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03:01
a lot of foliation theory too, in my understanding
@TedShifrin I looked it up, and it taught me something new, but the notation of a Hölder space is $C^{k,\alpha}$, where $k\geq0$ is an integer and $0<\alpha\leq1$.
@Clarinetist I find it a touch annoying that the top-rated answer there starts with "I have no idea about the level of activity in finite group theory."
oh and geometric group theory too
@BalarkaSen that's part of q u i k m a f s
Some stuff on profinite groups involves quite non-trivial knowledge of finite groups (after all a profinite group is just a inverse system of finite groups, with all the information boundled nicely in one object). People do care about profinite groups, see e.g. Galois cohomology
03:05
2+1=4-1=3 quick topology
hey @BalarkaSen
There is something I don't quite understand so flabby sheaves are ones where you can extend sections from a closed set onto the whole space X.
Constant sheaf is just if we pick a particular group G. Then to each open set $U \subset X$ we associate the map $p : U \rightarrow G$ right ?
where G is a group
No. Constant sheaves are locally constant functions
Sections over an open set are constant maps to your coefficient group
I think
03:11
no, that's not a sheaf
oh okay I see
Well sheafify it
yes, then you get what I said
yeah that makes sense that locally constant now I see why it is not soft sheaf.
@Mathei aha
03:12
sure
@MatheinBoulomenos do you know about etale cohomology ?
I tried to take a course on it, but it was too advanced and I dropped it
but I know a little bit, yes
I am interested in the construction do they use etale spaces or something ? I am just wondering because they have the same name.
curious
no, they don't use etale spaces. The "sheaves" that are considered are not even defined on a topological space
oh
The sheaves are defined over certain categories. It's really bad
03:15
that is weird how they consider sheaves on categories aside from topological spaces.
you give your schemes or whatever the Grothendieck topology, which is a category where the analogue for "open sets" are etale morphisms from other schemes I think
So it's not even a topology
yeah, that's correct
I mean schemes are defined using topology.
Cool I still remember this shit :)
i.e it is locally ringed spaces
I see @BalarkaSen
03:17
you can also define schemes as functors from the category of rings to the category of sets that satisfy certain conditions (I don't remember which exactly)
something something functor of points something
oh man that is high level shit when I will get to do this certain stuff in math, which I will I will probably be high all the time haha
@MatheinBoulomenos I see that seems very unnatural construction. I guess probably has uses in whatever advanced things people do.
it gives you a nice cohomology theory for varieties over fields other than $\Bbb C$ (and it gives you the topological cohomology groups of a variety over $\Bbb C$ only from algebra)
I see
"cohomology theory" okay wait hold on schemes are cool now where do you study them?
03:20
that's the main motivation. The whole thing with sheaves on categories actually became it's own field of study, called topos theory
have you seen this universal cohomology theory btw at some point ? I keep hearing that is something called motives.
I heard some stuff about it
but it too hard for me
@Daminark Ravi Vakil is best resource
hartshorn is sh!t
I agree, Vakil is awesome.
not hatcher
03:22
a nerve throbs dangerously on the front of his head
here edited it @BalarkaSen :P
looooooool
Lol, aren't they kinda different books though? I'm not completely fond of the idea of doing Hatcher anyway because I'd rather just invest in what I have hope in understanding, which isn't pictures, but like, hatcher is AT and Vakil is AG
Oh Hartshorne kek
Hatcher is awesome
03:23
@Daminark you want to know some commutative algebra before you start Vakil, though
@MatheinBoulomenos he can learn it as he goes
actually all of the commutative algebra is presented in his first chapter
if he can do all of the exercises in first chapter then he is good.
Mathein: yeah I'm auditing a commutative algebra class now. Originally I asked the prof for the AG class this quarter to send his notes (since he always TeXs notes for his class)
@MatheinBoulomenos Isn't that true of Hartshorne too? I thought/heard the background needed for Hartshorne is intimidating
for Hartshorne you don't need "some" commutative algebra, you need a lot
03:24
haha
But then yeah that happened. I don't think I'll be able to absorb all that background by... tomorrow morning
So yeah that's not happening, I'll hold off on AG until later.
For Hartshorne you need to master commutative algebra.
Get Atiyah-MacDonald bro
@Adeek I thought the first chapter is on category theory, not commutative algebra?
Speedread it
03:26
You need to know stuff like localizations, noetherian rings etc.
Though if so much stuff in AG involves cohomology theories I should def look into that subject eventually
@MatheinBoulomenos yeah, but he includes a lot of the commutative algebra that you need to know for later on.
Or if you want your life to be easier, Reid's commutative algebra
@BalarkaSen I second that
In particular he includes Localization, Noetherian rings, etc
03:27
Both Reid and A-M has equivalent amount of material, I personally found the first to be easier to read. They are both super thin.
