« first day (2490 days earlier)      last day (2530 days later) » 

12:00 AM
Yeah, both can't be satisfied at once.
 
Right.
 
What time is it in your country?
 
Uh $\nabla \times (\nabla^2 (\nabla \times \psi(\mathbf{r}) \hat{e}_z))$. This should be fun
 
:/
What do you know about $\psi$?
It is scalar, so it's not the end of the world.
 
12:05 AM
Nothing
 
Problem comes from Navier-Stokes
 
All I know is $\mathbf{v} = \nabla \times \psi \hat{e}_z$
 
$\phi$ or $\psi$?
 
12:06 AM
Trying to prove that $\nabla^4 \phi =0$
Oh
fixed
 
Not a very thrilling prospect in either case.
 
Index notation should do the trick
I guess
 
It should work, though the fact that it vanishes makes me wonder if there's something simpler to be said.
Is $\psi$ a function of $z$? I could see it depending only on $x,y$ corresponding to a flow within the plane.
 
The velocity vector field is in the $xy$ plane
 
Okay. So you also know $\psi=\psi(x,y)$.
...I say that, but do I believe it.
 
12:10 AM
Well not necessarily?
 
Well, you know that $\mathbf{v}$ is in the $xy$ plane and that (presumably) it's $z$-independent.
 
We can only say $\mathbf{v} = \nabla \times \psi_z$
Wait, what are you saying?
 
I'm saying that the velocity should neither have a $z$-component and that those velocities shouldn't change if you change $z$.
 
Yes that is true
 
Okay. What's $\nabla\times (\psi e_z)$?
 
12:12 AM
You get $\mathbf{v} = (\psi_y, -\psi_x,0)$
 
Right.
Now, suppose $\psi$ were a function of $z$ (along with $x,y$).
Then $\psi_x,\psi_y$ would also be functions of $z$.
 
Not necessarily
 
Yes, necessarily.
Oh.
Bleh, yes. Any function $f(z)$ added to $\psi$ would result in the same $\mathbf{v}$.
 
I think we're better off trying the index approach
 
This is a bit besides the point, though: In either case, the curl is of the desired form.
I think it's best to start from $\mathbf{v}$ alone, if possible.
 
12:17 AM
I've already written out the expression :P
 
Fiiine.
 
$\epsilon_{ijk} \partial_j \partial_m \partial_m \epsilon_{klm} \partial_l \psi \delta_{zm}$
Agree?
 
Whence $x_l$?
 
Oh, wow, I came in a few minutes ago and found that I was banned for saying "yeah you slow man" to Daminark. I can't make head or tails of this. Finally we have reached the point where I'm being banned for random messages, neither inappropriate with context or without.
 
@Semiclassical Argh, sorry bout that
 
12:19 AM
well, whaddya know
 
@BalarkaSen To be fair, 'slow' does have associations with mental deficiency in the US. On the other hand...yeah, context.
 
@BalarkaSen You seem to have made some sort of enemy
Not too sure if $\delta{zm}$ is how I want to write it. But does it make sense?
 
Probably better to do $\delta_{3m}$.
 
ergh
 
12:21 AM
Also, you've got $m$ four times in there.
 
Oh, and also, NSFW probably
 
Oh wow
 
An index should occur at most twice under the summation convention.
 
The first two $m$ are $n$
 
Mmkay.
If you eliminate the Kronecker delta, that simplifies to $\epsilon_{ijk}\partial_j \partial_n\partial_n \epsilon_{kl3}\partial_l\psi$
 
12:23 AM
Oh
I was gonna do the $\epsilon - \delta$ identity first
 
Eh, no reason to (that I can see).
I'd suggest eliminating $\delta$ first and then trading $\epsilon\epsilon$ for $\delta \delta-\delta \delta$ (with appropriate indices of course).
 
Okay, suppose order doesn't matter
I'm getting weird results
Oh wait I'm not
@Semiclassical I had to use $\partial_3 \psi = 0$ so you were right
 
12:39 AM
kk
 
1:32 AM
"Please give a technical/more concrete answer, not a 'Hatcheresque' one. I don't need intuitive images that much, I want a formal argument."
yeah no
 
Lol I remember hearing that Peter May bashed Hatcher for not being "theoretical"
He uses a mix of Hatcher and his own book for undergrad atop when he teaches
And apparently said on the syllabus that theoretically inclined students would be disappointed
 
May is not that bad of a person. He does concrete stuff with the algebra I think
but man, Hatcher >>> world
 
I was like "Oh, them's fighting words"
I mean he may do concrete things but he apparently views geometric intuition as being somehow at odds with theory. In particular, he griped that it took too long to define categories
 
We had this convo before
 
shrugs
Oh have we?
Wait yeah I remember.
 
