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12:00 AM
This is like IRC, but people have rep
Unfortunately for me it's not just about the journey. It's the badges. :D
 
If you're playing pokemon, it's both.
 
I gave Anupam his gold badge!
See:
Then only one I really cared about on SO I just got.
Unsung Hero.
Those were some yummy enchiladas.
It's like a spicy meat pie.
Dunno didn't get the badge, there should be a qualifier on the description that says it's also the highest score or top score per question, or something like that.
I tried sharing my answer on Reddit, but I think most of them came over and downvoted me for it.
 
@AaronHall Heh, I'm one vote away from Populist - one vote for the accepted answer.
 
Nope, the accepted answer has to be more than 10
So yours has to be at least 20
as I interpret it.
 
@KarlKronenfeld What is $\Bbb Q\otimes_{\Bbb Z}\Bbb Z$? @Mike
 
12:11 AM
@PedroTamaroff $\mathbb Q$
 
@AaronHall Yes, the accepted is currently at 10 (and for populist, yours must be more than 2*11).
 
@KarlKronenfeld Aha. Just a sanity check.
 
so that says more than 10, and yours must be greater than twice that
so you need 23, and they need 11.
unless the description's wrong, and it's 10 and 21
 
@KarlKronenfeld In general $\cdot \otimes_A A$ is the identity.
 
Cool ... @Pedro will soon understand my mini-lecture on tensors as twists :)
 
12:13 AM
@AaronHall Yes. 11 and 23.
 
Yes @Pedro ... For $A$-modules
 
@TedShifrin Sure.
 
@DanielFischer I am willing to bet you lunch (you have to get it in person) that the description is wrong.
 
@AaronHall I don't think so. From what I gathered on MSO, it works as described. The accepted answer must have a score greater than 10, and yours must be the highest voted, and have a score more than twice that of the accepted answer. There have been several questions on MSO where people wondered why they didn't get it when one (or both) was equal.
 
ok, well you have to collect in NYC, I work at One Bryant Park.
 
12:17 AM
@AaronHall I will collect it in due time, say fifty years or so.
 
@Daniel: Surely you'll visit us before I'm dead ;)
 
@AaronHall @DanielFischer I can be the middleman. Just meet me in Hell, MI.
 
LOL @Karl
 
@TedShifrin I wouldn't be too sure. It's damn far, and the ships are expensive.
 
Ships? Why not just hitchhike on a whale?
 
12:21 AM
@TedShifrin I'm afraid of Ahab.
 
A loony retort for everything! :)
 
I love this question title: "lebesgue measurable problem" :-)
 
Hey @Ted!
 
Hey @Studentmath
You'd prefer a non-measurable problem @Daniel?
 
@TedShifrin No, measurable is fine. Perhaps I even prefer Borel measurable.
 
12:33 AM
@KarlKronenfeld I guess I won the dumbest question of the day award today. Heh.
 
@DanielFischer: Oh, no! I'm at -15 today so far :-)
Off to the park. BBL
 
The best thing about this course is every time you finish a question you realise you have another one just tougher.
 
@Pedro: D&F's section on tensor products is really good
 
@FernandoMartin Good to know.
 
Hi @Ted
 
12:49 AM
@FernandoMartin @Mike
$\uparrow$ blocked 9gag.
Next step should be Facebook.
 
Yesss.
 
1:04 AM
>2014
>9gag
 
que mierda es el >
me pones nervioso
 
essplicame
 
No entiendo a que va el >2014.
 
1:20 AM
@Fernando Pedro is a bit behind the times.
 
Hi @Mike @Fernando
 
Hi @Ted, long time no see
 
Well, not that long :)
 
I've never actually seen you, so it's been an eternity.
 
@Mike WOW. SO DEEP. MUCH THINK. VERY THOUGHTS.
 
1:28 AM
But you know people who've known me a long time ...
 
Hey @Ted
 
@Ted What would you say your greatest asset is as a mathematician?
 
Oy ... You're sounding like skull.
 
I knew you'd say that
 
So why'd you ask?
 
1:32 AM
Because curious.
 
Someone asked me something similar.
Which was weird but regardless.
 
I have a solid foundation, I like to think about the interplay of different parts of math, and I'm insistent on doing/understanding examples whenever possible.
 
