@0celo7 The Lebesgue measure of a cube is just its volume. No one actually computes the measure like this

I think this book dissuaded me from a math minor.

@ACuriousMind Take an ellipse then!

Yeah, it's still the volume of the ellipse :D This is why physicists don't like being rigorous all the time, there's no point

I have more questions about these 12 pages than the entirety of GR texts I've read...

@ACuriousMind Lol what is the point of the Lebesgue measure then?

If we just define a metric space and use geometry for everything...

@0celo7 ::shrug:: We already told you nobody in physics really cares about how these integrals are defined.

This reminds me of the 4chan pic where some guy suggests a math curriculum with analysis in 10th grade.

You'd have a bunch of dead teachers.

@ACuriousMind Alright. Why is $$\{x\in\Omega:f(x)\le t\}=\bigcap_{j=1}^\infty\{x:f(x)<t+1/j\}$$ true? I don't see how the equality comes about. It is just that $$\lim_{j\rightarrow\infty}\frac{1}{j}=0?$$

I think it is.

@ChrisWhite One should not forget that the mathematicians also did calculus for quite a time only with these heuristics. The rigor grew out of them trying to make the same heuristic foolproof, so it's no wonder there are parallels.

@0celo7 Yeah, I think so, but you shouldn't take my word for it - I'm not a mathematician, much less a measure theorist.

@ACuriousMind LOL'd @ "An amusing exercise is to prove the facts that whenever $f$ and $g$ are measurable functions then so are the functions $x\mapsto \lambda f(x)+\gamma g(x)$, $x\mapsto f(x)g(x)$,..."

Ah, mathy amusement is the best amusement

@ACuriousMind Random unrelated question: Do you know of an intuitive reason why the Riemann curvature is the unique tensor with second derivatives of the metric?

@ACuriousMind , @0celo7, @alarge Hello everybody: who voted for closing this question? The user showed effort and needs just a hand of help.

@Sofia Homework questions need to do more than just show effort to be on-topic, they need to ask a conceptual question. "Where's my mistake?" or "What should I do now?" is not a conceptual question.

(one of the close votes is indeed mine)

I always like it when Shog9 comes in and posts

Is it weird that I really want to get into an argument with him?

@JimdalftheGrey Yes.

@ACuriousMind Heaviside liked to math concepts that he felt had merit even though they hadn't been proved. Like applying the quadratic formula to help solve some ODEs.

Did Feynman have some neurological disease? I was watching youtube.com/watch?v=wMFPe-DwULM and he squirms a lot.

@0celo7 I don't think Slavoj Zizek has any illnesses, that's just how he is.

@alarge Slavoj Zizek? I don't get the reference.

@0celo7 The philosopher. He has certain very distinct "mannerisms".

I know who he is, but what does he have to do with Feynman? Are you just drawing a comparison?

Nothing to do with Feynman per se, just a comparison.

@alarge I need a hand of help. There is a home exercise. I gave some indications to the user, but a certain Chris Drost solved the exercise for the user. I'd like to send to this Drost a message that home exercises should not be solved. But Drost is not known here, in the chat room. Bottom line, if I'd send him the message from here would he receive it?

@Sofia no he wouldn't

but I commented appropriately on his post

On that note, I have a book by Zizek (The Sublime Object of Ideology). I tried to read it a few years ago but I found it impenetrable. His talks and the documentaries are easy to follow though.

@alarge On what note? My horribly failing attempt to crack analysis?

Haha.

Have you found any video lectures online? I suppose they'd take more time than reading, though.

@JimdalftheGrey, @alarge is it possible to invite that Chris Drost to the chat room? I gave him a minus, but I don't see e how will he understand from that, that he should not solve home work? Does he have the right to participate in the chat room?

@alarge I haven't looked, but I have not exhausted my text resources. I wish these problems were worked out, some of them contain applications of concepts I don't get.

@Sofia Everyone with more than 20 reputation can talk in chat. That we do not solve homework is already covered by the relevant policy posts on meta, though, just link them in a comment

@Sofia I linked to meta in my comment. He can participate in chat, just tell him to join chat on the h bar. Or start a new room and invite him to that

@JimdalftheGrey yes, this is what I have in mind, to invite him here.

@Sofia You don't need to invite him to chat to explain why you gave him a minus and how he should improve his answer. That is exactly what the comments are for.

^ this

Hello.

hi

@ChrisDrost the question is viable; putting the solution to the hw problem in your answer is not

@ChrisDrost Hello Chris! You see, the riles here are quite severe about home exercises.

