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X4J
X4J
00:52
Is Jordan-Holder for finite group theory actually classifying homomorphisms with simple image?
 
7 hours later…
07:44
Hi everyone. Has this been studied previously, and if so, can anyone give references/link for the same?
Dragging points A, B, C, D gives some pretty patterns
 
2 hours later…
 
3 hours later…
12:41
hi
can any axiomatic system prove all the first order truths about natural numbers that involve a single unbounded quantified?
Godel's theorem says that no axiomatic system can prove all the truths. but what if we restrict the number of quantifiers allowed in the truth?
12:59
@SohamSaha it is beautiful
13:17
Anyone knows how is "fundamental neighbourhood" defined in relation to a continuous path, in Alg. Topology?
There is no reference in the books neither can I find it online.
context please
it is in a proof of uniqueness of the path lifting property
one moment
@RyderRude you might like this one too then
Let $p:E\to B$ be a covering, let $b\in B$, and let $e,e'\in p^{-1}(b).$
Then a path $f:I\to B$ with $f(0)=b$ lifts uniquely to a path $g:I\to E$ such that $g(0)=e$ and $f\circ g=f.$
The proof:

subdivide $I$ into subintervals each of which maps to a fundamental neighborhood under $f$, and lift $f$ to $g$ inductively by use of the prescribed homeomorphism property of fundamental neighborhoods.
where is that proof from?
13:24
May's concise course in alg topo
He defines that term.
where?
oh yes
found now
for some reason search did not show this
Interesting that the Lie derivative of a symmetric (0,2)-tensor field is an $L^2$-adjoint of the divergence
I once studied May's book. I liked it because it's really concise but couldn't learn new stuff because it's too concise.
It's not intended for a first exposition
X4J
X4J
13:52
If G is finite group and $Z(G) \cap G’ \neq 1$, is $G$ nilpotent?
I struggle to come up with a counter
14:02
$G^{\prime}=[G,G]$?
X4J
X4J
Yes
14:24
do you ever wonder what life would be like without $\infty$-cooperads? no? me neither
it's not true, it seems like SL(2,5) will be a counter-example
it's hard to imagine, I know
@BenSteffan the new patch finally added coop gameplay?
yes, and it has so many exciting features
like the trivial $\dashv$ primitive adjunction
$\infty$-coöperads, if you prefer the diaresis (you'd be wrong if you did but hey)
14:42
@BenSteffan this sounds like you're one step away from channeling Urs and talking about Dasein vs Being
 
