));
fprintf('Dual sine wave relative error = %6.2f dB\n',err);

%% try the same thing for white noise
x = 2*rand(n,1)-1;
y = xcorr(x,x);
y2 = xcorr(x(1:4:end),x(1:4:end));
d = y(4:4:end)-4*y2;
err = 10*log10(sum(d.^2)./sum(y2.^2));
fprintf('White noise relative error = %6.2f dB\n',err);
</code>
Comment: Thank you very much but the place of peaks in autocorrelation is important for me and therefore I'm not pretty sure that this code shows my problem. But as you point out while the spectrum changes, we face with small changes in time domain similar to what paper says.
Answer: For specific types of inputs the effect of frequency aliasing on the magnitude of autocorrelations may be negligible. However, I don't think this will be true in general.
For instance, for a bandlimited input or for white noise the under-sampling will not impact the shape of the autocorrelation (although it might change the scaling in a predictive way). The autocorrelation of white noise is a delta and it will remain to be a delta if down-sampled.
Now, the power spectrum is related to the autocorrelation by the fourier transform. So if your claim would be true it seems that you could also claim that frequency aliasing does not change the frequency contents of the input. And this is not true. But there might be exceptions (special cases).


Stone Shape Uses

Question: Specifically the Stone Sculptor Wild Talent for Earth Kineticists.
Can you create tunnels, or burrow with the stone shape spell? It says you may form stone into a shape that suits your purpose, but can you shape it into a tunnel to descend into the ground? Also, when you do this, is the stone you move displaced somewhere else?
Comment: A little over a week ago [this question](https://rpg.stackexchange.com/q/122314/8610) about the spell *stone shape* was posed. It may be helpful.
Answer: The spell stone shape (and, by extension, the kineticist utility wild talent stone sculptor special ability) can be used to create tunnels through stone. Being able to shape stone, though, isn't the same as having a burrow speed. If a creature uses an effect like shape stone to create a tunnel, after that tunnel's created the creature must then traverse that tunnel using its available movement modes normally. (That is, the creature may have to climb down a newly created shaft like everyone else, although its walls could be shaped so as to have handholds that make the climb is easier.) A stone shape effect does not, for example, allow stone surfing à la the Marvel Comics mutant Iceman:
 …but with stone (so more like Terrax)
Also, the stone shape spell only says it shapes the affected stone and doesn't say anything about, for example, dissipating unwanted stone or whatever, so the excess stone must go somewhere.
(Whether the spell stone shape can shape stone to such a degree that the stone is compressed into a smaller and far denser unit or even shaped into dust so that it can be easily dispersed is a question for the GM, but this GM would likely say no. The stone shape spell is typically only a 3rd-level spell, after all, and there are other spells that shrink material, annihilate material, or make neat holes in walls.)

Stone Shape Uses


How do I know if my bridge is actually useful to the Tor network?

Question: I set up a Tor bridge a couple of days ago, but it sees hardly any traffic.  How can I check that I did everything right and my bridge is actually registered in the Tor network?  And can I find out if it's actually useful?
Answer: You can learn more about your bridge at the Globe service: http://globe.rndm.de/
For example, you can learn how much traffic your bridge pushed in the last days or weeks.  You can also find out whether and how your bridge is given out to bridge users, like via email or https.
Also note that bridges that aren't given out are not useless.  These bridges can be given out to censored users manually, which means they're less likely to be blocked.  It makes sense to keep them running even if they're not pushing much traffic.  If you prefer your Tor node to push much traffic, you should consider turning it into a non-exit relay.  But bridges are important, too.
Answer: See my answer to a similar question here.
Briefly, search by nickname which you'll find on your ec2 instance in /etc/tor/torrc
Answer:
I Vidalia Control Panel select "view the network" :

2.Now you will see something like that :

Find yourself on the list on the left - you will see how "useful" are you for Tor, what's your internet connection speed, etc. , etc.
I hope I helped.
Comment: The list contains only relays. Karsten, however, asked for bridges.
Comment: This has nothing to do with bridges. They never show up on this list.