I like that answer.
 
And I believe I'm a good teacher :)
 
Sounds good.
:-)
 
1:36 AM
I knew skull was behind this ...
 
(They asked me the best way to study math, which I'm not in any way an expert in, but that's the same question I asked you in disguise.)
 
Inquiring minds what to know ;-)
 
maik you so sneeky
 
You're teaching him sneek @Pedro
 
Of course, I didn't tell him my real secret: my silky-smooth hair
 
1:40 AM
Uh huh ... While you have hair :D
 
@Ted I didn't learn about the weak-* topology in my functional analysis course. Do you know some notes where I can read about it?
 
What's the policy on asking a duplicate question if the previous one is a few years old and wasn't answered sufficiently?
 
Go for it.
 
No, it's standard. I think it's in Simmons and definitely in big Rudin.
 
Professor @TedShifrin have you thought about creating a youtube channel to collect together all your lectures?
 
1:44 AM
Nah, @skull. I don't even have control over what's being put up now. If we do the first half next fall, we'll try to organize it.
 
I know it's standard, but we still didn't do it :( Alas.
 
@Mike, it's ok to have stuff to learn in grad school!
 
Well, I need to know it now to get past page 6 of this book.
 
Have you seen any of Norman Wildberger's lectures on youtube Professor @Ted?
 
No, @skull, I really don't watch lectures on YouTube.
P. 6 of what? @Mike
 
1:54 AM
haha, this author is more insistent than most. in bold: (VERIFY!)
@Ted "C* algebras by example"
I haven't seen many of the promised examples quite yet.
 
I've seen underlined bolded italic verifies, as if the author is unsure himself that what he stated is true
 
@Mike Will you answer one question for me?
 
I can try...
 
Thank you!
 
Your questions are usually too hard for me.
 
1:58 AM
The joke was you said "Yes."
 
Oh, was that the one question?
 
But I couldn't type fast enough.
 
-_-
 
smacks silly Pedro
 
1:59 AM
Damnit I figured it out before you said it.
 
No need to add insult to injury.
 
Wow, he proves Gel'fand duality by page 8...
I figured that would be a big theorem I build up to.
 
This is stuff I know not @Mike.
 
Maybe there is a bigger one waiting for you :-)
 
@Ted The category of C* algebras is isomorphic to the opposite category of compact Hausdorff spaces. Apparently this is the motivation for noncommutative geometry.
 
2:05 AM
@Mike VERY ADVANCE.
 
Ah, yeah, this bells a ring.
 
add the adjectives "unital, commutative"
@Pedro Nah, google C* algebra. The definition is easy.
 
just kiddingg
 
ur makin fun of me
 
Could you guys tell me if the following argument is correct?
 
2:14 AM
@FernandoMartin Yes.
 
I want to see that if $1-x$ is invertible in $A[x]$, then $x$ is nilpotent
 
-_-
 
(I don't know if that's true)
 
In $A[x]$?
You mean $A$?
Try matrices.
I would say no.
 
no
I should clarify
 
2:15 AM
OK.
You're adjoining stuff?
 
OK, my choice of notation was terrible
 
Pretty sure I proved that in a pas life
 
nevermind, I'm stupid
 
WAI WAI WAI
I didn't understand your question.
 
@Fernando I'ma find you on Facebook.
 
2:18 AM
Good luck with that @Mike
 
That's what they said to the guy in Taken.
But he did find them, and he did kill them.
 
I fear I'm falling into the induction trap here. Given a connected graph G with n vertices, I am constructing G' where every vertex in G' represents a single spanning tree in G, and every pair of vertexes in G' are adjacent iff the corresponding spanning trees in G have n(G)-2 common edges. I am trying to prove that G' is connected and to find its diameter
I am thinking of proving by induction that if d(x,y)=a (where x and y are vertexes in G') than the corresponding trees in G have n(G)-2*a common edges..
Am I possibly falling into the induction trap in the n+1 step?
 
@Mike how the
 
I have a very particular set of skills
one set is math and the other set is looking at Pedro's friend list
 
2:31 AM
Wait, @Pedro has friends? Who knew?
2
 
@TedShifrin They usually last 2 months. I am too annoying.
 
