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12:00 AM
This must be one of the more controversial questions in terms of voting.
@tb at least three times. It could be 4 or 5 hundred :-)
@robjohn I mean that I always forget this kind of formulae so I need to calculate them from the Sarrus rule, wish there was some fast visual way...there must be...
@tb +4/-5. It does appear so.
the Sarrus rule is a visual way...
@MarianoSuárezAlvarez I just looked that up. I had never heard it referred to by any name. Nice to know what it is called.
12:03 AM
Is there any other way to remember that formula?
use it for a few hundred times
@tb hhh is using that rule to remember how to do cross products.
alternatively: view the $3\times 3$ matrix as the points of the affine plane over $\mathbb F_3$: then the terms in the determinant correspond to affine lines which are neither horizontal nor vertical
and there is some way to pick up the sign from the geometry, but I don't remember it
@robjohn Oh. this.
@tb yes. He has a handwritten image that he was using earlier, but that is the rule.
12:07 AM
Aww. I lost 20 points due to a user removal.
@AsafKaragila Never mind, 20 is a small matter.
I know.
@MarianoSuárezAlvarez Thanks, Mariano, I'll have think about it. But I presume I'll still be doing my zigzags 40 years from now. :)
@AsafKaragila darn them! don't they know that we'd rather see their carcasses out there than to lose the upvotes?
@robjohn Yeah! Those jerks!
12:09 AM
@tb Good LaTeX advice. The sentence does not sound right though. "to prevent the period from being moved to" is the minimally adjusted correct sentence I believe.
@tb do not take that too seriously, though :)
@hhh Draw a triangle with vertices i, j, k with arrows i->j, j->k, k->i. Multiplying two of these units is done as follows: if they are the same, 0; if they go with the flow of the triangle, the third in the line; if the go opposite the flow, the third one as well but with a minus sign
@tb that's how I do cross products. I just remember that if it is an even permutation of $i,j,k$ that the sign of $i=j\times k$ and $i\times j=k$ is right.
@anon that's pretty much it :-)
@WillHunting Thank you for the correction!
@robjohn I'm simple-minded: I just remember that the middle line has a minus in front.
$\begin{align}i&=j\times k\\j&=k\times i\\k&=i\times j\end{align}$
just cycle the permutation
12:15 AM
@AsafKaragila 50 here :)
@tb I wonder who that was.
No clue.
And why did he only upvote two answers of mine and not 500 of them??? :-P
whoever it was who left was the one who downvoted me. I lost 52
If it's any help, I point my right thumb in the direction of the first vector, and my right forefinger in the direction of the second vector. My right middle finger can then point in the direction of the cross product.
12:17 AM
@AsafKaragila Because he did not want to be suspended.
@anon @robjohn @MarianoSuárezAlvarez I gathered the ideas about cross-product to this q here.
@robjohn You'd lose 48 if that was the case, -5+1
@AsafKaragila Well, probably the payback for the Cantor set fluke I had yesterday...
@AsafKaragila no, the downvote was absorbed by capping, but then he removed the downvote and I got +2.
@robjohn But note that after capping one still can be downvoted. Order of voting is not commutative.
12:18 AM
@tb What fluke?
@AsafKaragila the one where you explained to somebody that $[0,1]$ was perfect.
Oh, that.
@WillHunting Yes, and that is why he counted for a total of +2 for downvoting and removing the downvote :-)
Robert Israel.
I know, I was trying to politely gloss over a goof, but I should have known better :P
12:20 AM
@WillHunting when you are downvoted after capping, the downvote is absorbed by other votes that were previously uncounted due to capping.
@robjohn Maybe I recalled wrongly or the system has changed.
@tb Either way, I think it's pretty effin' cool that the complement of the Cantor set is a union of countably many disjoint intervals but has uncountably many boundary points.
@robjohn then that has changed.
@tb It happened to me a few days ago, so the net effect of the downvote and its removal were +2 rep.
I believe you. :)
12:23 AM
There is probably no way of knowing who left.
