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3:01 PM
must the inverse of an element in an abelian group be unique?
 
hello , i want to see that if $x\in]0,1]$ then $x\in \bigcup_{n\in\mathbb{N}^*}[\frac1n,1]$
 
@Vrouvrou yes
 
can i say that from the density of $\mathbb{Q}$ in $\mathbb{R}$ there exist $n_0\in \mathbb{N}^*$ such that $0<\frac{1}{n_0}\leq x$
then $x$ is in the untersection of $[\frac1n,1]$
?
 
@Vrouvrou no because the density of $\Bbb Q$ is not related to the density of $\{\frac1n|n\in\Bbb N\}$
 
ohh
so how to do ?
we use Archimedean ?
 
3:11 PM
yes
Archimedean Principle: $\forall x \in \Bbb R: \exists n \in \Bbb N: n > x$
 
we have an other $0<a<b, there exist n\in \mathbb{N}; na>b$
but we have a strict inequality
and i need a large inequality
 
why do you need a large inequality?
$\Huge0<1$
a large inequality for you
 
because i need to find $x$ in $\bigcup_{n\in\mathbb{N}}[\frac1n,1]$
 
and?
 
i need to find that therer exists n such that $x\geq \frac1n$
and Archimedean give that $x>\frac1n$
 
3:17 PM
and if $x>\dfrac1n$ does not automatically $x\ge\dfrac1n$?
 
i think that no
 
why not?
 
i don't know
 
do you know what $x\ge\dfrac1n$ mean?
 
yes $x>\frac1n$ or $x=\frac1n$
 
3:19 PM
and if A is true, then is A or B automatically true?
true or anything is true
 
yes
thank you
 
now substitute A with $x>\dfrac1n$ and B with $x=\dfrac1n$
you are welcome
 
@Danu Hi.
 
@TedShifrin, the proof that the cohomology class defined by the curvature of the Chern connection is the so-called Atiyah class seems rather technical---do you think it's worthwhile to go through it in detail? Huybrechts provides a much shorter argument to show that the class does not depend on the Hermitian structure (but not that it's the Atiyah class).
@BalarkaSen Hi there
 
3:48 PM
@Danu: What in the world is the Atiyah class?
 
@TedShifrin Damnit Huybrechts and his terminology :P
 
That the cohomology class is independent of choice of connection is the famous Chern-Weil theorem (the proof of which leads to things like secondary characteristic classes).
 
The class of the curvature of the Chern connection :P
But Huybrechts has introduced it before curvature, giving it in terms of a cocycle
 
That can't be right, or your sentence is a tautology.
 
It was a joke :P
 
3:50 PM
Ohhh, for a line bundle only?
Your jokes are as bad as @Balarka's :P
 
No, for any rank
 
How does he do $c_k(E)$ with a cocycle?
 
$A(E)=\{U_{ij},\psi_j^{-1}\circ(\psi_{ij}^{-1}d\psi_{ij})\circ \psi_{ij})$.
No mention of Chern classes thusfar.
Here the $\psi$'s trivialize the bundle $E$.
 
A $1$-cocycle? So only $c_1(E)$?
 
@TedShifrin A $1$-cocycle indeed. I don't know about Chern classes yet, I keep on telling you! :P
But I guess yes...
 
3:52 PM
So this is actually just for line bundles.
 
No?!
 
It's what you get from the exponential sheaf sequence.
So that's a $1$-cocycle with values in what sheaf?
 
@TedShifrin $\Omega_X\otimes \operatorname{End} E$
 
Oh, so this isn't going to be the first Chern class until you take the trace.
I would say you should understand that what this does for a line bundle is what you get out of the map $H^1(X,\mathscr O^*) \to H^2(X,\Bbb Z)$.
 
I think Huybrechts discusses its connection to Chern classes in section 4.4 (fittingly called "Chern classes")
 
3:56 PM
OK.
Heya @apnorton
 
Long time no see, Ted
 
Heya @teadawg!
 
The proof that this cocycle describes the class defined by $F_\nabla$ looks really painful
Featuring a rather large commutative diagram of sheaves
and about 8-10 lines of computation
 
I still say you should work it out for line bundles only at this point. You can always look at Griffiths/Harris or my notes.
 
Hi everyone
 
4:05 PM
Heya @Alessandro
 
A prof briefly mentioned that one can think about a group as a set with $3$ operations, the "standard" binary one, an $1$-ary one that gives the inverses and a $0$-ary that selects the identity. Is there any motivation to do so?
 