Maybe I've never been one ...
 
@TedShifrin I'm doing some homological algebra in algtop, too. It's kinda gross :D
 
Univ coeff?
 
Yeah.
It's all easy to follow, if a bit tedious. I'm still not used to concerns about naturality.
 
2:35 AM
No comment :)
 
I'm not sure what you're not commenting about.
 
OK :)
 
:(
 
Hi @ted.
 
@PedroTamaroff I think you guys were too hard on this question. Obviously the OP needs to be reminded what a limit is and how it differs from a regularized divergent sum. However, there are some manipulations of sums that apply to regularizations (c.f. some arguments of Euler) whereas there are others that don't.
This leads into the meaningful and fruitful discussion of why some manipulations work and some don't. The answer, IIRC, has to do with a "graded" aspect to divergent series (in the words of Matt E) which may or may not be perturbed by certain manipulations.
 
2:49 AM
@anon That sounds deep.
 
@anon You're back. =)
 
yes, yes
 
I proved some stuffs about localizations.
 
Hi @Jasper
 
I drank some coffee.
 
2:51 AM
Hi @anon
 
@ted Since you wrote a multivariable calculus book, why not write a single variable one too?
 
@Pedro cool @Ted hi
 
Spivak already did it masterfully ... And I improved it slightly the last few editions.
 
@PedroTamaroff apparently the intermediate rings between a dedekind domain and its fraction field are precisely the localizations if and only if its class group is torsion
 
@anon That's too much for me =P
what's the class group?
 
2:53 AM
@PedroTamaroff as a initial exercise, try to classify the rings between Z and Q by hand without any help :P
 
that's what I don't know, actually
@anon OUCH
 
@PedroTamaroff the ideals modulo principal ideals under multiplication
@PedroTamaroff don't be intimidated, it's not at all hard
 
Classify?
 
@anon oh, the picard group?
${\rm Pic}\,\mathfrak o$.
"Pico" in Spanish means beak.
 
more generally, yes, the picard group
 
2:54 AM
Thought I'd put that out there.
@anon Yes, Lang talks about it briefly. I will probably do the exerciss he has on Dedeking domains so I can talk to your brainz,. =D
 
Pic to me means line bundles :)
 
@TedShifrin snif Peter is a geometry illiterate.
 
@TedShifrin :D
@PedroTamaroff That's what lurnin is for
 
@anon Anyways, today I proved that localization preserves UFDness and PIDness. Also, that if A is a UFD and p is a prime, A_(p) is a PID (I observed the ideals are (p)^n), and Karl more generally taught me that if A is local and the powers of the maximal ideal intersect at the zero ideal, every ideal is a power of it.
 
mmhmm
 
3:01 AM
I don't get this, I keep getting a false result. If d(x,y)=a-->n(G)-2a, and since $n(G)-2a\ge 0$ must uphold I get that $n(G)/2 \ge d(x,y)$, hence $n(G)/2 \ge diam(G')$, but that's not the result I should get...
 
do you feel like explaining what you're talking about (educated guess: graph theory?) or is this just a feelings post?
 
A combination, actually posted it a bit above. Erm, well:
 
ah yes, it is above in my screen.
all's well then.
 
@anon Been answering lots of compactness - questions - too.
 
Given a connected graph G with n vertices, I construct G' so that every vertex in G' corresponds to a single spanning tree of G, and two are adjacent iff the corresponding spanning trees have n(G)-2 common edges. Trying to prove G' is connected and to find diam(G')
Ah, haven't seen that, sorry.
I am supposed to get diam(G') $\le$ n(G)-1
I'm lost, any help will be extremely appreciated
 
3:09 AM
that sounds like a cool problm
 
It actually is, it's from Introduction to Graph Theory by Douglas B. West
Clear and solid book
I really can't see why the result isn't diam(G') $\le$ n(G)/2
 
I'd look at complete graphs.
 
Tries that, seems d(x,y) there is 1 for every spanning tree
At the complete 4 vertex graph
Or actually no, my bad.
 
There are definitely non-adjacent spanning trees.
 
Yes of course, looked at a different note.
 
3:19 AM
In fact I can find three non-adjacent vertices.
 
Seems so, there are few spanning trees there that have no edges in common.
 