Is it possible to program iPhone in C++

Question: I'm all for language diversity, but Objective C is insane. So I'm curious: is it possible to code iPhone apps with C++ while using the Cocoa API, etc?
Answer: Short answer, yes, sort of. You can use Objective-C++, which you can read about at Apple Developer Connection.
If you know C++ already, learning Objective-C would be pretty simple, if you decided to give that a try. More info on that topic is at the ADC as well.
Comment: @LeaHayes A free book called "From C++ to Objective-C".  Assumes you know C++ and then tells you how things are done differently in Objective-C.  http://pierre.chachatelier.fr/programmation/fichiers/cpp-objc-en.pdf (this is an English translation of the original French)
Comment: can you recommend a website or book that is aimed at experienced C++ programmers to learn Objective-C? I managed to mash a few test apps together, but I find the syntax a little backwards and I am sure that I am doing things that lead to memory leaks etc just through lack of knowledge of Objective-C
Answer: Although Objective-C does indeed appear to be "insane" initially, I encourage you to stick with it.  Once you have an "a-ha" moment, suddenly it all starts to make sense.  For me it took about 2 weeks of focused Objective-C concentration to really understand the Cocoa frameworks, the language, and how it all fits together.  But once I really "got" it, it was very very exciting.
It sounds cliché, but it's true.  Stick it out.
Of course, if you're bringing in C++ libraries or existing C++ code, you can use those modules with Objective-C/Objective-C++.
Answer: I use Objective-C to slap the UI together.
But the hard guts of the code is still written in C++.
That is the main purpose of Objective-C the UI interface and handling the events.
And it works great for that purpose.
I still like C++ as the backend for the code though (but that's mainly becuase I like C++) you could quite easily use Objective-C for the backend of the application as well.
Comment: @Spanky: How o I do what exactly. You can write normal C in *.m file. You can write normal C++ in *.mm files (Objective-C++). Inside Objective-C(++) files you can use normal `#include ` just like C/C++ files.
Answer: First off, saying Objective-C is "insane" is humorous- I have the Bjarne Stroustrup C++ book sitting by my side which clocks in at 1020 pages.  Apple's PDF on Objective-C is 141.
If you want to use UIKit it will be very, very difficult for you to do anything in C++.  Any serious iPhone app that conforms to Apple's UI will need it's UI portions to be written in Objective-C.  Only if you're writing an OpenGL game can you stick almost entirely to C/C++.
Comment: Sorry but ObjC is insane. The insistance on a completely different way of declaring functions, the use of brackets. It's being different for no benefit. They could have drafted the same effective language without the ridiculous syntax. They chose not to. Thus it's insane.
Comment: Since when was less documentation a good thing? "The C++ Programming Language" is very thorough and also documents the STL.
Comment: @Rhubarb - Though I like Objective-C I have to agree that it's weird syntax serves no purpose.
Comment: @Timmmm: Ever since K&R made that point in the front matters of their book. Objective-C is a much smaller language than C++, with many of the same benefits (and some more of its own). Everything's tradeoffs, of course. But minimal documentation makes Obj-C pretty easy for experienced C (or C++) people to get up to speed with.
Comment: Objective-C and C++ came about at around the same time, both with the goal of adding object oriented programming to C.  Objective-C followed Smalltalk's model of everything being dynamic dispatch, C++ chose to use function pointers (virtual methods) basically. The weird syntax serves to differentiate ObjC message dispatching from regular C, direct function calls. Not sure if it was strictly necessary, but it's one thing that makes it very easy to mix and match ObjC and C++ in Objective-C++.
Comment: @Rhubarb, Yup, remember Apple is the company that came up with that insanity, and Apple is different for the sole purpose of being different.
Answer: Having some experience of this, you can indeed use C++ code for your "core" code, but you have to use objective-C for anything iPhone specific.
Don't try to force Objective-C to act like C++. At first it will seem to you this is possible, but the resulting code really won't work well with Cocoa, and you will get very confused as to what is going on. Take the time to learn properly, without any C++ around, how to build GUIs and iPhone applications, then link in your C++ base.
Answer: You have to use Objective C to interface with the Cocoa API, so there is no choice. Of course, you can use as much C++ as you like behind the scenes (Objective C++ makes this easy).
It is an insane language indeed, but it's also... kind of fun to use once you're a bit used to it. :-)
Answer: I'm not sure about C++, but you can definitely code iPhone applications in C#, using a product called MonoTouch.
You can see this post for detailed discussion on MonoTouch Vs Obj-C: How to decide between MonoTouch and Objective-C?
Answer: I'm in the process of porting a computation-intensive Android app written in Java to iOS6. I'm doing this by porting the non-UI parts from Java to C++, writing the (minimal) UI parts in Obj-C, and wrapping the former in a  (small) C interface using the standard C/C++ technique, so that it can be accessed from Obj-C, which is after all a superset of C.
This has been effective so far, and I haven't encountered any gotchas. It seems to be a legitimate approach, since Xcode lets you create C++ classes as well as Obj-C classes, and some of the official sample code does things this way. I haven't had to go outside any officially supported interfaces to do this.
There wouldn't seem to be much to gain from writing my remaining UI code in C++ even if it were possible, in view of the help given to you by the interface builder in Xcode, so my answer would be that you can use C++ for almost all your app, or as much of it as you find appropriate/convenient.
Answer: Yes but Thinking that you can program every kind of program in a single language is a flawed idea unless you are writing very simple programs. Objective C is for Cocoa as C# is for .NET, Use the right tool for right job, Trying to make C++ interact to Cocoa via writing bridging code and trying to make C++ code behave according to Cocoa requirements is not a good idea neither expecting C++ performance from Objective C is. You should try to layout design and architecture of app keeping in view existing skills and determine which part should be written in which language then build accordingly.
Answer: I'm currently writing an Objective-C++ framework called Objective-X, wich makes PURE C++ iPHONE PROGRAMMING possible. You can do like this:
<code>#import "ObjectiveX.h"

