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5:00 PM
Orthogonal group? I don't think it's enough to have the determinant by $\pm 1$
 
having det 1 is SL
 
That's a much larger group, known as $\text{SL}^{\pm}_2(\Bbb R)$
sniped
rip me
 
yeah that's why I'm asking, first time working with them, no idea really how to write it as a set ?!
 
Orthogonal matrices are those such that $AA^t = I$
So it's just $\{A\in M_2(\mathbb{R}) : AA^t = I\}$
 
@BalarkaSen r e v i s i o n i s t h i s t o r y
 
5:02 PM
But yeah as an example, take $\begin{pmatrix} \frac{1}{2} & 0 \\ 0 & 2\end{pmatrix}$
 
OK, I guess I will try to work with that.
 
This isn't orthogonal, but it has determinant 1
 
Yes I understand
 
@Eric ur too deep into politisociology fam
OK gotta go now
Cya
 
i actually am and it's driving me insane
 
5:03 PM
the solution is to MAGA harder
 
uh oh
 
i don’t know why daminark didn’t say “columns hav norm one and dot product zero”
 
Seems it's a bit too late for me to edit the message and say that. Oh well, revisionist history has a 3 minute timer, it seems
 
@Daminark if only real life had such rules
 
2
Q: Complicated Proof of Free Group Generators by $n$ elements implies $\operatorname{Rank}(F) \le n$

user193319First I must point out the following corollary in my book: If $G$ is a finitely generated abelian group generated by $n$ elements, then every subgroup $H$ of $G$ may be generated by $m$ elements with $m \le n$. Here is the problem I am working on: If $F$ is a finitely generated free gr...

 
5:14 PM
So this is how a pump works
You can get from the left to the right but not from the right to the left
And you've got springs in there pushing the things out
Another type of vacuum pump
Gasses can get from the outside in, but not from the inside out
so you put like a hole in the middle for gasses to exit through
 
@Mathei @Balarka are you here?
 
$\approx$
 
@AkivaWeinberger It's cool!
 
Scroll pumps are great. I have a few in my lab. They don't need any oil or other liquids to work
So good as a first stage of ultra high vacuum pumping system
 
Hey @Mathein, @Alessandro, and @Akiva
 
Hey @Daminark
 
Mathein :D
Can i invite you fast to our room ?
 
5:38 PM
okay
 
@LeakyNun are you here
 
Guys can someone help me over my physics problem ? It's about a disc undergoing pure rolling
 
I want to find ${ 2 }^{ 100 }(mod\quad 1000)$, so I do:$\phi (1000)=400\\ { 2 }^{ k*400+1 }(mod\quad 1000)\Rightarrow k=2\\ { 2 }^{ 100 }(mod\quad 1000)\equiv { 2 }^{ 801 }*{ 2 }^{ 199 }(mod\quad 1000)\Rightarrow { 2 }^{ 200 }(mod\quad 1000)$. But now I'm stuck with $2^{200}$. If I repeat the process, I'll have a fractional $k$; Am I doing something wrong?
 
@Mathein oh so one thing I've been wondering
In commutative algebra we put the Zariski topology on Spec(R)
 
5:50 PM
But what exactly does that topology do for you? I've heard it as a buzzword before but it feels like it's an iffy topology
e.g. not being Hausdorff
 
So Mathei remember how you told me you can think about Noetherian normalization in terms of branched coverings? I'm trying to make sense of a short paragraph on the same topic in Reid's book
Hi @Dami
 
Hmm, you can think of the topology as enconding information about the partial order of ideals (that's just a reformulation)
but you can translate algebraic properties of your ring into topological statements about Spec(R)
and then you can use topological reasoning
 
The point is that algebraic functions are continuous, and (iirc) vice versa
 
If someone can help that would be great
 
Your topology shouldn’t have many more continuous functions than the ones that are relevant
 
5:53 PM
for example, Spec(R) is connected iff it's impossible to write R as a product of two non-zero rings
@MikeMiller are you talking about the Zariski topology on the prime ideals or classical algebraic geometry?
 