No.
 
great
 
Typical mathematician pedantry. :P
I guess that way the prof can say a homomorphism is compatible with all three operations.
 
I think he did say that
but I also think compatibility with the first one should be enough to get the other $2$ as well
 
4:11 PM
Indeed.
shrugs
 
I'm sure of that actually
 
Some mathematicians are in love with formalism. Some of us aren't so much.
 
Just as much formalism as you need. Sometimes that's quite a lot, though.
 
I grant that some people would say I am adept at my own fair share of formalism.
I try to give geometric interpretations of most things, but sometimes I can't.
Oh, and g'night, @MikeM.
 
4:34 PM
WHATZZZZZZUP ?
 
The room is sleeping.
 
Then I WAKE it up . MUHAHAHAHAHA
 
4:58 PM
hi chat
@TedShifrin I like geometric interpretations when possible. But myself I also like to find physical examples, especially if it gives some intuition for what's going on.
For better or worse, though, that tends to be less and less helpful the more technical the math becomes
 
5:21 PM
@Alessandro I think it's a neat way to say it. But there's probably not much in terms of motivation in it.
 
Hi everyone, [no intention to spam], I'm trying to figure out how I could evaluate an integral of the form $\int\log(ax+b)\exp(-0.5x^2)dx$. I have asked a question, for which I also started a bounty, [here](http://math.stackexchange.com/questions/1989341/on-the-evaluation-of-the-integral-int-fracba-frac1-ba-log-left). If you are interested, could you please take a look and lend a hand? Please be patient with me; I'm here to learn, not to spam :)
Thank you very much!
 
@TedShifrin 😴
 
@BalarkaSen @TedShifrin @Alessandro I guess there's that if you want to show that something is a Lie group or a topological group, you need that the inverse operator is continuous as well, not just the multiplication operator. But the "pick an identity element" operator is automatically continuous and smooth so I'm less inspired.
 
@nullgeppetto if you are interested in a numerical evaluation, what you can do is use that
$$\int_0^1 x^n\ln(x) dx = -\frac1{(1+n)^2}$$
and expand the exponential in a power series
 
Yeah, I guess it applies to any group object in a category.
 
5:32 PM
@s.harp, thanks for your response. The truth is that I would like to avoid going to a numerical solution (like using power series). I would very much like to have a form of solution, even if it's complicated.
@s.harp, I believe that some solution could be found, since there is an integral (here: en.wikipedia.org/wiki/…) where the Euler-Mascheroni constant appears and looks similar.
 
@nullgeppetto Mathematica can evaluate $\int_0^1 \log(x) \exp(-a x^2)dx$, the expression is -HypergeometricPFQ[{1/2, 1/2}, {3/2, 3/2}, -a], proofs involving such expressions are beyond me though.

Mathematica also can evaluate $\int_0^1 x^n \log(x)\exp(-a x^2)$, but it is more complicated.

It cannot handle $\int_0^1 \exp(-b x)\log(x)\exp(-ax^2)$ though
 
@Semiclassic: I like physical interpretations, too, when I can offer them. (Balarka just found one in my notes re geodesics on a surface of revolution.) But I don't know nearly enough physics for all the math that shows up.
Happy Halloween @robjohn :P
 
@s.harp, I tried some similar forms in WolframAlpha, but still didn't find what I want. Besided, I'm really interested in the proof. Anyway, thank you very much for your responses :)
 
Hi, @Ted, @Akiva
 
5:45 PM
Hi again, @Balarka.
Oh, DogAteMy is here :) Howdy.
 
I finished that English essay that I was supposed to have been writing last night rather than talking to you
 
DogAteMy, did recovery from the surgery go as well as had been hoped?
Talking to us is rarely a good idea. :)
 
Yeah. Still using the cane, but my physical therapist said I should try walking without it more.
Which I can do, but my walking is slightly uneven
There's one or two muscles in particular that I need to work on.
 
Well, physical therapists are usually smart :)
 
I don't know if I actually mentioned that before here — I'm using a cane!
 
5:47 PM
Yeah, I think you had told me.
 
They're fun to swing around and almost hit people with.
 
I'm scared of you as it is, DogAteMy :P
 
Yup.
 
Well, lots of scintillating math going on today, I see :)
 
5:55 PM
Interesting adjective
 
sin-e-tillating?
 
@TedShifrin Yes. Just not here.
 
Well, get lots done, @MikeM.
 
ok
actually, I'm converting to grading at some point, so maybe not
 
Now I just wish I was doing an experiment right now that involved use of a scintillator
 
5:58 PM
You'll just have to sin without one, @Semiclassic.
 

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