void GUIApplicationMain() {
    GUIAlert Alert;
    GUILabel Label;
    GUIScreen MainScreen;

    Alert.set_text(@"Just a lovely alert box!");
    Alert.set_title(@"Hello!");
    Alert.set_button(@"Okay");
    Alert.show();

    Label.set_text(@"Ciao!");
    Label.set_position(100, 200, 120, 40);

    MainScreen.init();
    MainScreen.addGUIControl(Label.init());
}
</code>
and compile it using GCC's appropriate commandline options. I've already compiled this helloworld app&it w0rkX0rz like a charm. ;-) It'll available soon on GoogleCode. Search for Objective-X or visit http://infotronix.orgfree.com/objectivex approx. a week later!
Updated (but apparently inactive) URL: http://code.google.com/p/objectivex/
Comment: Why not wait with posting until that is actually available?
Comment: According to SourceForge: "As of 2014-01-24, this project is no longer under active development." http://sourceforge.net/projects/objectivex/
Answer: It may be a bit offtopic, but anyway. You can program c++ right on iOS devices. Check out CppCode ios app - http://cppcode.info. I believe it helps to learn c and c++ and objective-c later.

Is it possible to program iPhone in C++


How do I tape a joint between drywall and plaster?

Question: I have old old plaster (on lath) bordering new drywall. Is it better to use mesh tape or just paper?
Comment: On a corner? On the wall itself? If the former, use caulk. If the latter, you might have a bigger challenge blending the two wall textures together seamlessly. Might be easier to cover it all with new drywall.
Comment: wall itself.  no, i'm not covering with new drywall...
Answer: Mesh tape works really well.  Some guys swear by paper though.
I prefer mesh because its really idiot proof and quite forgiving.
Plaster will crack though. Just a matter of time and how many tractor trailers roll by your house.
Answer: Mesh is definitively a better blending product.  I like to embed mesh with the lightweight setting joint compound and then use general purpose (premixed) joint compound for topcoat and blending.
AFA plaster cracking, there are large size mesh rolls meant for skimcoating and repairing plaster.  One I know of is 36"  by 150' by FibaTape


What does "Humidity at Less than 85% (no condensation)" mean in a camera's specifications?