@ChrisDrost Just advices are allowed, not solutions. I saw that you solved for him the Kinetic energy. You are not supposed to do that. This is the law.

Coincidentally, I am not very strict about home exercises. If they knew that their kinetic energy was wrong, then they probably know what the correct expression is, and even if they didn't, I killed (a) major derivation details and (b) semi-important coefficients.

! The Law !

@Jiminion Punishable by death!

@ChrisDrost by the minus, we signalize to the person who answers that he did something forbidden.

I'm sorry, @ChrisDrost. @Sofia has condemned you to the chopping block.

This is getting kinda children-of-the-corn-esque....

Since you know the answer, it is usually better to ask the OP a probing question that might help them to figure it out.

@Sofia No, something forbidden is signified by having the post deleted. A downvote signifies something is wrong or unhelpful (and should also accompany a mod flag)

Sofia: I understand that, but I stand by the existence of that equation in that reply. It contributes something especially meaningful (which is the astonishing fact that most of the terms cancel) and it is not the direct solution.

@Jiminion , @0celo7 you both calm down. I lost once 25 points because I didn't know the law.

@ChrisDrost it was proportional to the direct solution

@Sofia The term you're looking for is rule, not law.

Here we go again.

@alarge Fight! Fight!

I lost 100 points when my bounty expired. Didn't read the fine print on that one......

Ah, what has happened to the peaceful and quiet days of old!

@Jiminion Not to split hairs, but you lost 100 points when your bounty was posted.

@JimdalftheGrey The holy master has spoken. He split the hairs like a master.

ESL can sound a bit authoritative sometimes.... (We shouldn't be scaring off Chris....)

@Jiminion and @0celo7 term, rule, whatever the name, it's better that users know than to solve a problem, get points, and then loose all of that. I remember how nrvous I was when it happened.

@JimdalftheGrey well, not in general - I mean, there's no reason a mod flag necessarily has to be accompanied by a downvote (and certainly not vice versa). Though it is true that many categories of things that should be flagged should also be downvoted.

true true.

If you're asking me in the future to avoid solving homework problems that explicitly start "In Exercise ___, this happens" I'm totally OK with that. In this particular case I don't see a better way to answer this particular post. If you don't like the lack of answerability of the post that's OK; then the post should probably be closed.

@DavidZ Yeah, I thought about adding that caveat, but I figured the times you mod flag and shouldn't downvote were rare enough to ignore for now

@JimdalftheGrey I paid my money but didn't get anything. It should be refunded or at least not expire. (I ended up figuring it out myself, and couldn't even give myself the bounty.....)

@Jiminion , @JimdalftheGrey, @0celo7 , well you got the explanation from the direct source.

@ChrisDrost Now you're catching on. That's exactly the way we all think

The actual policy contains a quote, "Providing an answer that doesn't help a student learn is not in the student's own best interest, and if a solution complete enough to be copied verbatim and handed in is given immediately, it will encourage more people to use the site as a free homework service." My only point here is that I didn't do that.

@0celo7 ,@Jiminion what shall we do if the time is irreversible!? I am the last one to be pleased to give a minus. I have esteem for work done.

You could not copy what I did and hand it in verbatim, and to actually use the result you'd have to trace through the mathematics to figure out why, say, 1/2 \dot \theta^2 appears.

@Jiminion when you pay rep for a bounty, even if you get no answers, you get advertising and exposure for a week. The rep you pay is for the advertising; answers are a bonus

Well, GTG have a great weekend. Be safe and be warm.

@ChrisDrost If you think your answer wasn't a "feeding the bears" answer, then you have to convince a mod of that now. I don't have the power or authority to judge it, I merely raised the flag that it was potentially that situation. A mod came in and made the decision and it was deleted. On the plus side, that means the -1's don't count towards you

And the question probably will be closed

Are the mods determined entirely by rep?

qedstonybrook.weebly.com/the-chief-of-bali-hai.html

@0celo7 No, they are elected

@JimdalftheGrey I am sure of your good intention, but you were too hard with the poor fellow. "That's a shame" and so, why such words? He committed no crime, and didn't ask "solve for me the problem". The law here is be nice. It's a good law. And in fact we all are pleased to help.

I didn't even realize that @DavidZ had deleted it. That's sad. I vaguely remember one Math StackExchange user who just answered advanced questions by providing the (usually huge and complicated) final equations answering the problem, with no derivations. I liked her. :D

@Sofia I am nice and I'm pleased to help. It quite honestly and literally is a shame that his original solution wasn't correct.

@ChrisDrost yes Chris. This is the law here.

@Jiminion Noice.