I'm not sure what closing reason fits best.
too localized. What else?
Off topic?
Whatever, it'll be closed soon enough, I assume.
Not constructible?
12:24 AM
It can even be not a real question as it is overly broad.
People such questions on line to random strangers...
There are 9000 reasons to close such questions. Have your pick!
Oh, nice! Thanks, @hhh!
I am going to go with Off Topic.
One of you guys should vote to close as NARQ and the other as NC; then we'll let the fifth user find a duplicate! :-D
@AsafKaragila I had gone with too localized
12:30 AM
@robjohn Yeah, there's a second vote with Off Topic...
We should totally run an experiment what happens when five users vote on five different closure reasons.
Just as I was about to leave to walk my dog, I have a warm kitten snugged on my chest.
Also, it would be good if we can derive the corollary "Why can't I finish this effin' Whitehead thingie already?!!?"
@robjohn Do you have a really hairy chest?
moderately, but my shirt renders that moot.
@robjohn Moderately hairy, or moderately really hairy?
@AsafKaragila moderately hairy.
12:34 AM
Must be those Iraqi genes of yours.
@AsafKaragila my beard is what is really hairy, and why I've not been clean shaven for 35 years. :-)
@robjohn Are you shaving your chest?
Because Jerry Seinfeld did it in that Muffin Tops episode.
@AsafKaragila no :-p
Good. I was starting to think that you do everything Jerry did on that show.
@tb Yes not bad way to preprocess ideas first in Chat and then try to build quality threads that way -- making threads many times more deeper (and killing stupid qs) :)
12:36 AM
Next you'll find yourself in prison for not helping a really fat guy when he's being mugged.
@hhh I'm going to have to leave and remove my kitten. I am pretty sure that she will forgive me.
@robjohn Feel free, thank you for your help.
@Asaf, there is no such thing as Iraqi genes, everyone has exactly the same genes. Stephen Gould told me so
@EricGregor Obviously he was hoping to have the "immortality gene", but clearly he didn't have it.
@AsafKaragila I've lost 30 points.
Oh more people did.
12:44 AM
how much of a dopamine boost do you get per 10 points? what does this graph look like?
is there an easy way to see why a linear operator that has a left inverse and a right inverse in fact has a two-sided inverse?
i guess it is obvious if $|| I-T||<1$, where $I$ is the identity operator, that $T$ is invertible with two-sided inverse
there is an exercise in folland to this effect
@EricGregor that's a purely algebraic fact. Call $l$ the left inverse and $r$ the right inverse. Then $l = l(fr) = (lf)r = r$.
oh, i was forgetting that the set of operators forms a group
thanks @tb
No, it's not the group property, only the monoid property of multiplication
it's a ring (or an algebra).
yes, you are right
1:00 AM
Oh, they just stole an hour :)
1:10 AM
Now I almost finished with this.
I have skipped one proof and did not yet prove the independence of choice of resolution for Ext(A,Z).
However except those two I only have to write the set theoretic parts, which are easy peasy.
1:30 AM
How can I add a footnote in an answer? Like

\footnote{$(A+B)\cdot (A+B) = A\cdot A + A\cdot B + B\cdot A + B\cdot B$. The cross terms are $A\cdot B$ and $B\cdot A$. This has nothing to do with the cross product.}

I want to put a small notice as footnote, ideas?
This a here on which I am working on.
You can use '<sub></sub>' text on the bottom part, separate it from the main post with ----'s
@AsafKaragila How to make a reference to that then?
it is usually best not to use <sub>, for most browsers will render the text in a very small font
\tag{...} \label{...} -- perhaps.?
not suitable for long texts
1:34 AM
Add [1] and such to the text (or ^1 in LaTeX) and just use an ordered list in the bottom.