Question: Under the heading "Operating Environment", my camera syas:
<code>Less than 85% (no condensation)
</code>
What does it actually mean? Does it mean that cannot be used in a air-conditioned environment?
Answer: They are saying that the maximum "wetness" of the environment should be 85% relative humidity (RH).
As shown on the Psychrometric chart further down, 85% RH is an upper safety limit - air should usually be much dryer than this. At 85% RH you may need a drop in temperature of typically 5 to 10 degrees F to precipitate water out of the air. Not something you usually want to encourage.
Just cooling air will increase its RH (see below)
BUT cooling in an airconditioning system will usually decrease RH as the air is usually cooled so much that it comdenses water out of the air inside the air conditioner and then mixes the drier cold air with room air to produce a micture of lower RH than before.
Air can range from fully dry with no water content = 0% relative humidity (very rare) up to fully saturated with water vapor so that it cannot hold any more = 100% relative humidity.
The absolute amount of water in air at a given humidity will vary with temperature (among other things).  As the air is cooled the relative humidity will rise. If temperature continues to fall a point will be reached where RH = 100% and below that temperature water will be "condensed out" as liquid water.
As 100% RH is approached any inhomogeneity in the air or temperature  may result in some condensation at some locations so it is safe to stay away from the 100% boundary if it is desired to prevent any condensation occurring.
85% RH is a moderately safe upper limit if liquid water is to be prevented from being precipitated out of the air.
More information available on the technical aspects if desired.

100% RH can occur in clear air in a normal room or in open air.
In situations where air is relatively moist - possibly near sea or lakes or rivers or after rain, if temperature drops suddenly RH can go to 100% and condensation can occur.
  The classic example is taking a camera from a warm room outside when the outside air temperature is much lower. The misting which instantaneously occurs is condensation from the air. Mechanisms can be discussed as it may not be intuitive.
Some "light reading" :-)
Wikipedia Psychrometrics
Consider the chart below. Some of the arrows are somewhat drunken and the graph is not quite square as supplied - minor details which don't affect the points being made.
This is called a "Psychrometric chart. This tells you more than you want to know about the relationships for moist air between water mass, air mass, temperature, Relative humidity, energy and a few more things. I'll simply explain how it relates to the current question without any explanation of the underlying issues. A very important point will come out of this.
Line A B C D is a line of constant mass of water per mass of dry air. ie during the path from A to D water is not added to or lost from the air.
Point A is the starting point - 98F (98 Fahrenheit) - follow blue arrow down to temperature scale, and 40% Relative Humidity (RH) . This means that the air only holds about 40% of the water it could at this temperature.
The air is now cooled and we follow the path B C D. At B the temperature is ~ 86 F, at C it's 76F and at D it's about 69F. As we go from A to D the RH increases. It's 60% at B, 80% at C and 100% at D. If we cool this air below 69F water will start condensing out.
 You are unlikely to want this air inside your equipment when this happens.
A key point to note is that at 85 RH the temperature is about 75F.

This is only 6 degrees F above the temperature at which the RH reaches 100% ( = 69F) and water starts condensing out.

Even the original 40% RH = low is only (98-69) = 29 F above condensation point.
The important points to note are:

Even quite dry air can result in liquid water condensation with quite modest drops in temperature.
While the specification sheet allows for 85% RH non condensing this is an absolute safety limit and RH should be kept much lower. At 85% RH a drop in temperature of around 7 F can cause condensation - even less under some other conditions. (eg about 4 degrees F drop at about 55F.)