In like 5 chapters you'd know 80% of the background you listed
@Balarka this is what we're using in commutative algebra rn: web.mit.edu/18.705/www/13Ed.pdf
The first 5
A-M is pretty good
First 5 of Reid or A-M? Or both?
Or.
Their ordering of the chapters is the same
@Daminark this looks fun
03:30
I don't think you can do all the commutative algebra for Vakil without some other resources
iirc Atiyah wrote that book to teach himself commutative algebra lol
not sure if urban legend
It's believable, he comes from a geometric/analytic background
sure @MatheinBoulomenos I did commutative algebra before currently reading Vakil
You also want Galois theory
For what?
For the examples over non-algebraically closed fields
03:32
Anyone have tips to do well on math contests
I feel like I always quite before the last step or make stupid mistakes
sure
Just so annoying*
I've practiced under time pressure, without time pressure; No difference
There's a really cool story on a commutative algebra book and it's true: Zariski and Samuel, both algebraic geometers, wanted to write a book on algebraic geometry. They started with the appendix on commutative algebra. As they wrote on, the appendix grow larger. Eventually they said "skew it" and worked out the appendix, it's now a massive, two-volume book series on commutative algebra. (The work on alg geo never appeared)
rip
I have heard of Zariski-Samuel
It's like an epic tome
@BalarkaSen Is it also a monic tome?
3
03:38
Please stop
STOP
03:55
Oh shit someone's making a movie based on 1.Outside
a horror director called Agnieszka Smoczyńska apparently...
04:15
[Random word]
Galileo Algebra
My reaction to that is
Is there a means of finding a n-digit number whose digits' sum is x, and product is y?
I.e, a 7 digit number whose digits' sum is 60 and product is 1740
factorize the product
1740 = 2x2x3x5x29
lol what, 29
Well now that could be a problem...
04:54
@MatheinBoulomenos @BalarkaSen just bit confused on something. So, if we have a exact sequence of sheaf mapping $0 \rightarrow A \rightarrow B \rightarrow C \rightarrow 0$. We are asumming right from the bat that $\phi : A \rightarrow B$ is an injective sheaf mapping ? Or is it just injective on stalk level only ?
Yeah it is monomorphism whatever let us call it injection on stalk level
 
2 hours later…
06:50
@Adeek A sequence of morphisms of sheaves is exact iff it is exact at the stalk level
07:11
@Adeek unlike for surjective, an in injective sheaf map is actually injective on all sections
(Both of these statements are in Hartshorne II.1 - mine is exercise II.1.2)
Why is the commutative algebra/algebraic geometry crew gathering again? Go, shoo, scurry off
hi @MatheinBoulomenos @BalarkaSen
Balarka your geometric topology squadron is heavily outnumbered now
07:18
Oh shit not Daminark
Not you
@BalarkaSen given a loop $X : S^1 \to S^2$ that does not pass through $pt \in S^2$, how would you describe the processing of constructing the induced loop $X' : S^1 \to \Bbb R^2$?
This is 5 to 1 right now
@LeakyNun Compose with stereographic projection
@BalarkaSen but you can't compose if the codomain/domain don't match
The codomain of your dude is $S^2 \setminus pt$
that's the range, not the codomain
07:20
Nobody cares
If I ping someone not in chat, they'll still get the notification, right?
@CookieToast yes
The point is $X$ gives a map $S^1 \to S^2 \setminus pt$
@BalarkaSen Surrender immediately. Resistance is futile.
@BalarkaSen how would you describe the process of obtaining the loop $X'' : S^1 \to S^2 \setminus \{pt\}$?
07:21
Also, is there a way to make paragraphs aka "return" in chat without having to send the message and then write the next line?
@CookieToast shift+enter
$X$ factors through the inclusion $S^2 \setminus pt \to S^2$
Thanks Leaky :)
Are you happy now? Do you want me to draw a diagram for you?
Fucking hell
@BalarkaSen is "factor" a part of the subspace topology?
07:21
What?
$S^2 \setminus pt$ does get the subspace topology from $S^2$ if that's what you mean
The inclusion map is inclusion as a subspace
i.e. is it that you can factor because $S^2 \setminus \{pt\}$ is a subspace of $S^2$? what allows you to factor?
What do you mean, "allows you to factor"? You can write $X = i \circ F$ where $F : S^1 \to S^2 \setminus pt$ because $X$ misses the point $pt$.
In the category of topological spaces if you will.
I mean, what is the technical term for this? "because $X$ misses $pt$" sounds a bit arbitrary
There is no technical term for this because it is not technical
It is basic. Learn to speak English.