1:42 AM
lol what a trash question
 
we're the trash dudes
 
But yeah for what it's worth, in his midterm (writing a lecture) he did ask to start with motivation for the topic
So that's something
Anyway, what else is up?
 
If you're not able to take a (good) intuitive answer and make it formal, you probably need to do more.
 
@SemiC It really does depend on what formal means
notions of formality and rigor varies field to field in mathematics
you're not going to translate a picture into bunch of algebra everytime. might be an amusing exercise, but nothing more
 
1:59 AM
@BalarkaSen and when you go to integral domains who knows what formality means anymore?
 
Commutative algebra is one thorough branch of mathematics. But I'm certain even in algebra notions of rigor vary through the various subfield
 
isn't sure whether the pun came across
 
It just depends on what "proof" means. Is a picture allowed in how you parse the proof? That's true for topology. I am sure there might be varying cultures in mathematics which have distinct meaning associated to proof.
@Daminark I noticed it but I decided to ignore it
 
Just making sure. I'd hate for such quality humor to go unnoticed
 
Oh, good grief ... Balarka's messed up his un-sleep schedule already.
 
2:04 AM
sad nod
 
We were warned it was just for a day
Oh well shrugs
 
Demonark got Balarka banned? This is getting ridiculous.
 
i am sure it wasn't daminark. just my reply to him got me banned
which was like "you are slow man"
vOv
 
Wait you got banned again?
 
maybe Demonark is a fast woman.
 
2:07 AM
lol
 
Kek
 
@Daminark seems so
 
Someone is taking great amusement in this
Like there's no way that statement could offend someone
 
I am never amused, so it isn't I.
 
@ whoever is doing this: you slow man
 
2:12 AM
Presumably whoever is not currently here.
 
If that offended him, this too should.
 
Living on the edge, Balarka
 
pushes Demonark over the edge
 
30 minutes of solitude
and constructive work
 
Go to sleep, Balarka.
 
2:14 AM
falls in LaTeX
So wait whenever​ you're banned you decide the law of the excluded middle is false?
 
whose middle is excluded?
rehi Eric
 
tries to parse out statement and fails
 
I don't understand the excluded middle joke
 
Hey @EricSilva
 
2:16 AM
fails in LaTeX
 
You said constructive work
 
that's a stretch Amin
 
deposits Demonark in a stretch limo headed for the cemetery
 
rip
 
rip in pieces
 
2:21 AM
rip in latex
 
$rip$
 
Go to bed, all of you.
 
lies in bed and continues to chat
So actual math question though
 
it's only 9:30
 
it's only 8 AM
 
2:23 AM
glares at a Balarka
 
sry
 
i actually slept earlier, when i should've been doing galois stuff
 
glares at Eric
Good thing I have a month of vacation from glaring.
 
@Daminark So what's the math question
 
glares at the concept of glaring
 
2:25 AM
Oh, there's Fargle.
 
yo @Fargle
 
If you have a complex manifold, presumably it'd also be a real manifold under some canonically derived differential structure, right? If so, how much structure do you lose with that?
 
Heya @Balarka, @Ted
I feel popular now.
 
Hey @Fargle!
 
What do you mean? It's like asking if you consider a holomorphic function as a smooth function, what do you "lose"?
 
2:26 AM
You lose lots and lots of structure. This is like thinking of a holomorphic function on C as a smooth function on R^2
 
Hiya @Daminark
 
ahem
 
Oops.
Ted sniped me
 
You lose the information that it satisfies Cauchy-Riemann and is therefore analytic, for example.
But a complex manifold has a canonical smooth real manifold structure.
 
@Fargle What are you working on?
 
2:28 AM
Fargle, no doubt, is goofing off like the rest of you.
 
I guess I'm wondering how many nice properties of complex functions over real ones translate to those of manifolds
 
@Balarka: at this precise moment, nothing. In the grand scheme, everything.
But more precisely, lately it's been Artin and Ted's book.
 
Awfully vague, Demonark.
 
@Fargle Aha, that's a nice combination to have.
 
For example, on a connected compact complex manifold, the only holomorphic functions are constant. But there are zillions of smooth functions.
Can you prove this?
Fargle, I did send you algebra stuff, right? I think I did.
 