Much like I write bibliography in my longer answers.
e.g. in here
1:50 AM
@hhh: When editing the OP's comments into their answer for them, you might want to mention it's the *OP*s comments, not yours, in the suggested edit description box. Generally I (and others I think) would reject an edit that put someone's personal comments into someone else's answer, so you risked getting your edit rejected. Also, I still don't see how category theory is relevant to pictorial mnemonics. :)
Can anyone help me with this: math.stackexchange.com/questions/124108/…
just looking for another angle of attack on a problem i can already solve
@anon Yes that is good point, I wish I could somehow clarify that it was his comments. At the time of writing, I thought it was obvious.
@anon Wish I knew that :P (I just know that it can be done if really wanted...but sure perhaps a bit over-engineering, well hopefully some gets more inspired by my comments :P
@anon: now I tried to apologize the op due to my mischief, now better? Here. (hopefully the term "mischief" does not infer intentional because it was accidental choice of vocabulary)
@hhh: You've misunderstood me; there was zero issue with what you did, in fact it was nice of you to help out the answerer and it was totally clear to those reading the answer and its comments what was going on. However, the people who review suggested edits (e.g. me) don't get to see this context, so I didn't know they were the OP's own comments until I investigated your suggested edit. I only wanted to forewarn you for your own sake that you risked getting in rejected because a reviewer...
...might not have bothered to check what was going on like I did.
@anon What is the correct terminology here to use? Look I asked the question, can I still use the term "op's comments" (not referring to myself) when I want to make answers to my questions better/clearer? Perhaps in the future, I will use the term "answerer's comments" -- is that safe?
I would use "user" rather than "op," because the latter generally refers to the question-asker (even though technically it is still applicable to the user who submitted an answer).
2:05 AM
@anon "op"'s techical origin is?
origin + ?
Origin Player = OP
OP means "original poster." what the post being referenced is is a bit ambiguous though.
Please only use it to mean the person who asked the question.
2:30 AM
Am I being dense or has azarel totally missed the point of my question: math.stackexchange.com/a/124113/21725
Like you, I can't see that azarel's answer either answers the question or uses the hint.
thanks for checking it out for me @DavidWallace
Hello. I lost 10 points because "User was removed". What does that mean?
probably the user who voted you up quit or was removed for spamming the site
i can give you 10 points if you want
2:46 AM
Thanks eric. I just wanted to know what happened.
despite the setback i predict a bright future for you with lots of points rolling in
@JohnSmith Yeah, I lost 52 because of it.
@EricGregor: Thanks for the laugh! You made my day!
I do pop in occasionally to ask a question or two. It's just that I'm not doing research math much these days.
i couldn't upvote you because i'm only on math.stackexchange
@EricGregor: you're an analyst?
or a geometer?
2:53 AM
@JohnSmith i'm learning analysis, i'm just a student
We're forever students....
i'm just a student who talks to my betters too informally
you only stop studying when you stop breathing.
or are too drunk
@MarianoSuárezAlvarez: Don't drink and derive
2:55 AM
180 @MarianoSuárezAlvarez
what game is this?
pick a number
180 is the highest score you can get on the LSAT. it's used sometimes on the internet to mean exactly/perfect/well done
2:56 AM
at least at certain haunts i frequent
it only gets the point across the the relatively small subset who knows what LSAT is, though :)
never did lsat: one less lawyer in the world :-)
the L is for lawer?
or legal, I guess
you're going into law?
how on earth does one end up with a scale 0 -- 180?
you have to be militantly non-metric
2:59 AM
@MarianoSuárezAlvarez: Farenheight's relative perhaps
that scale is based on 180
temperatures are weird
and a special case, except for Kelvins
what is the highest possible math subject GRE score?
that can be the slang around here for well done
800 I think
USians have too many exams :)
all with different scales, afaict
@EricGregor it won't work. Surprisingly few people here care
3:01 AM
care about what?
This is the first time in 15 years I've heard mention of it.
why surprisingly?
no one has to care. the point is it's a funny thing to say when someone says something funny or astute
and it's easier to write a number than some variation on that
Don't forget that lots of non-Americans use these sites.