Even more useful, and complex, version here

Play with this:
Online Psychrometric calculator

Useful
Comment: @Jack - 100% RH can occur in clear air in a normal room. In situations where air is relatively moist - possibly near sea or lakes or rivers or after rain, if temperature drops suddenly RH can go to 100% and condensation can occur. The classic example is taking a camera from a warm room outside when the outside air temperature is much lower. The misting which instantaneously occurs is condensation from the air. Mechanisms can be discussed as it may not be intuitive.
Comment: So, can I assume that the 100% relative humidity is actually dark cloud that is going to start to rain?
Answer: It's hard to be sure from the way you phrased your question, but the way I'd interpret it is that the camera you're referring to should only be used when the humidity is less than 85%.  A/C reduces humidity so that shouldn't be a problem.
Comment: You could check the weather reports or carry a [hygrometer](http://www.amazon.com/b?ie=UTF8&node=13825071) everywhere you go. But I don't think this is a typical threat for most conditions — it's when you're in a tropical, high-humidity area for an extended time that you should take precautions.
Comment: But I would I know when the environment has a humidity of less than 85%?
Answer: It means do NOT operate the camera if the relative humidity is over 85%. Some water moisture may get inside and do bad stuff.
Note, you can easily cause a camera to be exposed to much higher relative humidity. Consider shooting in Alaska in the winter. You keep the camera, and yourself in a nice warm lodge. Inside the heated lodge, the RF is very low, maybe 10% Then you go out and shoot. The camera will cool down from the cold air (0 F is common) and the warm air that is inside the camera and lens, which was 10% is now colder but has the same water. The RH will skyrocket.
You can screw it up again, taking the cold camera into the lodge. All the water in the lodge air will condense on the camera.
Answer: It means when they did their humidity testing at 85%, they raised/lowered the temperature slowly to not cause any condensation during the test.
An environmental chamber has typically two set points, one for temperature and the other for humidity.
It achieves the humidity level by simply steaming clean water (as tap water can damage the equipment, but that's out of the scope of this discussion) and transfer it inside the chamber.
If the temperature rises or drops too quickly, it can cause condensation inside the chamber, which is not what the test engineers want (who knows what's gonna happen when the circuit boards are covered with dew), so they tested the circuits in a condition that has 85% humidity in the air, yet no water droplets present on the circuit boards.


Are darknets in the internet like Tor and i2p similar to darknets implemented for security like the CAIDA network telescope?

Question: I am planning to implement a darknet in an organizational network to work alongside Snort to detect malicious scanning activities in the network like the CAIDA network telescope/Internet Motion Sensor. I am a little confused with the darknets like Tor and Freenet which are in the internet and the concept which I want to implement. I know that there is a difference between them in terms of functionality and implementation, but will it be wrong to categorize both of them as darknet? I couldn't find a proper distinction between them anywhere.
Comment: Tor is "a circuit-based low-latency anonymous communication service" which operates using a protocol known as "onion routing". I2P is similar and variation known as "garlic routing". In essence both systems work by setting up a number of nested encrypted tunnels between the host, participating servers and an exit node. See http://www.onion-router.net/Publications/tor-design.pdf
Answer: TLDR:
 - The two are distinct concepts (as you noted)
 - The use of "darknet" in both cases is coincidental
 - As of 2015, "darknet" would be much more likely to refer to a private overlay network
In full:
In their paper about operating the Internet Motion Sensor (Bailey et al, "Practical Darknet Measurement") refer to the unallocated IP range as a portion of "darknet" and cite the website for a similar malware distribution monitoring system "The Darknet Project" (http://www.team-cymru.org/darknet.html) as the source of the name. They in turn cite no source and appear to have coined the term themselves.
In both cases the descriptions are clear about their use of the term as unallocated address space and do not relate it to the other common definition; an overlay network that is not directly accessible by standard protocols and applications as used to browse the rest of the WWW.
This appears to be simply a matter of the same name appearing independently in two contexts. The paper by Bailey et al, for instance, was written in 2006. At that point the word "darknet" was still largely up-for-grabs (the concept of Tor had been presented in 2004 and was not yet as well-known as it is now). That is no longer the case as the use of the word to describe an overlay network overwhelmingly dominates public consciousness. In research circles specifically focusing on internet traffic monitoring the use of "darknet" to refer to an unallocated IP range would likely still be understood, but I would personally avoid it in favour of one of the alternative names.


