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10:00 PM
You want to properly write the solutions down somewhere, and once you do that you'll start seeing things you don't understand
But I am too lazy so I answer in MSE instead
That gives a motivation for writing: fake internet points
You can also start teaching, that helps
All of those have a common theme: explain what you have or haven't understood (to yourself, to user#####s or to your friend/student)
 
I guess that's perhaps why I haven't gotten any further with algebraic topology. I get to the point where I can compute the homology of some spaces, then I don't actually see anything more.
It's like someone showing you a hammer and not letting you smash some nails.
 
Right, exactly
 
Often when synthesizing new info I'll basically mock lecture myself.
 
After some point you feel bad for having a hammer you can't use. I do that frequently (shh spectral sequences)
 
When synthesizing new information personally I basically give lectures mocking Fargle
3
Tough crowd today huh
 
10:08 PM
@MikeMiller So this is why my ears are always burning.
 
Heh
Is Bott & Tu good for exercises?
 
I found Bott & Tu to be mostly algebra than topology
I didn't do exercises from there
 
They have like one exercise per chapter lol
 
A bit more than that, but not enough.
Oh oh ... DogAteMy is back
 
What is a good geometry flavoured algebraic topology book with great exercises?
Or will I have to dig regardless?
 
10:18 PM
Hatcher
 
@Secret @Semi Hey, this is exactly what I was thinking about a few days ago:
 
I can't stand to read Hatcher half the time. :((((
 
Including the same diagram for a trefoil that I had
 
10:18 PM
I agree that the exercises in Hatcher are killer. The text is perhaps not so wonderful ...
 
Then do exercises from it
@Ted Hard disagree
:)
 
I agree with Ted for once.
 
You can't hard disagree ... I said the exercises are killer.
 
Which means the stars have aligned and Balarka is wrong.
 
but also it has Reidemeister moves, which is interesting
 
10:19 PM
@TedShifrin Hahah you got me
 
a @Balarka: Did you ever tell me why Hatcher defines a covering map not to be necessarily surjective?
 
If that's your only complaint I feel it's a weak one.
I agree the empty map is not a covering map, though.
 
Oh, no, that's not my only complaint.
 
It is against the Be Nice policy to state my opinion about people who don't like that book.
 
I didn't say I didn't like it.
I do like it.
 
10:21 PM
HAH @anakhro
 
I didn't say you're one of those people. :D
 
Oh no
 
"Perhaps not" killer is not condemning the text itself to purgatory.
 
We're Hatcher crusaders, don't mind us
 
I guess the stars have not aligned.
 
10:22 PM
It is our Bible
 
Most students find it very hard to learn from (in my experience at UGA). When I ran summer topology qual prep sessions, I had to provide a fair amount of intuition (and they'd already had the course and the book).
These students were not all Mikes and Balarkas.
 
Hi chat
 
Salut @Astyx
 
LOL
 
I happen to think my undergrad diff geo notes are carefully written and accessible, but students will complain about them, too. :P
 
10:24 PM
Those notes are beautiful
 
My brain seems to not be working right now : how do you call a function such as $\Bbb Z\to \Bbb Z, x\mapsto x+n$ ?
 
Did you define the wedge product with a factor?
(because that's the problem)
 
@Astyx Translation by $n$?
 
Translation, right
I couldn't think of anything else than transposition even though I knew that wa snot it
Thanks
 
Huh? @anakhro
 
10:27 PM
The usual "Where does one normalize wedge product"
Sometimes it's in symmetrization
Etc
 
If that was addressed to me, it's totally irrelevant.
The way I did differential forms in my blue book, there was no abstract tensor stuff, so wedge was very straightforward.
 
I'm just saying that's what the question was - but I assume it was a joke
 
Perhaps the real astral alignment is when Ted laughs at one of my jokes.
 
I'm perpetually misaligned.
 
:(
Might as well break out my tear-stained Kobayashi & Nomizu volume.
 
10:31 PM
What, are you trying to learn from that?
 
As I recall, their conventions make the volume of the unit cube equal to $1/n!$. I abhor that.
 
That is not good
 
K&N is encyclopedic but not the place to learn.
 
The only issue I have with Guillemin-Pollack is that
 
Yeah, that's a gross miscue by Guillemin, too.
 
10:32 PM
@MikeMiller it's another button on the Ted Shifrin control panel to press.
 
@Balarka: Here you go. So what's the easiest way to see the topology of $SL(n)$?
 
That's where I learned connections from.
 
Not sure what pressing those buttons gets you, @anakhro, other than my animosity.
 
Which is probably why my paper on connections is taking so long to write.
 
Even when I've taught, I've only done a few things following K&N.
 