3:02 AM
my point is that most people won't get the joke
180 for me only connects with a half-turn
i guess you could always say "fields medal"
but so few will get it even if you explain
I don't know what an LSAT or a GRE is; and certainly couldn't tell you (until just now) what each of them is scored out of. Can't we say "100%" or something sensible?
3:03 AM
fields medal, @DavidWallace
Thank you, don't mind if I do.
that's even weirder than 180...
I WROTE the GRE and I can't tell you what the scale is
give it a try, it will catch on
@EricGregor: fields medal is good. I aprove
3:04 AM
@JohnSmith - you know you can use up-arrow to edit. For a minute or two anyway.
i have to go afk for a bit. if anyone can help me here math.stackexchange.com/questions/124108/… i would be much obliged. even an upvote to give it more attention would be appreciated
see ya
@DavidWallace thanks, it works!
@DavidWallace: you never ask questions, omnipotent?
anyone know about transitivity in groups?
@JohnSmith I have never asked one yet, on any site. But it has been suggested to me, after a recent exchange, that I should ask one on linguistics.se.
I wish I was you. The only reason I came was to ask question
Usually, if I want to know the answer to something, I want it immediately. If it doesn't already exist on Stack Exchange or Stack Overflow, I go and look somewhere else.
Which is kind of selfish of me, and unfair to future users who might want an answer to the same question. I make up for it by answering more than my fair share of questions.
3:13 AM
i joined via stack overflow because the question I had had never been asked before.
and did you receive a satisfactory answer?
for that question yes. But there is an occasional stupid answer
There always is. Often though, answers that are not themselves complete contain a grain of truth that can lead one to a correct answer.
Occasionally, someone will post an answer to their own question, after they've read the others and thought about it a bit.
@DavidWallace most all answers seem to leave something out. i usually get a part of the answer from each posted answer
The stupid ones for me are those where the answerer accuses me of not understanding the question.
@Jeff I wish there was a way to split rep among the answers
3:20 AM
@JohnSmith - I just looked at your first ever question. It looks like you got three really good answers, which was fortunate.
@JohnSmith agreed. but that's been discussed here and rejected
@DavidWallace: on math?
Umm, on StackOverflow. The thing about UIView and NSTimer on iPhone not releasing memory. Was that not your first ever question?
I just upvoted all three answers and the question, by the way.
@DavidWallace: yep and I lost a lot of hair on that question.
I learnt there that most books on Objective-C programming are crap.
I can imagine (about the lost hair). I'm glad I don't develop applications for mobile devices.
3:24 AM
I don't
It was just a hobby project, which got me a job
@DavidWallace you wanna see a video of it?
Umm, I'm not that curious, sorry.
But post the video if you want to. Somebody here will watch it, and make kind remarks about it, I'm sure.
1 hour later…
4:45 AM
Hi @David
5:33 AM
Hi Rajesh.
Watz up
@David : If i click on your photo icon in the chat it shows that your primary account is on StackOverflow, but it showed the other day as math.SE and even you said that you never posted anything on stackoverflow
I have never posted any question on any SE site. SO is the first site that I joined. I don't believe it would have shown up as Math.SE.
Sorry if I seem taciturn, Rajesh, I'm kind of busy doing some work in another window.
6:11 AM
Hm, -10 from a user being deleted. I wonder who it was.
Everyone here has been speculating on this.
6:39 AM
Q: Sum of two closed sets in $\mathbb R$ is closed?

ro44Is there a counterexample for the claim in the question subject, that a sum of two closed sets in $\mathbb R$ is closed? If not, how can we prove it? (By sum of sets $X+Y$ I mean the set of all sums $x+y$ where $x$ is in $X$ and $y$ is in $Y$) Thanks!

6:57 AM
@DavidWallace I know the 6 answers of mine that the deleted user upvoted. I see no trace except there is one comment by user13838, who no longer exists.
7:08 AM
Yes, I think the basic algorithm is wrong. If someone's reputation points are supposed to reflect reputation, then the votes (up or down) of former users should count just as much as the votes of current users.