That graph is gonna be huge though: there are 16 spanning trees.
So I guess showing that the diameter of that graph is 3 will make you happy.
But I wouldn't look much higher...
 
I can see it is now, which of course collapses the previous idea of diam(G')..
Which means I did something wrong with the d(x,y)=a-->n(G)-2a common edges of course
Which just makes me more stupid since I can find the faulty there, guess I will head back to the notes. Thanks @mike!
 
Don't call yourself stupid.
But good luck with the problem!
 
Inexperienced*
Thanks!
 
3:47 AM
Got it :) it's a really nice question
 
3:58 AM
The main point is that every tree has n-1 edges, it all falls down from there
 
@PedroTamaroff Innit past your bedtime
 
4:13 AM
For any positive integer $k$, let $S_k$ denote the sum of the infinite geomet-
ric progression whose first term is $\dfrac{k-1}{k!}$ and and common ratio is $\dfrac1k$. The
value of the expression $S_1+S_2+\dots$ is ?
I think it is $e$
But am not sure as $S_1=0$ and $S_2=1$, $S_3=1/2!$, etc. So, it could be $e-1$.
What is the right answer?
 
4:37 AM
@FernandoMartin Want to know something cool?
 
No
jk, tell me
 
First I'ma define a C* algebra.
A C* algebra is a Banach space over $\Bbb C$ with a unary operation * that satisfies:
$(A+B)^* = A^*+B^*$
$(\lambda A)^* = \overline{\lambda} A^*$
$A^{**} = A$
 
ok, sesquilinear and involutive
 
yeah
oh also it's an algebra. shoulda mentioned that.
 
4:40 AM
$(AB)^* = B^*A^*$
we also have $||AB|| \leq ||A|| \cdot ||B||$
 
Someone Please answer me too!
 
ok, that holds in Banach algebras in general right?
 
and $||A^*A|| = ||A||^2$
yeah, that's part of the definition of a Banach algebra
 
because you want multiplication to be continuous
anyway that's the list of properties
Let's say you've got a compact topological space $X$. Then the space of continuous functions to $\Bbb C$, $C(X)$, is a $C^*$ algebra (with pointwise addition, pointwise multiplication, $^*$ the adjoint operator, and $||\cdot||$ the $\sup$ norm.
 
4:43 AM
Ok, so?
 
That's not hard to see, the list of properties is easy to check.
 
This assignment $X \mapsto C(X)$ is contravarantly functorial.
 
via precomposing, right?
 
Yah.
What's cool is that it's an equivalence $\text{Compact Topological Spaces} \rightarrow C^* \text{-alg}^{\text{op}}$.
 
4:44 AM
that's someone's duality
Gelfand?
 
Right.
The inverse functor topologizes the set of maximal ideals on a $C^*$ algebra.
And this fact is pretty much the motivation for the field of noncommutative geometry.
 
That's cool
though it's pretty outside of my depth
 
Yeah, I just learned the proof
The proof isn't too hard, but you need to know some functional analysis
Shit
Okay so I lied to you.
I always make this error when saying it :P
 
hahaha
What was the lie?
 
The equivalence is between compact topological spaces and the opposite category of commutative unital C* algebras.
There's also a similar equivalence between $\sigma$-finite measure spaces and von Neumann algebras (C* algebras with a predual)
 
4:50 AM
don't know what $\sigma$-finite means yet
nor a predual
 
countable union of sets of finite measure; banach space whose dual space is the space you're talking about
I love when you get these sorts of totally unexpected relationships
 
5:18 AM
@JoseAntonio, will please help me here?
 
How can the dimension of the representation of a finite group be infinite dimensional?
Grr
 
How do you define the dimension of a representation?
 
@FernandoMartin Beat me to it, you devil.
 
I do not know either.
It is said that the "natural" representation of the Monster group is infinite dimensional
 
Well, if you don't know what the word means, how can you be surprised?
 
5:30 AM
I thought dimension was just the vector space dimension of the image under the representation
But in that case, this would be impossible
 
remember that a group representation isn't a map to a vector space
it's an action on a vector space (via a map to $GL(V)$)
 
ok, according to wikipedia
the dimension of a representation is just the dimension of $V$
 
dimension is defined by the dimension of $V$ usually (at least, that's how I've seen it in the past)
 
then every group admits an infinite dimensional representation
 
Oh, so even though our group is finite and its image in the GL(V) is finite, we might not be able to find a smaller subgroup of GL(V) that contains this image?
 