10:35 PM
@TedShifrin The maximal compact subgroup of SL_n is SO(n)
That's how I see it
 
I wonder what that OP actually knows.
 
@TedShifrin one of these days we will rub shoulders.
 
Doubtful, @anakhro. I have no mathematical existence.
 
I'd write an invariant differential form on SL_n as a vector in the Lie algebra, which I understand
I don't know what the question is asking for precisely
 
I saw that question and didn't like it
 
10:38 PM
You are thoroughly annoyed by me, so I think you have passed the Turing test in the least.
 
First deRham cohomology. In non-compact cases, it's not clear whether every closed form is cohomologous to an invariant one.
 
You always pass to max compact first
 
It's easy to compute singular R cohomology, that's H^1(SO(n); R)
But in de Rham I guess I'd pass to maximal compact by Poincare lemma
 
Easy with all the machinery you have ... Again, that OP sounds like he/she knows little.
 
Then on SO(n) your argument makes it easy to see that H*(SO(n)) = H*(so(n))
Then calculate the latter :)
 
10:40 PM
Hey, they are almost nice.
The least you can do is return the favour.
 
You need to know that you can compute the cohomology from the invariant complex for compact groups, yeah.
 
G-averaged differential form is cohomologous to the original form, I suppose?
 
Right, @Balarka. Not necessarily valid in the non-compact case, since you can't average.
 
@Ted which is why you deformation retract
 
Sure @Mike ...
I just doubt that person knows that much. I dunno.
 
10:42 PM
Note that H^1(g) = g^ab :)
Well, good luck to them then ;)
 
LOL, you'll note I'm not answering it either.
 
I pass
 
I guess @Fargle disappeared for being mocked.
 
Rip
 
He did?
 
10:43 PM
Does de Rham behave nicely with the Lie correspondence in some way?
 
Oh, perchance not.
 
yes if compact.
 
I've just been busy on the side.
 
You can take cohomology of the Lie algebra, because differentials on the functionals are just defined by the brackets
 
I'm disappointed to find that I do not deserve your full attention, @Fargle.
 
10:44 PM
Oh fancy.
 
Maybe that's wrong
Yeah for G connected compact H^*(G) = H^*(g) and I am unwilling to do more thought
 
That's what I would expect
 
In general you can say something about what the subcomplex of invariant forms computes
 
One of my favoritest results is that for a compact symmetric space, the cohomology ring is just the complex of invariant forms.
 
Oh wow that's like a cohomology of a dga is isomorphic to dga thing
Rational cohomology something something
Suillvan something
 
10:48 PM
In particular, all invariant forms are closed. So the cohomology collapses.
Sullivan-Griffiths-Deligne
 
Oof @ Deligne
 
This is far more classic than that stuff.
 
! I didn't know that
 
Oh speaking of classic, any seminal papers in geometry or topology you would highly recommend at least one read, @TedShifrin?
 
that explains some miracles I have seen in practice
 
10:49 PM
Unfortunately (or fortunately), it tends to fail for homogeneous spaces that are not (locally) symmetric, @MikeM. That's what made my Ph.D. thesis interesting (I suppose).
 
Hi.There's this question. By $K(x)\K$ do they mean the extension $K(x)/K$ or something else?
 
Right, I know how to compute it in general: it's simply the cohomology of the complex of $H$-invariant cochains on $\mathfrak g$.
It's just a miracle those are so nice in the symmetric case.
 
Setminus
 
Oh okay, I thought there was a typo
 
@MikeM: Not really a miracle.
 
10:53 PM
@TedShifrin if I wrote short expository notes on differential geometry, would you ever take a glance over them and give me suggestions?
 
No. My taste is very particular and I don't want to do this sort of thing.
 
Understandable.
What made you fall in love with mathematics, @TedShifrin?
(this is not an interview)
 
you're very odd today
 
I'm not sure I have a coherent answer for that, and I'm not going to answer.
 
@MikeMiller is this a negative thing?
 
10:57 PM
about half of odds are
 
Hope the odds are on my side.
 
strange fella
 
Damn, there sure is a proliferation of questions from Guillemin & Pollack on main.
 
Yikes
 
Exam season.
 
10:58 PM
It's early stuff in the book, so I dunno.
 
I've been answering emails about linear algebra all day.
 
@TedShifrin Is it just the same one guy?
"Idonotknow"
 
And this OP is sadly very misguided.
I just volunteered to "teach" my best AoPS calculus student an informal reading course out of my blue book next year. I wonder how disciplined he'll be. I suggested he try to get as much as possible out of the current course for the rest of the year. :P
 
Oh speaking of which, is there a good book that does complex analysis from a differential geometry perspective?
 
I always make sure I broach my silly questions here, so that I can be picked on in real time. :)
 
11:00 PM
I don't know if such a book exists.
 