@ChrisDrost Math.SE has a different homework policy than us. They allow that because solving math is what they're about. We're about physics, so if a question boils down to solving math, it's not our purview

@ChrisDrost it's a pity that someone invests work, and the work is lost. Unfortunately, we can't reverse the time, i.e. to warn in advance people not to do this. I also underwent such things.

@Jiminion how? .... I'm impressed. I may have to give you a title in the Jimpire for that

@ACuriousMind I'm reading another (mathematical physics) book now and it makes sense how the Lebesgue integral works on $\mathbb{R}^1$. I'm still not sure how to measure crazy shapes in $\mathbb{R}^n$ though.

@Sofia the answer is still there. Anyone with 10k+ rep can still see it. And if he successfully petitions a mod, it can be undeleted easily

@Jiminion what is the meaning of this picture?

@Sofia chat.stackexchange.com/transcript/message/20172796#20172796

@JimdalftheGrey I have hardly 3k. But I don't need that solution. Well, you are right, it is not lost. But is unavailable for the students.

@JimdalftheGrey I didn't understand the hint. What you "see" in this picture? Feynman as a monk?

@JimdalftheGrey and what he keeps in his hand? A thunderbolt?

Chris White challenged us to find a good quality version of the picture he linked

Jiminion did that

in his hand is a feynman diagram

@JimdalftheGrey Aaaaa! A Feynman diagram. Hi, hi, hi!

@JimdalftheGrey what kind of a hat he wares? Is it typical to some ancient priest?

I have no idea about anything to do with his outfit

@JimdalftheGrey, @0celo7, @Jiminion , @ChrisDrost , @ACuriousMind , @alarge , and @DavidZ to everybody Good night! I go to a movie and then to sleep.

@ACuriousMind What is the $\operatorname{End}$ operator? (In the context of linear algebra and tensor products.)

@0celo7 Given a space $X$, $\mathrm{End}(X)$ is the set of all morphisms $f : X \to X$. In linear algebra, $\mathrm{End}(X)$ is just the space of all linear maps from $X$ to itself.

End stands for endomorphism

@ACuriousMind Makes sense, given the context. Oh that really makes sense :)

@DavidZ hey, I altered the reply that you deleted and you said it was only "temporarily" deleted -- would you be comfortable reinstating it now?

@ChrisDrost no, we don't undelete homework-like answers when they're edited. The old version is still accessible from the revision history.

So it was not "temporarily" deleted then.

What makes you say that?

Because you just indicated that it is permanently deleted?

No I didn't

"we don't undelete" seems to say that. Is there any way for anyone who is not "we" to undelete it?

"We don't undelete... when they're edited "

Is there a way to modify an answer without editing it?

no

what I'm saying is, the fact that you edited your answer does not affect the timeline for its undeletion

Ah.

So what does affect said timeline, and what is said timeline?

Usually a few weeks

but it's intentionally not specified in the homework policy

Confusing. If I may ask, what made this particular example different from the ones that you've highlighted before as being "great homework questions"?

Well, that's an entirely separate issue from deletion of the answer

But it doesn't seem clear what it's asking

I guess now I am surprised to find out that my answer was "bad" independent of the sort of question that was asked. o_O

Well I wouldn't really say your answer was bad, just that it gave away too much

That's not a bad thing?

Not really. (Unless you equate "bad" with "shouldn't do on this site".)

Well, yeah. I guess I just view it practically. It contained some sort of flaw (in this case, an expression which was proportional to a term which was part of the answer) which was sufficient to say "nobody shall be allowed to see this upon this site."

Meh.

Yeah, pretty much. If the homework question asks for the kinetic energy, and you give an answer that includes an expression for the kinetic energy, this is what happens.

Sort of. The homework question asked why a naive calculation of the Lagrangian based purely on the generalized coordinates was wrong, and I gave an expression proportional to the kinetic energy to show what sorts of terms were being missed.

It looked to me like the homework question (not to be confused with the question posted on the site) was asking for the kinetic energy.

Oh no, @DavidZ. Not even. I can see 10.12 on Google Books by searching that book for 10.13; 10.12 asks for the equations of motion.

Presumably this question asks the same.

Ah, that might make a difference...

That's part of why I was giving T which is equal to L if U = 0.

(we do tell people to quote the actual homework problem)

Well, I'll just wait a few weeks and try to be a little less sensitive to these things.

Just found it weird that I'd quickly gotten two -1's and then a delete on what I thought was a straightforward not-too-revealing answer.

Just something to get used to on this site, I guess

'kay. Thanks for your help/clarifications!

no problem :-)