5:35 AM
simply use the trivial map $G\rightarrow GL(k^X)$ with infinite $X$
 
Is this kinda the idea?
Yeah, but I think they mean something more in this context than the trivial map..
 
I don't understand your question
 
My question is: If it is so easy to construct an infinite dimensional representation, why are they making such a big deal out of constructing an infinite dimensional representation of the monster? Any math major can do it
 
it's easy to construct trivial ones
 
But, there are several known finite representations of the Monster. Why not extend these to infinite dimensional ones?
That's non-trivial, right?
@Mike Thank you for your help. I think I kinda get it now. :)
 
5:39 AM
I have no clue tbh
 
well a) who's "they" and b) just because you have an infinite dimension rep - which might be the easiest way to define it - doesn't mean it's obvious how to show that this is just an "extension" of a finite dimensional rep the way you describe
whoever you're reading might be mentioning it's infinite dimensional just because the easiest way to work with your thing is with this infinite dimensional rep
i'd also like to point out that groups are for nerds
 
How about $V$ is a finite dimensional vector space and $\rho:G \to V$. Then, add basis vectors to get an infinite dimensional space. Then $GL(V)$ is a subgroup of $GL(W)$( Its actually the subgroup that fixes $V$). Will this work?
Well, I will gladly convert to nerdery then :)
Oh, they refers to Wikipedia
I meant $\rho:G\to GL(V)$, sorry
 
 
1 hour later…
6:47 AM
This was probably not important enough for flagging or posting on meta, but maybe some of the mods will notice this here in chat.
There is a question on meta which is identical copy of the question on the main. This question was closed.
Wouldn't a better solution be delete the meta question or migrate it to main and close as a duplicate?
I am not sure whether Community user bumps closed questions, if he does this question will reappear on front page of meta periodically. (I have asked about this on meta.SO.)
 
r9m
7:41 AM
hucum answer getting more upvotes than downvotes (+5/-2) .. deleted it :P [period]
 
8:09 AM
@robjohn yes, i understand that the question needs editing, but I do not know how should I...in that case...what should I do?
 
 
1 hour later…
9:28 AM
Huh. Someone just went through and upvoted eight answers of mine in the space of a few minutes -- each one with +9 and checkmark before the mass upvote.
 
@HenningMakholm Done that to me too a good half-hour ago.
 
Maybe a late April fool?
 
hello, who can help me please math.stackexchange.com/questions/737829/…
 
@DanielFischer Sounds just about the same time as mine, currently at 38 minutes ago.
They're probably going to disappear tonight when the serial-voting detector runs.
 
@HenningMakholm Yes, pretty much the same time, 41/42 minutes for me.
 
9:34 AM
@HenningMakholm or @DanielFischer can you help me on eigenvalues ?
 
@Vrouvrou just state your question here, if someone wants to help they will :-)
 
I looked at that question, but it's not something I feel competent in.
 
@DanielFischer Heya what countour would you use to solve $\int_{-\infty}^\infty e^{-ix}/(1+x^2)\mathrm{d}x$? Semi-circle in the lower or upper half plane ?
 
@N3buchadnezzar Lower. You want $\lvert e^{-iz}\rvert = e^y$ to be small.
 
Thats what I thought, thank you =)
 
9:40 AM
Bill had a slew of +9/accepted upvotes too.
 
@Daniel have you seen oldjohn?
 
@skullpatrol Not for a longish time.
 
:-(
 
@skullpatrol But "seen 13 hours ago"
So he's just not been to chat.
 
Thanks.
 
9:48 AM
"how many sides does a dot have" Huh?
 
Infinite.
 
$-\frac{1}{12}$
9
 
@DanielFischer Not cool
 
-_-
 
@N3buchadnezzar Sometimes, you just have to prick where it hurts.
 
9:51 AM
Damn string theory is chocking everybody.
 
String theory: Hendrix > Clapton.
 
Ba dum tss
 
@DanielFischer Cream
 
A string had a theory once but it was to thin to be tested :D
 

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