I used the word "pullback" but Mathein commented that it was too convoluted
pull back the loop $X : S^1 \to S^2$ along the inclusion $g : \Bbb R^2 \to S^2$
07:24
Yes, Mathei knows how to speak English, even though he's a German algebra machine.
good morning
well i dun now hu tu spek english
oh hey @ManolisLyviakis
speak of the devil
if $X \subset Y$ has the subspace topology. then for any map $f:A \to X$, $f$ is continuous iff the composition with the inlusion $X \to Y$ is continuous. This follows immediately from the definition of the subspace topology, but you can also call it "universal property of the subspace topology" if you like to give things fancy names
sorry for leaving yesterday had a boat to catch
@ManolisLyviakis anyway, from $X:S^1 \to S^2$ you can construct the loop $X' : S^1 \to S^2 \setminus \{pt\}$ because $X$ misses $pt$. Then, since you have $g : S^2 \setminus \{pt\} \to \Bbb R^2$ and $h : \Bbb R^2 \to S^2 \setminus \{pt\}$ being inverses of each other, you obtain the loop $X'' = g \circ X': S^1 \to \Bbb R^2$.
07:27
So far Leaky hasn't even raised a topological point. His confusion about the so called "restricting codomain" is set theory.
since $\Bbb R^2$ is simply connected, you obtain a homotopy $X''' : S^1 \times I \to \Bbb R^2$
oh I thought the set theory part is clear
@BalarkaSen I don't like using concrete categorical terms, i.e. I don't like invoking the set-ness of the objects (cc @MatheinBoulomenos)
:theenk:
let $\iota : S^2 \setminus \{pt\} \to S^2$ be the inclusion map
07:28
oh ok @LeakyNun
then, $X'''' : S^1 \times I \to S^2$ gives you the homotopy of $X$ to a constant map, where $X'''' = \iota \circ h \circ X''' $
im reading what you previously said
"factors over the inclusion map $S^2\setminus\{pt\} \to S^2$" and "misses a point" is the same thing
@LeakyNun I don't like using my left leg. I will thus stagger around with my right leg instead of walking with both legs.
@ManolisLyviakis In one message:
07:31
@LeakyNun Wait what, why?
From $X:S^1 \to S^2$ you can construct the loop $X' : S^1 \to S^2 \setminus \{pt\}$ because $X$ misses $pt$. Then, since you have $g : S^2 \setminus \{pt\} \to \Bbb R^2$ and $h : \Bbb R^2 \to S^2 \setminus \{pt\}$ being inverses of each other, you obtain the loop $X'' = g \circ X': S^1 \to \Bbb R^2$. Since $\Bbb R^2$ is simply connected, you obtain a homotopy $X''' : S^1 \times I \to \Bbb R^2$. Let $\iota : S^2 \setminus \{pt\} \to S^2$ be the inclusion map.
Then, $X'''' : S^1 \times I \to S^2$ gives you the homotopy of $X$ to a constant map, where $X'''' = \iota \circ h \circ X'''$
Leaky wants to write mathematics using pages of homotopy type theory.
@Narcissusjewel because the whole point of category theory is to forget that the objects are sets
even in category theory, we sometimes prove things in the category of sets, then use Yoneda or some forgetful functor (for algebraic categories, the forgetful functor preserves limits, filtered colimits and is conservative)
07:32
@MatheinBoulomenos I was explaining to others, so I need to use concrete terms :P
@LeakyNun wut, that's an absurd statement.
@LeakyNun No, it is not the point. Concretizability is a very very useful tool. See what Mathei said.
In that it doesn't even make sense
the proof that filtered colimits commute with finite limits (which can be quite useful) in certain categories (groups, rings, modules, etc.) is really "1. prove it in the category of sets 2. use the forgetful functor"
07:34
Also note, 'forget' and 'object' are loaded terms when talking about category theory, so you should be careful with such claims.
@Narcissusjewel lol come on
You have a forgetful functor from set to... fuck
to dementia
you forget all the mathematics you ever learnt
(I'm shocked)
07:36
I'M SHOCKED
But I'm old
I am in the state of immense dismay which is similar to what a human being feels when he or she is getting electrocuted
Wait what, why?
@Balarka you're definitely not expressing it in the same way that someone being electrocuted does
I know how you feel though. I have to mark 35 assignments that are 10 pages long and riddled with errors now
@Narcissusjewel You don't get the meme?
It's a verbose meme
07:38
LOL!
Very well done
I think I missed it because it more or less says "I feel as though I am shocked"
Good lord
Very strong meme
Well played
07:41
Now to get drunk while marking these assignments, wish me the tenacity to get them done
Like I have seen that Jake Paul verbose meme a million times
So you guyz ready to help with some more topology?