Look at Krantz, @anakhro.
 
what does that mean...?
 
Or if it would be more likely to be like Miranda's book, and more algebraic geometry.
 
@Fargle: Not counting the ones you email me directly :D
 
Shh
Nobody has to know
 
11:01 PM
busted
 
:O
 
There's a lot of serious overlap of differential geometry and 1-dimensional complex analysis. The Schwarz lemma is really about hyperbolic geometry. There are various books, but, as I said, look at Krantz for starters.
 
Complex Analysis: geometric persective?
 
i'm out
 
Or Explorations in Complex Analysis
See ya Mike
 
11:03 PM
Geometric Viewpoint, I think, is the subtitle.
 
Si. Thanks I will check it out.
 
If you put a polynomial in a different plane, the one you get out is homogenous to the first?
 
say what, @CaptainAmerica?
 
That doesn't makes sense?
 
Nope.
 
11:09 PM
I don't know how to word this! It's in my head!
Polynomials in the Cartesian plane are different to the ones in the projective plane, yes? Geometrically at leas?
but there are some that are homogenous?
Dang it.
I don't know how to explain this.
 
Nope.
 
I think I'm using the wrong word. Which part is wrong.
 
To define a subset of projective space as the zeroes of a polynomial, you need the polynomial to be homogeneous. But the polynomial, per se, does not define a function on the projective space.
 
Oh oh! That's what it was. The thing I was looking at was finding zeros for inflection points :D. Then it started talking about homogeneity and I couldn't get it right in my head.
 
This is what happens when you read stuff that's too far removed from your knowledge base.
 
11:13 PM
It was an accident.
 
That's not to say it's bad to stretch yourself.
Just be sure to check with someone more knowledgeable than you (such as Ted) to get it right.
 
Ted loses patience.
 
As do most teachers.
Students know how to learn in too many ways.
More than we can teach in.
 
Curiosity can breed apprehension, but it can just as well breed misapprehension.
 
That's why you check with others.
 
11:17 PM
;-;
 
Weird, I was totally reading that with the other definition of apprehension in mind.
 
Well, the FBI certainly reads that one way.
:^)
 
I have no idea.
 
@Fargle I just realized I don't understand what you mean by that.
 
That's cuz apprehension has two totally different meanings.
 
11:20 PM
Wait the fbi reads????
 
@CaptainAmerica16 You can find things out by being curious, but you might just as well get stuff wrong.
 
But apprehension also means nervousness and insecurity ...
 
Two of my most prevalent qualities.
 
I think I'm gonna go play in the kitchen instead. Bye.
 
Goodbye.
 
11:21 PM
Bye @Ted
 
Love that Ted.
 
Yeah, he's so nice.
 
Amen.
Where are you reading about varieties, @CaptainAmerica16?
Hi again, @MikeMiller
 
I need a dummy check on a variational calculus issue
Consider $$S = \left(\int f(t) w(t) \right)^2 / \int w(t)^2 \, .$$
What $w$ maximizes this?
 
11:39 PM
Oh no, look, a physicist!
 
::hides::
 
@BalarkaSen are you not even the slightest interested in physics?
 
I am, I don't know much about it
 
@BalarkaSen So eh... how 'bout that question...
 
Have you ever seen how the symplectic form comes up?
 
11:41 PM
Don't think I know the answer to this one, @DanielSank. Off the top of my head vaguely, Cauchy-Schwarz seems relevant
 
It's actually cute.
 
@anakhro I think the analogue of the gradient vector field is the Hamiltonian vector field in the symplectic world, right?
 
@anakhro No specific books on anything. It was just in a math Discord server.
 
That's probably where the physics comes in
 
@BalarkaSen effectively, you take a function to a vector field.
But it works out pretty nicely.
Like every property.
 
11:44 PM
Interesting
 
Like you only want the vector field to depend on dH, the differential of your Hamiltonian H.
And you want it to be linearly dependent due to Newton's laws.
 
Naturally the object you choose for this has to be a bilinear form.
You want it then to be non-degenerate otherwise your vector field is not always defined.
And then conservation of energy translated into this situation gets alternating.
 
@TedShifrin wait what else does it mean?
 
And then you don't want it to depend on time.
 
11:47 PM
@anakhro Ahhh
 
And so you get that it has to be closed.
And that last one troubled me for eons
(eons = a year)
 
I should learn this properly and explain it to one of my fellow first year who was asking me about symplectic geometry
He's incidentally also interested in physics
 
I'd be willing to go over it in detail if you wanted.
Whenever
I know it's like 5 there now
 
Excellent, I will ask you sometime this month.
Ya I should sleep
 
Goodnight. ;)
 

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