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3:13 PM
Morning @mike. Good night @ted.
 
Morning.
 
Morning, @Mike.
 
Hi @MikeMiller
 
lots of hellos today
 
3:17 PM
@MikeMiller well, Ted's not here, so you get all the hellos.
 
Meh, the feeling when you answer a question nicely and then people vote to close the question. Bah :p
Silly question, but I liked my solution..
 
i think it's a bad quesiton but your answer is good
 
hello mike
 
bananas! how's your study of van kampen going?
 
3:20 PM
@MikeMiller Indeed. Like mentioned in the comments, I think OP just forgot to add the limits.
 
good, van kampen is awesome
computing fundamental groups of wedge sums etc now
 
cool.
 
but had to learn about free groups etc first
 
try computing fundamental group of a genus g surface by considering connected sums.
 
what's a genus g surface?
 
3:22 PM
a sphere with g handles.
 
the connected sum of g tori
 
It's a genie.
 
0
Q: A list of the basic properties of smallest grammars.

Enjoys MathSee here for an intro. So there is not really a lot of research into the problem (say as much as research into FLT) at the moment. I thought we could come up with the obvious properties that smallest grammars must have. Here are some: Prop. 0.0 There exist no unused rules in a smallest gramma...

 
How is that fit for meta?
 
3:42 PM
because it's a list
 
Hi guys
 
Are u all mathmatician?
 
define mathematician
 
3:48 PM
i am an apple.
not sure about mathematician, though
 
@iBalarkaSen o/
 
A mathematician is a person with extensive knowledge of mathematics who uses this knowledge in their work, typically to solve mathematical problems.
 
i am not a mathematician, for sure, then.
@ɧɿρρԹʅȝՇԵՐՎԾՌ heya
 
me neither then
 
Then what are u then?
 
3:50 PM
already said that : i am an apple
 
At most a student
 
I plan on becoming a mathematician
 
At least a jellyfish
 
But yeah, for now I'm just a student
 
u jelly?
 
3:51 PM
i see
 
iwriteonapples
 
haha @iwriteonbananas
 
Tead: Which level of maths are u at now?
 
over 9000 bro
 
3:53 PM
Level 6, the boss Ted is at level 10^7 :D I haven't beaten him yet
 
@ʙᴀᴅᴀᴛᴍᴀᴛʜ hehe
 
@yswong How would you define "level of mathematics"?
 
I've been told many times that I have a mathematical "gift," so much so that I gain nothing from being complimented as such
@yswong Complex analysis, I would say
 
@teadawg1337 If I tell you you're awful at maths, will that help ? :P
 
@teadawg1337 Opposite for me. Most people say I should quit math because I'm too dumb
 
3:54 PM
stop bragging teadawg
 
@ʙᴀᴅᴀᴛᴍᴀᴛʜ :/
 
Level as in which topicl in maths are u all studying right now Eg: Calculus, Linear Algebra, Real analysis etc
 
I'm studying algebraic topology.
 
I'm not bragging, quite the opposite to be frank
 
@ʙᴀᴅᴀᴛᴍᴀᴛʜ you should. i recommend you drop outta school, smoke weed and take pills
3
 
3:55 PM
@iwriteonbananas That sounds like a better life
 
lol
 
Im only at Linear Algebra
 
@BalarkaSen Are you using Hatcher
 
Yes, I am.
 
@BalarkaSen My friend told me he learned AT better by using 2 separate books for homology theory and homotopy theory
 
3:56 PM
I've never used any math book except school books
 
@teadawg1337 why?
 
@ʙᴀᴅᴀᴛᴍᴀᴛʜ Haven't studied homotopy theory yet.
 
For me im learning maths so that it would help me in the understanding of Physics
 
@ɧɿρρԹʅȝՇԵՐՎԾՌ French schoolbooks \equiv Research monographs.
 
Espically Quantum Mechanics
 
3:57 PM
@BalarkaSen Uh not at all
 
Im more interested in Quantum Mechanics actually
 
@BalarkaSen French schoolbooks are bad. They're full of funny pictures and easy exercises
 
Oh?
 