2:31 AM
@BalarkaSen Agreed. I'm learning a lot from both. It's just that I think my bias is finally revealing itself--I think I prefer algebra to analysis.
 
(This is a proof of the fact that compact complex manifolds do not embed holomorphically in C^n for any n)
 
@Ted, you absolutely did.
 
in a similar vein to Daminark's question, what are obstructions for a real even dimensional manifold to have some sort of complex structure
 
(I think you need $b_2 = 0$?)
 
That's a harder question, Eric. You need to start with an almost complex structure.
 
2:32 AM
Ted can confirm this stuff
 
Nonsense, Balarka.
Try $S^2$.
 
Aw snap
 
rip
 
You're thinking Kähler constraints on the homology.
But there are lots of non-Kähler compact complex manifolds.
 
Ok, I think compact complex manifolds can't have free fundamental group, except Z.
 
2:33 AM
Right, for an almost complex structure I feel like reduction of the structure group of $TM$ should be enough
 
No, that's wrong, too. There are tori.
 
Those are free abelian.
Not very free
 
Oh.
 
Well, if it's compact you know it's bounded on the manifold, which is almost Liouville. I would guess that either charts or analytic continuation (guessing the former) allows us to push toward full power Liouville?
 
I actually don't know that.
Demonark: Charts are only local, so Liouville is not going to work.
(Remember that in the real setting a ball is diffeomorphic to all of $\Bbb R^n$, but that's false for the complex case.)
 
2:35 AM
@EricSilva S^6 admits an almost complex structure, but it's not known whether it admits a complex one, I think
 
Oh snap
 
Right, still unknown.
But think about more local tools you have in complex analysis, Demonark.
Not that you've studied it yet.
 
"Liouville's theorem" is not a bad intuition to have on that problem, I think
 
right, didn't Atiyah put out a paper on that that stirred controversy
 
But it's wrong for reasons I said. You don't have $\Bbb C$ charts.
 
2:36 AM
so i heard
@TedShifrin True enough
 
Yes, Eric, but there's no belief that it's right.
 
right
 
You can read Bryant's stuff on Chern's work on $G_2$ structures ...
 
@Fargle Algebra is good. What are you reading there?
 
Demonark: So what more local sorts of theorems do you know?
He said Artin, @Balarka.
 
2:38 AM
Yeah I was just asking what in Artin
 
Trying to catch up with my notes, but I got to chapter 5. I find it odd, but interesting, that he builds so much machinery into the dihedral groups.
 
There are even dihedral groups, too.
 
The distinction being?
 
what are obstructions preventing almost complex structures from being complex?
 
#Daminarkpun
 
2:39 AM
Oh. OH
BOOOOOOOOOOOO
 
rofl
 
(Good pun.)
 
Why did Demonark's name go on that? Cuz it's that bad?
 
The things I've "seen" in complex analysis are the early results about C-R, real and imaginary parts being harmonic, Cauchy integral theorem/formula and Liouville, singularities, Laurent series, branch stuff, and Christoffel-Schwarz
 
There's an integrability condition (sort of like Frobenius, Eric — in fact, if you assume real analyticity, it is Frobenius). Look up the Nijenhuis tensor.
 
2:40 AM
The further you go along that chain, the sketchier my grasp becomes, starting from "sketchy"
 
Demonark: Nothing on maximum principle?
I would expect that the physics guy talked about the mean value property of harmonic functions and the maximum principle.
 
The name doesn't ring a bell right away
 
Well, it's relevant to our discussion.
Or open mapping.
 
ooohhh sweet @Ted
 
Huh, I hadn't heard of the maximum principle either. That's neat.
 
2:42 AM
It's possible that came up very briefly
In one of the early lectures he was blasting through results like crazy
 
For harmonic functions it's an exercise in my book :P
 
20 theorems in an hour and a half
 
You need a real course.
 
So maybe that happened then
 
I <3 harmonic functions
 
2:43 AM
A real course?
 
harmonic functions best functions
 
On complex analysis?
 
@Daminark pls stop
 
get outta here
 
Lolol
 
2:43 AM
Eric, did you make a note about my comment about Bochner-type theorems? Wu (at Berkeley) wrote a nice book on them.
 
