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21:01
differential forms are very natural
my intuition for differential forms is that in order to define integration you have to have linear data in order to approximate the area under the curve so that is why for instance the definition of differential forms are natural in terms of sections of the tangent bundle.
I like derivation definition the most I get the most feel
yes probably, it's from all I've seen (which isnt that much) an extremely elegant and useful theory, but not if they come out of nowhere and you've been introduced to manifolds 4 weeks earlier
no chance that I'll by two volumes, if I feel the need to learn the stuff again and in more detail, I'll grab some good lecture notes
@KarimMansour I am guessing you mean sections of the cotangent bundle
sorry sections of the cotangent bundle
21:08
@user2103480 "buy"
hahaha *no chance I'll sit in front of my laptop hastily scrolling through two volumes
while seeing alexandra waving at me constantly
lol
I must remember to thank her in my thesis
I think that technically, our bookstore is kazakhstani, isn't it?
libgen is Russian, Alexandra (and so sci-hub) is Kazakhstani
In June 2020, a study found that articles downloaded from Sci-Hub were cited 1.72 times more than papers not downloaded from Sci-Hub.

lmao
21:12
Differential forms are not scary and they are very powerful.
he's not dissing the subject, he's talking about poor lecture style
@user2103480 If there is more than 3 clicks needed to access the paper why bother reading and citing it, too much effort
Well, OK, I was absent. Poor lecturers ruin pretty much every subject.
Recently I was reader a paper where "for the convenience of the reader we will present here some proofs from [18], but note that the construction presented there is much more general" and then [18] are some unpublished notes that appear to not be available online...
@AlessandroCodenotti personal communication
21:17
I've been told in a dream that...
21:40
Hello everyone. I was taking a look at square-integrable functions.

$f\colon \mathbb{R}\to\mathbb{C}$ is square-integrable iff $\int_{-\infty}^{+\infty} \left|f(x)\right|^2\,\mathrm{d}x < \infty$.

Maybe because I was studying with the help of Wikipedia it is kind of unmotivated and I can't see it, but why is the square part important?
Some functions are square integrable, while others are not
The answer really is that using this gives you more generally an inner product, whereas no other exponent will do that.
In begin{align*}...end{align*} environment I'd like to put in some parenthetical remark on one of the equation lines, how do I do that
(Wow, how did ChatJax render that)
Just put it with a quad space and \text{} ...
Ah, OK, fair enough.
21:46
Now, if you want it not to affect the alignment, in regular LaTeX there's a way to do that, but I forget, and I don't know that it works here.
Thanks @Astyx and @TedShifrin.
Oh, you probably mean in regular LaTeX.
Yeah
Not in MathJax
I think \hidewidth might work, but maybe not in that environment.
Align junk is such a hassle
21:47
I'm rusty. It's been 10 years since serious book-setting.
Well, I like alignat with multiple alignments, @MikeM, but it is dopey getting it to work.
Yeah I should maybe read how to use LaTeX someday, if I end up writing worthwhile drafts.
I just wing it lol
These days most everything is google-able, Balarka, but I do have 3 or 4 books I bought centuries ago, and I still look at them.
Yeah. Anyway this is only if I write anything worthwhile any day
Low probability event
Depends on the meaning of "worthwhile." :P
Anything which is even remotely original :)
21:49
Is semi-locally simply connectedness preserved under continuous map?
Exposition is, in my opinion, worthwhile if it is well-written and enlightening.
I doubt it, @love_sodam.
What? No more false diff geo today? :D
@TedShifrin I'd say that is a form of originality.
@love_sodam Of course not, find a map from the interval which traces out the Hawaiian earring
Meh, I dunno. "Originality" in academe has a very different connotation.
Keep examples in mind when studying new concepts!
21:52
Right, I know, of course.
I didn't mean that
I was thinking of the same example, @MikeM, but as a quotient of $\Bbb R$ with loops at each integer.
In that case, @Balarka, I think you've probably already contributed worthwhile stuff on this site.
That's very kind of you haha
So \hidewidth is apparently from Plain TeX, not LaTeX, although I know I've used it in LaTeX before.
@MikeMiller Oh, right. Thanks. @TedShifrin Unfortunately I have topology test soon.
Maybe I could label it and show label
21:55
I don't like that, @Balarka. I try to be approximately a purist.
Then it probably overfills at the right, anyhow, but I dunno.
Oh I guess yeah
I forgot
I bet \hidewidth works, as it works in tables and matrices and such.
Try a test.
You put the thing in brackets after the hidewidth.
OK let me see
21:57
Oh right. \align isn't LaTeX, anyway. It's from AMS-TeX.
\begin{aligned} ... \end{aligned}
oh wow it typeset that
Also, @Balarka, you could also do it as a multiple alignment with the field empty for other lines
Oh tag maybe
Right yeah
No, tag is what you were talking about with labels.
Yeah... I guess equation labelling. It doesn't overshoot though!
Haha
I was thinking of using showlabels package, which does overshoot
22:06
@TedShifrin Are you available elsewhere for a few questions?
hidewidth does not work in the align environment, but it does work in matrix. Shrug.
22:51
I want to show that the sequence $a_n=\frac 1n -2^{-n}$ is decreasing. I have tried to show that $a_{n+1}-a_n<0$ and $\frac{a_{n+1}}{a_n}<1$ but inboth cases I faced difficulties to continue. Which way do you suggest to show that?
23:34
0
Q: How to integrate this improper integral.