Presumably then, when a person quits the site, all their questions and answers disappear. If the purpose of SE/SO is to compile a database of questions and answers, then this is the completely wrong thing to do. Imagine if somebody set fire to everything Isaac Newton ever wrote, on the day that he died. Or Thomas Jefferson. Or anyone else who's made a great contribution to the world.
@DavidWallace No, the Q&A don't disappear!
OK, that's good.
7:23 AM
Nice one. Reflects my feelings at best!
1 hour later…
8:51 AM
Hi all of you
@KannappanSampath Haha, then you are full of feelings!
@WillHunting : )
@KannappanSampath But sometimes I accept an answer even if it does not really solve the problem, as long as it is good enough.
@WillHunting solve the problem means solve your particular problem or it solves your problem but not fully? In the latter case, I see no issue, in the other case, I think you may have to think if you should really wait!
@KannappanSampath, Good morning Kannappan! Or afternoon as it may be in your case.
8:56 AM
Good morning, How have you been? Did you do that exam well?
Yes, it is the reason I am in such high spirits today! I aced it :)
@FortuonPaendrag @FortuonPaendrag Great!!!
I will interpret that as a very vigorous 2 hand shake
but, thank you
Yeah, kind of!
How was your week? Did u celebrate ugadi?
9:00 AM
No, I do not celebrate it, it is a kind of regional: It is widely celebrated in Andhra Pradesh, but not in Tamil Nadu!
Hi @Mariano.
Ah, I see.
Finally Will stabilizes!
@MarianoSuárezAlvarez ayt? I need a mod.
Oh, something serious?
Hi @Matt
9:05 AM
@MarianoSuárezAlvarez Can you turn this into CW before I get any more votes?
the answer?
The answer can be CWified by yourself!
@KannappanSampath Hi
@MarianoSuárezAlvarez Lovely, thank you!
@KannappanSampath How? I tried to edit it more than 10 times but that didn't work.
9:07 AM
No, you just click on edit and search for a check box that lets you do it and check it!
@KannappanSampath Doh. I see. Thanks : )
Ok, later folks. And thanks!
10:08 AM
@DavidWallace I think the formula in question is right!
So I can't differentiate?
OK, I'll try again.
Upto some additive constants I guess, the formula is right.
@DavidWallace Oh, sorry. I did not mean to say that! Probably, you are worked up by something else and are not giving this what it needs! : )
@JoeZhou what do you mean by =_Q in your recent question?
Nothing to apologise for; if the formula's right, then it's right. If this is the case, then I'm glad you pointed it out. I have removed my comment from the question.
Yes, I've just managed to prove that the formula is right.
Thank you @Kan for pointing this out. You've saved me from making a complete fool of myself.
Now all I have to do is figure out why the OP got the wrong answer!
10:17 AM
I think OP must have naively assumed $x=2t$ and failed to notice $dx=2dt$.
Just a factor of $2$ at some place tempts me to say that!
I now can't view the question properly. MathJax is screwing up. Does it work OK for you?
and more to the point, is martini's answer correct? Because if it is, I won't bother trying to answer the question.
MathJaX works fine for me.
And, yes martini's answer is right.
OK, thanks again for the save.
I'm off to bed. Good night, Kannappan.
Good night! Sleep well!
1 hour later…
11:28 AM
@robjohn see here for who it was.
11:38 AM
@WillHunting Hi
@Skullpatrol Hi!
@WillHunting I like the new look of your avatar.
@Skullpatrol Where does the name "skullpatrol" come from?
From a guy named Skully.
@WillHunting Your back!
12:13 PM
@robjohn Are you too busy to talk about the OED definition of "trolling?"
12:26 PM
@tb I assume that that answer was one that got downgraded. This adds to my suspicion that it is user13838.
@Skullpatrol why worry about the OED definition. If people don't like something, it doesn't really matter what it's called; it's unpleasant to them.