1 hour later…
X4J
X4J
15:43
@Thorgott could you explain why you’ve considered specifically SL(2,5)?
I checked some of the dihedrals and these arent a counter
I figured it would be easy if $G=G^{\prime}$, so I googled "perfect group with non-trivial center", got SL(2,5) and then checked on the wiki that it's not nilpotent
(I am not suggesting whatsoever that that is an in any way optimal counter-example or route to achieve it, but I was not willing to actually expand any of my own thought on the matter)
@Thorgott such are the horrors of unstable homotopy theory
all of this for Quillen's Lie algebra model :(
X4J
X4J
Oh I see I was just interested to know if its true because the last step of a lower central series
It seems that it is a subgroup of that intersection
@SohamSaha nice
16:12
The thought of having an oral exam on Monday about all of this fills me with happiness and joy. I am not at all worried about it :l
I thought you'll have an exam on 3-manifolds, not infty-cooperads
"hmm, I should look up examples of comonads" opens MO "The universal covering is an idempotent comonad on topological spaces" closes MO
@Thorgott It might come as a surprise to you that people sometimes have more than one exam :^)
3folds was today
Koszul duality is on monday
X4J
X4J
16:29
@Thorgott It should be right?
16:47
"quality time" indeed
17:38
Hey, quick dumb question, is there a nice intuition to make Itos lemma feel a little less 'gross'? My brain just wants it to be df = f_x (dX) + f_t, which I know isn't right but my brain keeps defaulting to it
17:51
@BenSteffan I've been out of the game too long :P
how did 3-manifolds go?
@Thorgott it went great. as soon as they asked me to do something using geometric intuition I got completely stuck but luckily they only did that once and it didn't affect the grade lol
I've just decided to postpone the monday exam to march
@Thorgott lol
18:24
Long time no see @BalarkaSen, how's life? What are you up to these days?
@BenSteffan nice! no Borromean link tho?
@Thorgott thank god no
I had to draw a hopf link
that I managed, but had they asked for more I would have failed :)
18:41
the Whitehead link is also a funny one
yes, that one would also have killed me
although there is a really ugly way to draw the borromean rings I think I could manage
it's really not that bad unless you challenge yourself to not erase parts of the lines when you realize something has to pass over
It would take me an embarrassing amount of time figuring out what goes over what
my capacity for geometric thinking is really poor
always has been
spatial reasoning is the term I guess
I don't think mine is good either, but at least that much I can manage (I hope)
somebody I know went through phd applications this term and in some place (I think it was something like Oxford or Cambridge) they only asked him about knot theory in the interview
that is a scary prospect :')
but then again he works on the geometric side of things
19:21
Hi
hoping that the knot theory talk I've put on my cv won't come back to bite me
Hi
Anyone know the structure theorem here?
binky: structure of what?
ah yes, the structure theorem
describing the structure of... reality at large
19:25
For linear systems
I don't think that's a name in common use
yeah, i don't know of anything that is commonly referred to by that name. even "linear systems" could be a couple of things. what is a situation where you might want to use the theorem?
@BenSteffan Do you know the Assembly language?
the one and only? of course :)
Practically it says that gives a linear system $ Ax=b$ and $v_0$ a solution of it , Then the solutions are of the $v=v_0+w$ type where $w$ is a solution of the homogeneous system associated with $Ax = b$
19:33
@BenSteffan I will have to program the Motorola 68000, I hope it's fun :-)
@Pizza ah, I've only worked with x86
but asm is fun, in it's own twisted way :)
binky: ah, OK. i think you could safely assume that many people in the chat are familiar with it
👍
19:52
(1). If $X$ is a smooth manifold and $Y$ is a T2 second countable space and $f:X\to Y$ is a local homeomorphism can one give $Y$ a smooth structure? (2). What about if $f$ is a map $Y\to X$?
I think true both cases
20:14
@monoidaltransform Obviously not for 1 — take Y to be a disjoint union of X and something which is not a topological manifold. Yes for 2: smooth structure is a local structure, and you just transport it via local inverse of f.
surely you'd want to assume that $f$ is surjective for 1
I don't think (2) is clear either
yeah
what does "smooth structure" mean here?
it's obvious that $Y$ is a top manifold, but the transition maps of the induced atlas can get a bit wacky
I mean, can one construct an atlas for $Y$ making it a smooth manifold?
20:25
define "manifold"
So for $(2)$ it is true?
second countable hausdorff space that is locally euclidean??
ah, you're assuming $Y$ is second countable
I should just go to bed :)
I don't know if you can, try it
If i am understanding things correctly, math.stackexchange.com/a/350743/1226695 shows that its not true?
@a9qd0wf i dont think (2) is true.
this is going in the wrong direction...
20:32
I might be missing something but I dont see why it is false then (if f is surjective)
(2) I mean
Hello,
Is it possible that the covering map $p:\tilde X \to X$ may not carry over the entire homotopy into $\tilde X$?

Where $\alpha$ is null-homotopic on $X$, and $\tilde\alpha$ lifts to $\alpha$ through covering map $p$.
@monoidaltransform nobody said it's false
@flowian this question is not clear to me
ok i am convinced (2) is true. Charts in X $(U,\phi)$ that are small enough should be transported to $Y$ by $(f^{-1}(U),f^{-1}\circ \phi)$ or something
sorry, I got confused by chatGPT. With this.
Given a covering map $p:\tilde X \to X$ and a loop $\alpha:[0,1]\to X$ based at $x_0$. Suppose $\tilde \alpha$ is a lift of $\alpha$ with $\tilde \alpha(0)=\tilde x_0$.

I'm trying to show that if $\alpha$ is null-homotopic in $X$, then either $\tilde \alpha$ is null-homotopic(which makes sense through $p$ sending open nbhds to open nbhds homeomorphically) in $\tilde X$ or it is a loop that does not bound in $\tilde X$
@monoidaltransform yes, but as I said it's not so clear how the transition maps behave
@flowian my advice is: never use chatgpt for any serious mathematics
20:43
Good advice, it just generated bunch of Qs I've got to check
The question is: Prove that if $\alpha$ is null-homotopic in $X$, then either $\tilde \alpha$ is null-homotopic in $\tilde X$ or it is a loop that does not bound in $\tilde X$.