How is mining of bitcoins done

Question: What makes this mining process so computationally intensive and can this mining activity be substituted by other hard labors in physical world?
Does this mining effort alone adds intrinsic value to the bitcoin like gold?
Answer: The mining process is similar to a brute force attack and, assuming you already appreciate the nature of such attacks, mining is computationally intensive.  More specifically, mining involves repeatedly, and as fast as your computer allows, guessing at the solution to a problem that cannot be solved quickly by the human brain.  There are now dedicated computers, made specifically for the Bitcoin community, able to make billions of guesses per second.
The mining process is hard by design as it does make the Bitcoin generation process harder, almost like mining for gold.
The mining activity cannot be substituted by other hard labors in the physical world.  BUT, you can buy Bitcoins on an exchange with the dollars you earned in the physical world.
Comment: thanks , but why not try to solve the problem with a creative algorithm rather then brute force just to simulate hard labour? If algorithms are invented that solve the problem without much effort (P=NP) then this system will fail?
Comment: You might enjoy https://en.bitcoin.it/wiki/FAQ where the proof-of-work issue is linked to security. My understanding is that the entire system relies heavily on the decentralization of 'power', in this case the power to solve complex problems. If the problems could be solved more easily the official 'record' of transactions could be altered/subverted more easily. I acknowledge that this answer is now in the realm of interpretation.

How is mining of bitcoins done


How to change network connection order?

Question: I have multiple network access in the same area.
How can I reorder the priority to which the connections are made ?
Answer: Go to your System Preferences → Network.
Alternatively, you could click the WiFi icon in the menu bar, then choose Open Network Preferences.
In the preference pane, you can make your Mac prefer Ethernet connections over WiFi, for example, or the other way around.

If you click the "settings" symbol next to the + and - buttons, you'll be able to select "Set Service Order":

If you continue to WiFi → Advanced you can set the order of WiFi networks specifically by dragging them around:
Answer: System Preferences → Network → Wi-Fi → Advanced
Answer: Go to System Preferences>Network>Advanced>Wi-Fi:

As you can see, you can drag the networks into the order you prefer.
Comment: The "drag in progress" screenshot is a nice touch.
Answer: Open System Preferences, and select the Network preference pane.  Once opened, select the Advanced button.

From within the resulting sheet, you can drag the order of your networks around to fit your needs.

Once done, make sure you hit Apply on the bottom right of the Network pane to save the changes.
Answer: Once you've accessed the Network>Advanced pane that allows you to reorder the priority of your network list as described in this thread, then drag your desired network connections to the top of the list to give them priority over others.

How to change network connection order?


Feynman diagrams vs Feynman integrals?

Question: It is well-known how one can translate a (physical) Feynman diagram into integrals of kind:
$$I(p_1, \dots, p_n) = \idotsint \prod_{l=1}^{L} \frac{d^D k_l}{(2\pi)^D} \frac{\text{scalar products}}{\text{propagator factors}},$$
where the propagators are of kind $D(q, m) = q^2 - m^2 + i0$ for $q$ a linear combination of loop and external momenta. However, is the converse true, i.e. that an arbitrary Feynman integral can be depicted via Feynman diagrams?
Comment: Mainly, because diagramatic Feynman integrals are better studied and have properties, which the general integrals may not have. For example, for the diagramatic ones is known that they admit Hopf algebra structure, as hinted in http://arxiv.org/abs/hep-th/0202110v3 or http://arxiv.org/abs/1504.00206 ...
Comment: Well, what definition for "Feynman integral" do you use that's not "integral obtained from a Feynman diagram", so that this question isn't tautological?
Comment: I've seen many definitions. For example, in a book about Feynman integrals by Smirnov they are defined as integrals, whose integrand is a product of propagator-like factors, i.e. $E(\{k\},\{p\}, m) = \sum A_{ij} \, (q_i \cdot q_j) - m^2$, where $\{q\} = \{k\} \cup \{p\}$ denote all loop and external momenta (and $A_ij$ are parameters).
Comment: Sorry for offtop, but could you maybe explain a little why this question is important to you? Because it is much like asking if any hat can be used as a boomerang.
Answer: There are Feynman integrals that don't come from diagrams. These integrals occur naturally when trying to use tensor reduction to remove the numerator in some Feynman integrals. The generalization of a graph/diagram is called a Matroid and in Feynman integrals (as you described them) where the momentum flow in the denominators can not come from a graph, it can come from a matroid.
For more details, see:
Kreimer, Dirk, and Karen Yeats. "Tensor structure from scalar Feynman matroids." Physics Letters B 698.5 (2011): 443-450. [link]