The latter, but with prime ideals in mind still
 
You can also show that if you divide out by the nilradical, then the topology on Spec(R) doesn't change
so this shows that $R$ is a product of two non-zero rings iff $R/\operatorname{nil}(R)$ is a product of two non-zero rings
you can show this also algebraically
but it's a bit messy to lift idempotents
(of course, there's still algebra involved in proving the statements that relate algebra and topology)
 
How does one get a ring homomorphism $k[W]\to k[V]$ from a polynomial map between varieties $V\to W$?
 
you pull back regular functions
 
Hi can anyone help me out with my problem please
 
5:59 PM
makes sense
 
@Daminark there's other stuff you can do as well. For example, irreducible components of Spec(R) correspond to minimal prime ideals, so if you can show that Spec(R) has only finitely many irreducible components, then you proved that R has only finitely many minimal prime ideals
this works e.g. for Noetherian rings
Also Spec(R) is Hausdorff iff R is 0-dimensional, i.e. every prime ideal is maximal
there are a couple more of these facts that relate algebraic properties of R and topological properties of Spec(R)
they can be really handy
but I think the main reason to care about the Zariski topology is that you can define sheaves on it
 
@MatheinBoulomenos I can see this for finite rings for sure, since every point is closed in a finite space => discrete
 
Spec(R) might be finite, but not discrete
 
can someone tell me what is a compact group ( in easy way )
like very short description
 
it's a topological group whose underlying topology is compact?
 
6:04 PM
I know what a compact topological space is
 
that probably doesn't help
 
it does not since I did not take topology
><
 
Right, yeah, topological groups are a thing
 
our teacher talked about that in rep theory
 
maybe you know compactness from analysis?
 
6:04 PM
yes
 
the thing with open coverings?
 
The group $\Bbb R/\Bbb Z$, or addition mod 1, can be thought of as a circle
 
Well i have a small idea about that
 
A circle is closed and bounded and thus it is compact
 
I meant if R was finite, then Spec(R) is a finite $T_1$ space, that should imply discreteness, no?
 
6:05 PM
did not get it very well from analysis either
akiva
the way our teacher presented compactness
was more than bounded and closed
 
@Daminark yeah that's true
 
who is starring the names i write?
is that you dami ?
 
Nope
 
-.-
Ok i wont bother you guys
 
I usually star witty or r/nocontext things
 
6:07 PM
I ll keep doing my algebra thing , see ya later :D
 
@Mathein How do sheaves come into play?
(Though note that my idea of sheaves is very vague, just as a way to track local data that somehow has to do with contravariant functors from the poset of open sets of a space to sets)
 
@Daminark going to eat dinner now, I'll explain that later
 
Yeah sure, see you around!
 
I am a student and interested in Math - Can I ask some questions if something is unclear? Even if maybe I can google it easily? Because so often it comes that a tutorial doesn't explain it as detailed as I want it. And here I'd have the possibility to ask as much as I want.
... and as detailed as I want :-)
... hopefully...
 
don't ask to ask; just ask
 
6:22 PM
clear enough - thanks :D
 
do isomorphism preserve open sets?
 
@AkivaWeinberger Eureka Forbes gang Eureka Forbes gang Eureka Forbes gang / spent 10 racks on a new pump / etc
 
Skortya: isomorphisms of what? topological spaces?
 
Between $\mathbb{R}^4$ and the 2x2 real matrix.
 
homeomorphism always sends open sets to open sets
 
6:32 PM
cool
 
@Daminark: Given a ring A, one defines a sheaf of rings on Spec A. One has a nice basis for Spec A given by D(f) := Spec A \ V(f), f \in A. Now one can show that it is somewhat sufficient to define a sheaf on a base of the topology and that uniquely extends to a sheaf on the whole space. On the base you now define the sheaf O to work like O( D(f)) := A_f, the localization of A on the set {1, f, f², ...}
Now (Spec A, O) is what geometers call an affine scheme and the central object to study in Algebraic Geometry are ringed spaces (Spaces with a sheaf of rings on the space) that locally look
Maybe that helps... if not I'm sure Mathein can tell you more than that :)
@Daminark ah it may be interesting, that given the construction above one finds that O(Spec A) = A. So you don't lose any algebraic information when changing from A <-> (Spec A, O). That is, for example why it is important to have the structure sheaf together with Spec A: Spec A itself certainly encodes algebraic information topologically, but it doesn't quite catch all the information of A
 
6:48 PM
yeah, that was pretty much what I was about to say. If you're not familiar with the notation O(Spec A)=A means that you can think of elements of A as "functions" on a the space Spec(A)
 
Okay so I'll just try to absorb this, give me a second
 
did someone say schemes ?
do you want me to tell you a story about schemes ?
@Daminark
 
Well wait for a bit, I'm still a noob at this and just wrapping my head around what's being said
 
schemes are just topological spaces that have algebraic local data
nothing more nothing less
 
Sup guys?
 