ANUPAM BISHT I am trying to integrate this: $$\int_0^\infty\frac{ 1}{x^2} \, \mathrm d x$$ I was trying to convert it into a complex integral. But did not know how to proceed. My original question is : $$\int_0^\infty\frac{1-\cos x}{x^2}\, \mathrm dx$$. Can someone give me a hint on how to proceed. Thanks...

Since $f(z) = \frac{1-e^{iz}}{z^2}$ has no singularities in the region enclosed by the piecewise smooth curve $C$, does that mean $\int_{C} f(z)dz = 0$?
@AttractorNotStrangeAtAll as a side note to what ted said, for square integrable functions f,g, the integral of f*g is always finite, and this integral can be thought of as an extension of the usual dot product of two vectors - to functions, which are basically uncountable vectors
Doesn't $\int_{C} f(z) dz = 0$ just follow from Cauchy's theorem?
Care to add hypotheses?
That sounds a lot like Cauchy's Theorem itself, without hypotheses.
Yeah, I think it is...$f(z) = \frac{1-e^{iz}}{z^2}$, and $C$, which is described in the link, doesn't seem to contain any singularities of $f$, so I believe the integral over $C$ should be $0$.
Got a question. Note I know absolutely nothing about complex analysis. If $i$ is the imaginary unit, $t \in \mathbb{R}$, and $k$ is some positive integer, what are the necessary conditions for $$\int_{0}^{\infty}e^{-x[1 - i(t/k)]}\text{ d}x$$ to converge?
(Even better, I'd like to know what the relevant theorems are)
23:45
It's Cauchy's Theorem precisely when $C$ is a simple closed (appropriately smooth) curve and $f$ is analytic/holomorphic inside $C$.
Hey Ted!
Great, thanks @Ted
I have a lot to study come winter break
No, Clarinet, that wasn't to you.
Oh lol
All you need to know is that $|e^{it}|=1$ when $t$ is real.
23:47
AH it's that trick
Thanks
You certainly know that absolutely convergent implies convergent.
Hi, Demonark.
How's it going?
Got cussed out by a brat who has no idea how to ask a question on main :P
Rambling for five paragraphs about differential geometry. And the only question is a beginning multivariable calculus/linear algebra question about orthonormal bases. Ugh.
23:55
Does $\frac{1-e^{iz}}{z^2}$ have a simple pole at $z=0$?
user: expand the power series
Ted: ugh that's annoying
I guess it would be a double pole...
Well, don't guess.
But how can Jacky Chong use the fractional residue theorem when it has a double pole (see the above link).
?
Back up. You are old enough to be able to answer your own question by taking five seconds.
23:57
My version of the fractional residue theorem says the pole has to be simple.
I have no idea what the fractional residue theorem is, but stop talking and do 5 seconds of thinking/writing.
Oh, lol...yeah I am an idiot.
What are you up to today, Demonark?
The trouble with the chatroom (and with so many posts on main) is that people write without thinking just a little bit!
Today's been sluggish so far, but I need to keep reading Helgason's notes
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