(Also @Balarka what was your pun from earlier? I actually didn't pick up on it)
 
It wasn't mine, it was Ted's
 
But yeah I do intend to do it eventually
 
I think Balarka and Demonark belong in a dungeon punning at one another.
 
yes! I usually write down some of the sage advice I receive in here to look at later @Ted :P
 
2:44 AM
some
 
OH LOL
 
well when I have pen and pad lol
 
@Fargle can you classify groups of order 4
 
isn't the maximum principle for holomorphic functions just a corollary of the fact that they're open maps
 
if yes, replace 4 by 6. if yes, replace 6 by pq. if yes, prove the Sylow theorems
 
2:45 AM
@BalarkaSen $V$ and $\Bbb Z_4$.
 
which I think @Daminark should have proved with Schlag unless he did different exercises
 
Geez, Balarka. Sylow Theorems are a few chapters later.
 
good, good
 
$S_3$ and $\Bbb Z_6$.
 
yeah I was kidding
 
2:46 AM
Yes, @EricSilva.
 
Well, we proved that polynomials are
Oh well I guess you extend to power series
 
Power series aren't as useful as you think.
 
I can't classify for $pq$ yet, but I do know thanks to theorems on product sets and so on that $\Bbb Z_p \times \Bbb Z_q$ is one such group (if you meant for $p,q$ to be prime).
 
Theorem: Whatever is true for polynomials is true for power series. Corollary: Power series have finite degree.
 
And I also know that $\Bbb Z_{pq}$ is isomorphic to that group.
 
2:48 AM
Woo! :P
 
Balarka really needs to go to un-unsleep.
 
@Fargle Only when $p \neq q$.
But yeah.
 
I figured that was implied. :3
 
@Daminark the idea is that there are coordinates that make any holomorphic function look locally like a really simple regular polynomial.
 
Group actions is the coolest stuff, Fargle.
 
2:48 AM
lol sorry for notation.
 
You'll find a bunch of my questions on that.
 
@TedShifrin Absolutely. There are so many clever results that the shift in perspective leads to.
 
\('-')/ group actions!
 
Artin has great exercises, Fargle, so enjoy them.
 
group actions are love, group actions are life
7
 
2:49 AM
I intend to. I just need to wade out of this chapter on linear transformations...
 
But yeah @EricSilva that this allows us to extend makes sense
 
@Daminark you should properly learn something about harmonic functions eventually. They're very useful for a lot of things.
 
Physics.
Harmonic analysis.
 
*harmonic functions*
*harmonic analysis*
Checks out
 
geometry!
of particular flavors
 
2:52 AM
But yeah at some point in life I do intend to do so. Do they come up in granola?
 
We already know Demonark hates geometry, Eric.
 
depends who teaches, they definitely should in complex analysis.
smh :(
 
@Ted Where'd you get that idea from?
 
I inferred it from your abhorrence of things concrete.
 
I was intending at some point to read up on Chern's diffgeo book
 
2:53 AM
the book Schlag lent me?
 
See how you do with my baby stuff in boot camp first.
 
I don't have abhorrence of concrete things so much as that I'm not good at it yet
Lol for sure
 
I like that book
 
Which book, Eric?
 
lectures on differential geometry
 
2:54 AM
Chern/Chen/Lam?
 
yup
 
Yeah
 
Still weak for no exercises.
 
yes
 
Wait none? Kek
 
2:54 AM
that's how i was feeling
 
None.
That's why you need my reams of homeworks :)
 
Your exercises could supplement well though
 
I need to keep reading Riemannian geometry
[cracks open book]
 
Does it at least leave out details?
 
LOL, no.
But it's not wordy.
 
2:55 AM
(That statement, in isolation, is rather bizarre)
 
I could give you some stuff from Clelland in the bootcamp if you really want to read that book @Daminark
 
/r/nocontext
 
Hi @Dair
 
@Ted Hi
 
@Dair interesting, didn't know that exists
 
2:56 AM
@BalarkaSen there is even /r/evenwithcontext
 
The exposition is quite clear imo, it is terse enough that you can fill things in which is helpful
 
Thanks @Eric! I'd be down for taking you up on that offer once we get through curves/surfaces
 
but reeaally needs exercises
 
@Dair I knew about the former, not the latter
 
Schlag also lent me something by Cartan "Riemannian geometry in an orthogonal frame" but it was waaaay harder going because it was so alien
 
2:57 AM
Cartan wrote several little books. That title doesn't sound quite right.
 
Oh, I forget the French title, but it's different.
 
ah I see
I'll read this at some point, hopefully in the French so I have some reading practice
 

« first day (2490 days earlier)      last day (2530 days later) »