6:56 PM
you can do cool things with schemes
 
Well the point is more, I should understand the formality behind it and why that translates down to what you're saying
 
yo @MatheinBoulomenos
your familiar with fibrations right ?
yeah @Daminark
 
yo @Adeek
 
yo yo yo
@MatheinBoulomenos I was wanted to discuss something related to fibrations and chow groups
 
@Adeek I'd give that a 0/10 if that's your answer to why schemes are interesting.
 
6:58 PM
Anyone here familiar with surreal numbers?
 
@BalarkaSen it is your personal opinion. Actually one can get a lot of interesting things out of that intuition
 
@Adeek I know basically nothing about that
 
The statement "you can do cool things with schemes" carries no intuition. Don't be delusional.
 
I am not delusional
I don't know why your rude sometimes
attacking people who don't hold similiar ideas as you is a form of nazism
 
Lol
 
7:02 PM
As far as I can tell this chat is for people to share their ideas not to get attacked for thinking in certain way.
@BalarkaSen For the record, I didn't say you can do cool things with schemes as the intuition. I said topological space with local algebraic data.
You can use this intuition for example which tells you schemes over $\mathbb{C}$ admits mixed hodge structure.
 
Has anyone read "Analysis on Surreal Numbers?" One of the definitions in there gives me an interesting connection with schemes
 
You are not getting attacked for holding ideas different from mine. You are getting criticized for passing off word salad as mathematical pedagogy/motivation, which is basically a form of intellectual dishonesty (an opinion not just possessed by myself, but many others)
 
@lush okay so to my understanding, sheaves are supposed to be the sorts of things that you can glue together (I'm getting all this by thinking of smooth functions on a manifold), so I sorta buy that yo can do this. Now let's say you're talking a finite union just to be a bit easier
 
Schemes allow us to use "geometric" reasoning to study algebraic objects, among other things. They also generalize and shed light upon the objects of study in classical algebraic geometry. (E.g. there are moduli spaces for algebraic varieties which are schemes, but not varieties)
 
I am saying what I believe is nice intuition of thinking about schemes
 
7:06 PM
@Adeek wanna help me understand why projective space as a scheme over an alg. closed field behaves like you'd imagine?
 
Arguing over someone else's personal intuition on a concept is pretty rude imo
3
 
@Daminark a finite union of what?
 
You clearly haven't understood my argument if you think I was arguing over personal intuition.
 
So would you have something like, $O(D(f_1)\cup \ldots \cup D(f_n))$ is the localization of $A$ at the set of products of the f_i?
 
I mean this entire conversation is practically informal, not some academic conference here
If it was then I could see your aim
 
7:08 PM
Daminark: In general it is a little bit more difficult to calculate the sections ( <-> O(...)) over non-base sets
Daminark: Actually you have D(f_1) \cap ... \cap D(f_n) = D(f_1 * ... * f_n)
Think of it like this: the "smaller" your open subset, the "more" functions you have on it. If your ring behaves well (e.g. A is a domain), A_f embeds into A_fg, so by building the intersection of the D(f_i)'s you're making your subset smaller and A_{f1*...*fn} is a "bigger" ring
 
The theory of schemes created a displine that was able to absorb ideas (sometimes directly, sometimes as analogs) from classical algebraic geometry, commutative algebra, homological algebra, category theory, number theory, algebraic topology and differential topology. In some of those cases, algebraic geometry also "gave back" to the other subjects and in the case of number theory, it even shaped the subject decisively.
(That's not something I came up with, it's stuff I read. Though I've been able to make sense at least of parts of that claim.)
 
The elements of O(D(f) \cup D(g)) "correspond to" elements of (x,y) \in O(D(f)) x O(D(g)) such that they are "equal" when "restricting" them to D(fg)
 
@Zal My aim is to point out that Adeek was hijacking a pretty perfect, coherent, concrete answer by @lush explaining why a certain mathematical object was interesting by posting a loose intuition that doesn't really carry much meaning or motivation, following with a comment which basically boiled down to "these objects are cool" patching up the failure of the previous one. This has happened multiple times before, otherwise I would not have commented.
I hope that gives some context to what I was arguing against.
If it's anything that irks me, it's unhelpful pedagogy.
 
I see, I can understand your stance then\
 
@Adeek wAT, this is actually like, a crazy thing to say dude
7
 
7:18 PM
@Eric even more so, given the context that it wasn't an attack, but a constructive (perhaps a little harsh) criticism.
But shrug
 
so much drama
 
well well that was an interesting read.
 
To say the least
 
Do regular maps ever raise the genus?
 