@Skullpatrol That is a 6 year old draft addition. Did it make it; was it changed?
@robjohn That draft is what is currently available on the OED web site.
@tb This answer was downgraded and it also has a comment from user13838.
Some other dictionaries list that sense, too. For example, Chambers (the 11th edition) has this: "intransitive verb To make a conscious attempt to provoke controversy or disagreement on the Internet", Merriam-Webster's Advanced Learner's Dictionary says “a person who tries to cause problems on an Internet message board by posting messages that cause other people to argue, become angry, etc” for troll,
Cambridge Advanced Learner's Dictionary says “to leave an insulting message on a part of the Internet for no reason”, and NOAD has "send (an e-mail message or posting on the Internet) intended
@Skullpatrol why worry about any definition. If people don't like something, it doesn't really matter what it's called; it's unpleasant to them.
@robjohn I agree.
12:37 PM
And what I gather from all those definitions is that there are many definitions of 'trolling'; none of them seem real positive :-)
@robjohn There are 66 more in the Urban Dictionary ;-)
Definitely a word that people like to use, so they like to define it.
not to mention Wiki
@robjohn Hm, do you think the answer is correct now?
I mean this.
1:05 PM
Could someone explain why I am missing one term there? (Notice the question mark) Related to polar coordinates and partially things we have done with @robjohn.
@robjohn From the thread I linked you to you can easily infer that the user's handle was percusse. Mainly linear algebra and quite a bit of editing
Only true case is when $\frac{\partial_r}{r}=0$ but that is non-sense -- is it? Must be some err...
Hi @tb
@robjohn And to add to that, I got a nice answer badge from that thread and I know I got a vote from that user.
Hi Asaf
1:14 PM
Hi all of you!
Why is Will's browser crazy or is he? : )
@AsafKaragila How can one know about the Borel graph theorem without knowing about the $1-1$ correspondence between selectors and transversals?
@KannappanSampath Do you know any hard problems involving simplifying expressions, and factorizations?
I am having problems finding difficult ones.
@tb Beats me. I think we just talked about some theorem that says that if a set is $\Pi^1_1$ (lightface, of course) then it can be used as a selector kind of set or whatnot.
My memory faded, it was like two weeks ago...
I see. But the annoying thing is that MG's answer is already accepted...
@N3buchadnezzar I am not sure if I can get anything on the top of my head. But, yes, I can if I got to my house. Look up some of the books I solved when I was young! : )
1:18 PM
Gotta go again.
@tb I concur.
Should this be closed as dupe of this?
Hi @anon
OK. I presume, this is not the right duplicate! I'll write an answer!
Moved my q here.
1:23 PM
lol, I was typing up an answer in chat. looks like I'll get points instead
@anon LOL!
@KannappanSampath I was thinking of problems along the line of $x(x+1)(x+2)(x+3)-1680$
@N3buchadnezzar Hmmm. Possible, but I would not like to trouble people with radicals. Numbers should be nice when done in the right way! : )
@KannappanSampath And that problem is! Clever factorization leads to a polynomial of second degree and so on.
@N3buchadnezzar Right, I have not thought enough, but if you can wait for a month or so, when I go home, I can let you know of some good ones!
1:29 PM
Or problems such at $\sqrt{3 + 2\sqrt{2\,}\,} = 1 + \sqrt{2}$
@KannappanSampath Thanks =)
@N3buchadnezzar This is a nice one. I have a good collection of related ideas at home!
I have started making a small list of my own
Not much time for it though, because of school ^^
Good, I am typing up an answer. BRB.
1:44 PM
@AsafKaragila Did you look at that call for Duplicate, I feel guilty for having to retype the same argument, I'd like if you please went through and told me if they are the duplicates of each other or not!
Regarding the permutation questions, the older question you linked seems much more general than the recent question, and at too high a level for the op (probably).
Oh, I see. Then, I'll not vote to close that question. But, essentially, argument is all the same!
(I have used that question, thrice until now!)
@anon Thanks for telling me!

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