@yswong You should frequent the physics stackexchange if you like physics more
 
I thought French highschool education was good.
 
3:58 PM
@ɧɿρρԹʅȝՇԵՐՎԾՌ Can I see some of the pictures in your textbooks
 
But now i have to brush up on my maths first
 
@BalarkaSen HS education is nothing exceptional. College is really good.
 
Before i can even start learning physics
 
@iwriteonbananas I get so much attention for my math skills that I'm starting to question the validity of the "praise."
It makes me wonder whether I'm truly as gifted as everyone says I am...
 
@BalarkaSen That being said, we're way more generalist (not as specialized)
We don't choose majors and minors. The only choice we have is for the last two years, Scicence/Economics/Litterature
 
3:59 PM
Yes, as Arnold said...
 
Arnold ?
@BalarkaSen @ʙᴀᴅᴀᴛᴍᴀᴛʜ editions-bordas.fr/feuilleteurs/9782047329276/demo
 
@ɧɿρρԹʅȝՇԵՐՎԾՌ It doesn't load for me :(
 
@teadawg1337 i see i see
 
Chargement de l'application, merci de patienter quelques instants.
 
@ʙᴀᴅᴀᴛᴍᴀᴛʜ That means 'loading' xD
 
@teadawg1337 i dont believe anybody was born with innate ability to be good at math
no special snowflakes
 
@ɧɿρρԹʅȝՇԵՐՎԾՌ omg the cover of chapter 1...it's from a bad American crime TV show about fake math...
 
@ʙᴀᴅᴀᴛᴍᴀᴛʜ LOL
 
damn it, it's still loading.
 
I guess it's nice that everything is..colorful
 
4:02 PM
@BalarkaSen @ʙᴀᴅᴀᴛᴍᴀᴛʜ The book is a math book for the last year of HS, scientific section
 
Teadawang 1337:This is the physics that i will be doing
 
@ɧɿρρԹʅȝՇԵՐՎԾՌ I hope university textbooks are not like this
 
We don't have textbooks at all in college
 
lol that book looks ridiculous btw
 
Problem solved
 
4:03 PM
What does it seems like to u
 
@teadawg1337 Give me a tiny bit of your math gift such that I can have a better performance ... :-)
 
It seems like it's loading @yswong
 
@iwriteonbananas That's the typical book HS students use in France
 
@Chris'ssis If anything, you should give back the math gift that you stole from @ʙᴀᴅᴀᴛᴍᴀᴛʜ
 
@yswong That looks pretty bad to me if it's for beginners in quantum stuff
@yswong Is it for beginners ?
 
4:05 PM
@teadawg1337 I have no math gift, unfortunately. But I think I can stand for that. ;)
 
Can say that it is taken by college somphores
 
sophomores*
 
Im just wondering if having a good maths background could help me in understanding all these stuff
 
@yswong But have they done quantum stuff in 1st year ?
 
@yswong Maybe at MIT or Berkeley, since that appears to require multivar-calc and linear algebra as well as basic knowledge of topology
 
4:07 PM
No
 
Then it's pretty bad
 
You don't start a quantum stuff course by talking about momentum operators out of nowhere, and h (supposed to be h bar=$h/2\pi$)
 
@teadawg1337 did you calculate it without pen and paper? $$\int_{-\infty}^{\infty} \frac{e^{a x}}{1-e^x} \ dx, \space 0<a<1$$
 
That's on page 17, which is just about the second page of the notes if you remove the summary
 
4:10 PM
@Chris'ssis That looks like it would require contour integration, which I'm not familiar with at all
 
@yswong Oh wait... are those course notes, or just exercises ?
 
@teadawg1337 No, it doesn't necessarily require contour integration. I did it in 3 different ways.
 
Those were course notes
 
@yswong Yikes....
 
But what's the part from page 1 to page 14 ? It looks like the real course only starts at page 1. (not the pdf pages, the text's pages)
 
4:11 PM
BBL
 
Later @Chris'ssis
 
@BalarkaSen @ʙᴀᴅᴀᴛᴍᴀᴛʜ Why do you guys even need textbooks/books ?
 