Hmm
I know that if a map raises genus it can be homotoped to live inside the 1-skeleton of the codomain surface
But
 
7:30 PM
Let's assume that the regular map is constant
Edit: non-constant
 
Do all regular map between surface have to be branched covers? I think so
Yeah
It's locally polynomial
Covering maps always decrease genus (Euler characteristic of domain multiplies by degree of the map). I'm thinking if I can generalize the argument to branched covers (i.e if I throw away a bunch of points)
 
0
Q: Are all cell decompositions useful?

PerturbativeBased on the definitions and conventions used in the book Introduction to Topological Manifolds by John Lee, I can do the following. Let $X$ be a Hausdorff space, then define $\Gamma = \{\{x\} \ | \ x \in X\}$, that is $\Gamma$ is the collection of all singletons of $X$, and is trivially a parti...

 
lush is right
That is not a cell decomposition
The 0-skeleton of a CW complex always has discrete topology
A concrete example of a space which does not admit a cell decomposition is the cantor set.
 
@BalarkaSen But aren't you using the definition of a CW Complex being inductively built up (like in Hatcher)?
 
if the topology of the CW complex is not the topology on your original space, then what you have is not a cell decomposition
 
7:42 PM
@Perturbative how is a cell decomposition defined in Lee?
 
Can you reiterate Lee's definition? I don't have the book with me. Any sensible definition should imply that the 0-skeleton of a CW complex has discrete topology.
 
Damn I'm on mobile now :( lemme see if I can make a plan
Basically what Lee does is define a cell decomposition of a topological space X as being a partition of X into open cells of various dimensions
And then the open cells of dimension n >= 1 have characteristic maps defines the usual way
*defined
 
Why is {x} open in X?
If you assume that, you just say that X is discrete
 
And then he says a Hausdorff space X together with a cell decomposition of it is a cell complex
 
Eric Wofsey just posted an answer saying Lee's definition is not useful.
Strange.
I have never heard anyone distinguish cell complexes from CW complexes
Ohhhh
I see what Lee does
He defines a cell complex as a pure partition into cells, and defines the weak topology + closure-compactness later
Or at least I bet that's what's happening. Let me copyleft the book
 
7:48 PM
Yeah
 
Nailed it
 
Sniped me as I was typing out a similar message
 
Hahah
So yeah that definition is worthless
 
Aight
 
Well, only of conceptual use anyway
Good question (+1)
 
7:49 PM
Thanks
I'm off to bed now, thanks for the help everyone!
Night
 
@BalarkaSen for characteristic classes should I do milnor?
 
heheheh
Hatcher dawg
 
I'm not wild about hatcher's Alg Top book
 
I think Milnor-Stasheff has more material. I don't know much about char classes beyond the basics
 
7:54 PM
(i don't hate it either)
 
I think you algebraists would like M-S's style better
But get VBKT as a side-reference while you're at it :)
 
im probly not an algebraist lul
thanks for the link :)
 
>im probly not an algebraist lul
>is reading Szamuely
 
I just read the stuff that seems interesting at the time
I was pretty into analysis at some point, I arguably still am. I just haven't really done much in a while.
 
reading stuff that seems interesting is totally what algebraists do
we topologists only read the bible
 
chastity dawg
 
lol
 
@Daminark can you confirm this one?
 
I think most topologists read the book "how to not really prove a result but just claim its intuitively clear"
:-P
 
I dunno if the manga of topologists is analogous to a holy book but...
 
7:58 PM
I have no problem with intuitive arguments for examples
But when the proofs get too handwavey, I get a bit spooked
 
just draw a picture of a 2-sphere and prove it in that context. The case for general topological spaces that satisfy the right adjectives (which are not stated explicitly) follows in the same way
2
 
^^
our algebraic topology class 2 in a nutshell
 
hahaha @MatheinBoulomenos shots fired
 
y'all have seen Serre's diss on topology papers, right?
 
8:01 PM
nope
 
Hatcher's font style is funky
 
really wanna see that now :D
 
that's from a person who got a field medals for topology
that lecture is so great
11/10
 
@MatheinBoulomenos (mumbles) really tho, homotopy theory...
 
8:02 PM
okay, "modern topology" :P
 
Well super influential to topology nonetheless
 
Lol I'm taking AT next quarter, taught by my commalg prof, so that'll be fun
 
@Daminark you should have gotten Farb
sadface
 
ahaha :D
 
@ZalTukhara I think that font is 10/10
 
8:05 PM
Funky in a good way of course
 
ah then you are spared
 
is $\left(\begin{matrix}0&a\\ b&c\end{matrix}\right)$ $a,b,c\in\mathbb{R}$ open in $\mathbb{R}^{2\times 2}$ it's not right?
 
iirc it's Lucida Bright
 
@Skortya Right, because it is closed and the space is connected
 
Thanks
 
8:09 PM
@Daminark who?
@BalarkaSen one time benson thought he discovered the standard construction of the universal cover and it was p funny
 
@MatheinBoulomenos Had a chance to read about positive bases yet?
 
the space of paths thing?
starting at a basepoint
 
yeah
 
@TobiasKildetoft ah sorry, I've been a bit buy
 
that's a 10/10 construction.
 