But everytime I die a little bit inside
@Narcissus good luck
@Narcissusjewel are you also being verbose
07:42
@Narcissusjewel which class are you grading?
Ok @TedShifrin, here you go!
We have
$$\int_{-\infty}^{\infty}\frac{ue^{u}e^{yu}}{(1+e^{2u})(1+e^{yu})}du$$
$$ =\int_{-\infty}^{0}\frac{ue^{u}e^{yu}}{(1+e^{2u})(1+e^{yu})}du + \int_{0}^{\infty}\frac{ue^{u}e^{yu}}{(1+e^{2u})(1+e^{yu})}du$$

Looking at the first integral, substitute $u = -v$. We get $du = -dv$. The integral becomes
$$-\int_{\infty}^{0}\frac{(-v)e^{-v}e^{-yv}}{(1+e^{-2v})(1+e^{-yv})}dv$$
$$=-\int_{0}^{\infty}\frac{v}{(e^{v} + e^{-v})(1+e^{yv})(1+e^{-yv})}dv$$
Multiplying through by $\frac{e^{v}e^{yv}}{e^{v}e^{yv}}$ gives us
@ManolisLyviakis just ask
@LeakyNun YES
i know that if 2 spaces have the same homotopy type then their fundamental groups are isomorphic
is the reverse true?
if they are also path connected*
Take something like $S^1$ and $S^1 \vee S^2$
Take $S^2$ and a point
S^2 is not homotopy equivalent to a point. Non obvious fact!
The homo sapiens that is self-conscious and is capable of referring to itself as "I" has its brain sending chemicals around which causes the conscious homo sapiens to feel an uncomfortable state that a typical homo sapiens would refer to as "dismay", and the chemical sent is of high frequency which causes the feeling to become immense, and
trying to describe it, the homo sapiens uses analogy, which is comparing similarity between two concepts, and likens it to a physical feeling, which is feeling generated by nerve receptors on the skin, and the source being electrocution, which is applying
I think you need some other invariant (like homology or higher homotopy groups) to produce an example
07:51
@LeakyNun This is 10/10
$S^2$ and a point are not homotopic equivalent?
Let's compile them into a verbose meme
@ManolisLyviakis define "homotopy equivalent"
Can someone repost the long lost Hippa meme?
I want the imagery
Two topological spaces X and Y are homotopy equivalent if there exist continuous maps $f:X->Y$ and $g:Y->X$, such that the composition $f \circ g$ is homotopic to the identity id_Y on$ Y$ , and such that $g \circ f is homotopic to $id_X$
07:54
@ManolisLyviakis so S^2 -> pt -> S^2 must be a constant map
and that cannot be homotopy-equivalent to the identity map on S^2
@Leaky No...
It has to be homotopic to the constant map
@MatheinBoulomenos Two space can have the same $\pi_n$ for all $n$ without being homotopy equivalent I think
It takes work to prove the identity map S^2 --> S^2 is not homotopic to the constant map
@Alessandro False
oh wait
No, Alessandro is right
07:55
5 mins ago, by Balarka Sen
S^2 is not homotopy equivalent to a point. Non obvious fact!
so this is wrong, since S^2 and pt are homotopy equivalent
@MatheinBoulomenos No.
@LeakyNun They aren't; I typoed above
yes $S^2$ and the point are homotopic equivalent
@Daminark Calc II
07:56
the theory of Postnikov systems would be a lot easier if you only need to know the homotopy groups ...
@Balarka you're probably thinking weak homotopy equivalent, and even then I'm p sure that's false
@Narcissusjewel "I'm SHOCKED!" -- Ted Shifrin
And you can define contractible spaces by checking if they are homotopic equivalent to a point
You guys are all confused. State what you are thinking one by one.
Are $S^1$ and the pseudocircle homotopy equivalent? I have vague memories of weak homotopy equivalence
07:57
1) S^2 is not homotopy equivalent to a point
@ManolisLyviakis but S^2 isn't contractible
2) Two spaces with isomorphic pi_n for all n need not be homotopy equivalent
@ManolisLyviakis oh right they aren't
S^2 isn't contractible
im confused
So Alessandro said that two spaces can have the same $\pi_n$ without being homotopy equivalent
lmao
topology
@BalarkaSen That's what we're saying!
Oh.
Fuck.
That's like mega true
Well I stated my truths!
I just can't read
I can write
07:59
S^2 is simply connected meaning that its fundamental group is only a point
doesnt that make it contractible?
@ManolisLyviakis no!
No.
@AlessandroCodenotti No, $\pi_1(W) = 0$ if $W$ is the pseudocircle.
Simply connected means \pi_0, \pi_1 vanish right?

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