Something like a summary of the subject
 
@yswong Not that, see after the summary, there is a list of problems/solutions
 
@ɧɿρρԹʅȝՇԵՐՎԾՌ Where else will I learn from?
 
4:14 PM
@ʙᴀᴅᴀᴛᴍᴀᴛʜ Lessons. Teachers.
 
@ɧɿρρԹʅȝՇԵՐՎԾՌ My school doesn't offer some of the courses and most professors don't teach well or don't like helping students
 
That's a shame really :/
My teachers are GREAT :D
 
So the only other viable option is textbooks
 
Oh the author is just trying to tell the readers of what kind of problems student are expected to encounter in the subject
And show some examples of howto solve it
But the real course only starts after pg 14
 
The course seems to give a lot of unproven results though. Which I wouldn't like.
 
4:17 PM
Hello @DanielFischer!!!
I want to explain why the execution time of partition at an array of length n is $\Theta(n)$. That is the function partition:
partition(A,p,r){
  x<-A[r]
  i<-p-1
  for j<-p to r-1
       if A[j]<=x then
          i<-i+1
          swap(A[i],A[j])
  swap(A[i+1],A[r])
  return i+1
@DanielFischer Could we say the following?
We suppose that assignments, access to elements of the array, swaps require constant time.

The for-loop is executed $n-1-1+1=n-1$ times. Therefore the time execution of the pseudocode is $\Theta(n)$.
 
hi @hippa @Chris'ssis
@Chris'ssis have you been connected since I left ?
 
@Ramanewbie she's BBL, she was there a few mins ago
 
ok @ɧɿρρԹʅȝՇԵՐՎԾՌ
 
Ice storms sure give me a lot of time to mull over math problems :3
 
4:41 PM
Hi @DanielFischer do you maybe know how to show that the limit $\lim\limits_{y \rightarrow 0} \frac{\sin(y^2)}{y^2} = 1$ follows from simply noting that
the limit $\lim\limits_{y \rightarrow 0} \frac{\sin(y)}{y} = 1$?
 
0
Q: Small question about condition in the paper: https://eudml.org/doc/125333

VrouvrouI have this condition: $$ \forall v\in \ker(-\Delta_p-\lambda)\setminus\{0\}:\\(p-1)\int_0^{\pi} h(x) v(x) dx< \underline{F(+\infty)}\int_0^{\pi} v^+(x) dx+\overline{ F(-\infty)}\int_0^{\pi} v^-(x) dx$$ where $h\in L^{p'}(0,\pi), p'=\frac{p}{p-1}, p>1$ and $$F(x)=\begin{cases}\frac{p}{x}\...

an idea please ?
 
4:51 PM
@Ramanewbie Why?
 
@Chris'ssis no, @hippa told me you'd been out...
 
@Ramanewbie I'm in a meeting with some mathematicians BBL
 
@Chris'ssis ok
 
@Chris'ssis how do i find $$\sum_{k=1}^{\infty} \frac{1}{k^3(k+1)^3}$$
 
4 mins ago, by Chris's sis
@Ramanewbie I'm in a meeting with some mathematicians BBL
 
4:58 PM
he'll read it later
 
@iwriteonbananas $$\frac{1}{k^3}-\frac{1}{(k+1)^3}-\frac{3}{k^2}- \frac{3}{(k+1)^2} + \frac{6}{k}-\frac{6}{k+1}$$
 
I would assume that's by partial fraction decomposition?
 
@Chris'ssis that's equal to $\frac{1}{k^3(k+1)^3}$?
 
@iwriteonbananas Isn't it obvious?
 
5:07 PM
@iwriteonbananas The two are equivalent, the latter is the former's partial fraction expansion
 
yeah i see
 
@iwriteonbananas What result did you come up with? I worked it out from there if you need more help :)
 
Is the pushforward measure a categorical-theoretic pushout?
 
@teadawg1337 10 - pi^2
 
@iwriteonbananas Yup!
 
5:20 PM
You mean pushout, @Alyosha? I don't see where the two maps we would be pushing out on come from.
 
Good night @Mike
 
Hi Ted.
 
Morning.
 
Hello @Ted!
 
McTedster!
 