Feynman diagrams vs Feynman integrals?


Adding goals to reports/dashboards?

Question: i am setting up some reports for the creation of dashboard that show revenue etc vs. goals for different time periods. I am currently working on quarterly revenue vs. goal broken down by Account owner.  I set up a matrix of opportunity owner vs. amount for closed won opportunities in current quarter.  I have this displayed as a bar chart and want to be able to add a goal value for each of the 5 opportunity owners in the report and compare that to actual revenue. What is the easiest way to add these goal values?
Answer: Unless you already have a field in your data that contains the goals or some "factor" used against a previous period's closed-won opportunities that's been set for each user, there isn't an easy way to do what you're asking.
The only way I can possibly think of would be to create a formula field based on an existing field and you might be able to use that. However, you'd need it to operate on a custom field unless each account owner's goals used the same constant as the "factor" to use on the reference value you'd use for creating their goal (e.g. Amount of previous period's closed-won opportunities) since the formula would apply to all of your account owners unless it was contained in a custom field for each one (or perhaps each account).
In the Reporting Workbook or other documentation, take a look at "Create a Formula using the PARENTGROUPVAL Summary Function" (Tutorial 3, step 2) and " Create a Formula Using the PREVGROUPVAL Summary Function" (Tutorial 3, step 3) as those would seem to be the formula functions that would apply to what you're trying to do.
Answer: Have you seen this tip on Creating Sales vs. Goal Reports and Dashboards in the Salesforce success community? https://success.salesforce.com/answers?id=90630000000hs6gAAA

Adding goals to reports/dashboards?


Taxes and childcare expenses deduction for a dual-income couple in Alberta?

Question: We live in Alberta and need to know if we should obtain a receipt for childcare.  We paid $2400 last year.  My common law husband made $49000 last year and I made $28500. Is this worth it?  Will our childcare provider be taxed as well?
Comment: Related question: http://money.stackexchange.com/questions/1034/child-care-expenses-how-much-deduction-can-be-claimed-on-tax-return-canada
Answer: Generally speaking, child care expenses are deductible on the tax return of the spouse with the lower net income, and at your income level, a $2400 deduction would be worth a reduction in taxes of $600, on your 2011 tax return.
So if you'd like to reduce your taxes by $600, do ask for that receipt – and yes, a receipt is required.  Refer to Canada Revenue Agency's Form T778 - Child Care Expenses Deduction for 2011.  Quote:

Supporting documents – The individual or organization who received the
  payments must give you a receipt showing information about the
  services provided. When the child care services are provided by an
  individual, you will need the social insurance number of the
  individual. Do not send receipts with your return, but keep them in
  case we ask to see them. If you file your return electronically using
  EFILE, show your receipts to your EFILE service provider.

Whether or not you get a receipt (and if you don't, you forgo the $600), your childcare provider is still required to declare their self-employment income, and their net income (i.e. after any costs incurred doing the business) would be subject to income tax.
If you think you'd be doing somebody a favor by not asking for a receipt, you'd potentially be helping somebody commit a tax fraud.