8:10 PM
he got really excited thinking and then someone told him it was the standard def and he was just like "o"
 
@MatheinBoulomenos That's fine
 
ayy lmao
actually i understand him
i never read the definition beyond the first two lines until i figured out why it was natural
 
yeah it sort of goes to show you that it's a good construction
 
I guess the way to think about it is
if you have $X$ with an open cover $U_\alpha$
look at $\bigsqcup U_\alpha \times \pi_1(X)$
 
I mean, you know what it should be as a set
 
8:15 PM
@EricSilva Rohit
@BalarkaSen I don't think the guy often does AT, usually rep theory or AG
 
and then identify along the overlaps by requiring if $\gamma$ is a loop at $x_0$ of nontrivial homotopy class then it should lift to a path above
 
oh sweet
 
The fiber above x being pi_1(b, x)
 
right
 
The open cover just helps you know what it is as a space
 
8:18 PM
Actually it's exactly like the construction of analytic continuation of a holomorphic function. If $\gamma$ is a path on $X$ starting at $x_0$, make up the space above little-by-little by gluing a Cech-path of $\{U_\alpha\}$ that hits $\gamma$
If you have a loop below it just opens up above because... there's only backward identification not forward
(U_k is glued to U_{k-1})
So that should justify why we should appropriately topologize a space of paths
 
You guys are great. Enjoying math! Wish I were young(er). Sanity seems to prevail here (a nice reprieve from some of the stuff in main). I'm glad I checked the chatroom out tonight. Thanks for the link to Serre's lecture @BalarkaSen. G'night!
 
Nobody but me is gonna read that message Balarka
 
Let $G=\sum_{i \in I} \Bbb{Z}$ be the direct sum of an arbitrary number of copies of $\Bbb{Z}$. Let $k \in I$ and define $H_k = n \Bbb{Z}$, and define $H_i = \Bbb{Z}$ for all $i \in I \setminus \{k\}$. Is $H = \sum_{i \in I} H_i$ a subgroup of $G$?
 
Haha @JyrkiLahtonen glad to see you around! Please come here more often
@MikeMiller Oops lol I should point to a more coherent discussion here for those interested.
please excusefully my hiccupy incoherence topology
 
@JyrkiLahtonen Hi. Don't usually see you here
@user193319 Yes, that is a subgroup
 
8:24 PM
@TobiasKildetoft Thanks!
@TobiasKildetoft Actually...the only thing that bothers me is that $H$ contains elements which have infinitely many nonidentity components. Isn't that a problem?
 
@user193319 why would it contain those?
 
Well, if $I = \Bbb{N}$, and $k =1$, then $H$ contains the element $(2,1,1,....)$ which apparently isn't in $\sum_{i \in I} \Bbb{Z}$.
Also, I'm taking $n=2$.
 
@user193319 No, it does not contain that element, since you have defined it to be a direct sum, rather than a direct product
 
So $H$ isn't even in $\sum_{i \in I} \Bbb{Z}$...Shoot...
 
yes it is
Unless you mean to use the symbol $\Sigma$ in two different ways here
 
8:32 PM
It is? But surely it's not a subgroup...I'm using $\sum$ however Hungerford uses it. He uses it in the definition of the (external) weak direct product.
 
right, and you use that both for the group and for the subgroup, so why would you get things in the subgroup not in the group?
 
Oh...Yeah. I see. So $H$ is a subgroup of $G$. Then is it true that $G/H \simeq \Bbb{Z}_n$, given how $H$ is defined?
 
Very good! Thanks for the help.
 
8:54 PM
Hey @BalarkaSen just wanted to apologize for saying what I did about with holding my ideas
I thought you were criticizing my intuition of things I am sorry for that
 
@Adeek I was perhaps too harsh in my criticism. I apologize on my part too.
 
I respect discussing stuff with you. I wouldn't want that to go away for silly thing.
:)
 
@Adeek Do you want to explain this message in more detail to me:
2 hours ago, by Adeek
You can use this intuition for example which tells you schemes over $\mathbb{C}$ admits mixed hodge structure.
I'm actually interested in that
 
sure
 
His highness is certainly a well respected member of the chat room community :P
 
8:59 PM
@skullpatrol old meme
 
yup, classic
 

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