5:29 PM
hi Jasper, @teadawg, @Hippa, bananas
 
@TedShifrin o/
 
Hi @ɧɿρρԹʅȝՇԵՐՎԾՌ@TedShifrin
 
hi @ted
@iwriteonbananas I saw chris'sis message don't worry
 
Salut, @Gato @Ramanewb
 
@TedShifrin if I iidentify $L(E)$ where $E=\Bbb{R}^2$ to square matrices $2\times 2$. The norm of $\Vert A\Vert_{\infty}$ is the sup of the sum of the coefficients ?
 
5:43 PM
@Gato o/
 
@ɧɿρρԹʅȝՇԵՐՎԾՌ Si tu identifies les applications linéaires de $\Bbb{R^2}$ dans $\Bbb{R^2}$ aux matrices carrées $2\times 2$ muni de la norme $\Vert \cdot \Vert$_{\infty}$ de quelle norme parlons nous ?
Du sup de la somme des coeff en valeurs absolue ? ou simplement du max des coeff ?
 
hi ted
 
@Gato max des valeurs abs des coeffs
 
Donc par exemple si je prends la matrice A de coefficients 1,2,3,4 et u=(1,1) que vaudrait la norme infinie de A, 4 ?
 
@Gato: That's not my definition of the norm. Isn't it $\max_{|x|=1}|Ax|$?
Je ne suis pas d'accord avec M le @Hippa.
hi @semiclassic
 
5:53 PM
Je ne suis pas d'accord avec M le @Ted. :P
checks
 
@Hippa @Gato: Si on parle des applications linéaires, ça devrait être indépendant de base.
 
@TedShifrin aha !
 
@TedShifrin the subordinated norm ? Yes it's my definition.
 
OK, that norm makes sense only for matrices, not for $L(E)$.
 
afk dinner. Gotta eat more roasted infants.
 
5:55 PM
@TedShifrin J'identifie $L(E)$ aux matrices carrées.
@ɧɿρρԹʅȝՇԵՐՎԾՌ donc c'est la somme !
 
So your norm depends on choice of basis, @Gato.
awfully early for dinner, @Hippa
 
@TedShifrin Not in France...
 
In France most people eat dinner at 8 PM, @teadawg ...
or later ...
I guess Tennessee is having an inordinate number of cancellations because of the ice, @teadawg. I heard on NPR today that Kentucky is in terrible shape, too.
 
It's 2 am here. I just ate dinner.
 
So if I want to compute the infinite norm of a matrix $\Vert A\Vert$ what is the norm ? @TedShifrin
 
5:58 PM
I don't know, @Gato. I don't know your definitions.
 
@Ted Kentucky and far northwestern middle TN got over twice the amount of snow than was forecast
My area got around 3-4 inches
At least the roads are clear-ish this time around
 
Where I live in Georgia, we have to wait for everything to melt ... no road clearing at all, except for the interstates.
 
@TedShifrin Ok, in fact I am trying to prove that the inequality $\Vert l(x)\Vert_F\le \Vert l\Vert_{L(E,F)}\Vert x\Vert_E$ it's not necessary true if we don't take the 'natural' norm $\nu(l)=sup_{x\in E, x\ne 0}\frac{\Vert l(x)\Vert}{\Vert x\Vert}$
 
@Gato: That's the norm I was using earlier. And that inequality is quite easy, then.
 
@TedShifrin I should say what I was doing.. To find an counter example I am trying to find a matrix $A$ such that $\Vert A(u)\Vert_E> \Vert A\Vert_{L(E)}\Vert u\Vert_E$
 
6:04 PM
Oh, for a different norm, @Gato?
You mean $\|A\|$ in the second term.
 
@TedShifrin Yes
for both lol
 
And for some $u$, not all, I presume.
 
@TedShifrin Yes, I tried to take A = 1,2 and second line 3,4
and $u=(1,1)$
 
with the norm you and Hippa were discussing?
I don't know.
 
6:09 PM
@TedShifrin Yes, but I took the sup of the sums it's a subordinated norm so it will not works right ?
 