I don't know anything about second part of this Q.
bound what?
20:57
No idea, seems like chatGPT's gibberish
sorry
I guess what I really wanted to ask is if the lift of null-homotopic loop can ever be NON null-homotopic?
that question is still a little bit imprecise, because the lift of a loop need not be a loop
and paths are always null-homotopic since the unit interval is contractible
so I suggest you introduce fixed endpoints
21:14
"lift of a loop need not be a loop" Do you happen to have an easy example for this?
consider the universal covering of $S^1$...
oh right
thanks :)
21:39
> Consider $f$ continuous on $[0,1]$ and suppose $$\int_0^1f(x)x^n\,dx=0\quad (n=0,1,2,\ldots).$$Then $f(x)=0$ on $[0,1]$.
Apparently this is due to the Stone-Weierstrass theorem, since there exists a sequence of polynomials $(p_n)$ such that $p_n\to f$ uniformly. The text goes on to say that since $f$ is bounded, $(p_n)$ is uniformly bounded, meaning there exists an $M$ such that $|p_n(x)|\leq M$ for all $n$ and $x\in[0,1]$.
I wonder, why does $f$ bounded imply that $(p_n)$ is uniformly bounded? I know of the theorem/proposition that every uniformly convergent sequence of bounded functions is uniformly bounded. I guess each $p_n$ is continuous, hence bounded on $[0,1]$, so indeed they would be uniformly bounded according to this result, but I don't see how $f$ being bounded would imply $(p_n)$ being uniformly bounded.
Whatever. I can live with this error.
Maybe they meant to phrase it differently.
$|p_n(x)|\le|p_n(x)-f(x)|+|f(x)|$
why do you assume it's an error? forget about continuity, maybe it is confusing things for you because boundedness can be thought of as a consequence of continuity on [0,1] in this case. prove that if a sequence of bounded functions converges uniformly to a bounded function, then the sequence is uniformly bounded
on any subset of R (or your favorite R^d), not just [0,1]
or see if you can generalize this framing to something that seems even more natural to you
it is very very very close to the definitions
21:59
ok 👍
@Thorgott hmm, but $|p_n(x)-f(x)|$ might not be less than $\epsilon$ say for $n$ large enough, or?
maybe it doesn't matter
that's the definition of uniform convergence
ah right :) I mistyped
I meant to say, for finitely many $n$, $|p_n(x)-f(x)|$ might be larger than $\epsilon$, so we have a problem with finding a bound that does not depend on $x$.
we don't, finitely many exceptions are never an issue
ok
apparently ZF sans C is consistent with there being no injective abelian groups whatsoever
learn something new (and horrifying) every day
22:20
Suppose I have a homomorphism $\phi$ from the fundamental group $\pi_1(X,x_0)$ to $\pi_1(Y,y_0)$. Does there exist a continuous map $f : X \to Y$ such that $f(x_0) = y_0$ and $\phi$ equals the homomorphism induced by $f$?
In general, no.
There is no $f$ realizing the interchange-of-factors map on $\pi_1(\mathbb{R}\mathrm{P}^2 \times \mathbb{R}\mathrm{P}^3) \cong \mathbb{Z} / 2 \times \mathbb{Z} / 2$, for instance
@BenSteffan what is an injective abelian group?
injective $\mathbb{Z}$-module
an injective object in the category of $\mathbb{Z}$-modules :)
@BenSteffan AH, okay. Thank. I had a feeling this was so. Thanks!
welcome :)
22:25
@BenSteffan lol
101
A: Is the axiom of choice really all that important?

Asaf KaragilaImportance is a relative thing. For a computer scientist, or an applied mathematician, or a combinatorialist working with finite sets the axiom of choice might be the least important axiom in mathematics. As instances involving only finite sets will never require the axiom of choice. We can see...

@user193319 mathoverflow.net/questions/166153/… is a kind of 'high tech' point of view of this, for compact connected manifolds
@BenSteffan oh wow haha
@leslietownes the answer actually applies to any CW-complex
it's not just for compact connected manifolds
but the answer is also, well
any space with the homotopy type of a CW-complex
something something we don't know whether every manifold is a CW-complex
you have to get pretty damn lucky for all the relevant groups to vanish
it's always cool when somebody links an answer by someone you have a paper of open in another tab
22:35
oh no
...should I be afraid?
 
1 hour later…
23:40
@BenSteffan Yes.
What are we talking about?
people posting links to MO answers by people you happen to have a paper of open in another tab
"Denis Nardin? That name sounds familiar... wait a minute"

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