PDE was exciting today, @Ted: we essentially showed existence and uniqueness to weak solutions of the Dirichlet problem on a bounded domain. Using the huge amounts of functional analysis we built up and some Fredholm theory...
 
Hi!
 
@MikeMiller Are you using Evans?
 
No. We're most closely following Hormander.
 
Hormander is very hard.
 
6:11 PM
It's not so bad given guidance, e.g. lectures for which the proofs are comprehensible.
 
There is Taylor as well.
 
@Gato Ah oui, c'est bien la somme q_q
 
Once you have that and you look at a proof in Hormander you understand all the details he skipped.
 
@ɧɿρρԹʅȝՇԵՐՎԾՌ En fait la norme somme infinie, c'est un théroème je suis trop bête. Si je calcule $\Vert Ax\Vert_{\infty}=\max_{1\le i\le n}\vert \sum_j a_{i,j}x_j\vert$ alors c'est équal au max de la somme des coeffs des valeurs abs.
 
I got a question about C++... can anyone is here help me?
in*
 
6:16 PM
Go Sobolev spaces, @Mike.
 
hi all. I have a question regarding $L^AT_EX$
 
There is a TeX.SE :P
 
@G-man Wrong way to type that.
 
I'm a fan, @Ted. He also invoked an inequality I learned from reading Warner... so I got something out of that :D
 
@G-man Use \LaTeX.
 
6:17 PM
@ABeautifulMind that's cool
 
@G-man As cool as me.
 
rolls eyes
 
combs hair
 
rolls Ted's eyes
 
combs Hippa's hair
 
6:18 PM
That's inappropriate, @ɧɿρρԹʅȝՇԵՐՎԾՌ.
 
@ABeautifulMind :O
@ABeautifulMind At least it's easier on me than on Ted :3
 
So what I was asking was that if there is some way to hide some parts of the answers on MSE, because sometimes they get a bit long, and I use a phone, so that's a bit of a problem..
not much screen real estate
 
That's not really a $\LaTeX$ question. That's an issue with displaying MathJax on your phone.
 
no, it displays math just fine,
 
Well, the rest of the text here isn't in LaTeX. :) I don't know what it is.
 
6:24 PM
but what I was thinking was that I could add some kinda button in my answers which would toggle the visibility of certain sections
 
You can make things hidden (for example, if one gives a hint and wants to hide the solution), until someone mouses over the hidden portion. That doesn't work on phones/iPads.
 
takes me a lot of time to type on this darn phone and i keep missing responses..
 
Yeah, phones are not meant for SE.
 
that happened again
and again
@TedShifrin how does that exactly work?
 
6:28 PM
Phones are not meant for the web SE. They work reasonably well with the app.
 
@TedShifrin: going back to that question i've been bouncing around re: closed trajectories on surfaces
 
@Ted aight thanks. @Mike yeah I know, imagine how I type math on MSE with an android
 
what would be the relevant keywords to search for that? i imagine a lot must already be known
 
I enjoy reading MSE on my phone. I cannot write answers on it.
 
i can write very short answers on my phone.
 
6:31 PM
closed geodesics will work, @Semiclassical, but also closed orbits for dynamical systems (differential equations)
 
but that doesn't happen often
ah. closed geodesics sounds a little too restrictive (after all, lines of latitude aren't geodesics of the sphere)
but then again the real 'ill-posed' part of my question as it stands is that i just say "pick a coordinate chart", which is probably way too generic
 
Of course, it's a hell of a thing to try to understand closed orbits...
 
@Semiclassical: For your edification (not necessarily answering your question), learn about the Poincaré-Bendixson Theorem.
 
hmm, interesting
 
If you don't already know it, look for Hirsch Smale Differential Equations, Dynamical Systems, and Linear Algebra.
 
6:36 PM
i guess what i'd like is a generalization of that to other surfaces than subsets of the plane
which is an obviosu thing to search for
 
I have a woefully poor knowledge of dynamical systems...
 
A fancier book, @Semiclassical, is Philip Hartman's Ordinary Differential Equations. In there is a statement of A. J. Schwartz's generalization of Poincaré-Bendixson to 2-manifolds.
 
Sigh ... I have to get rid of some of these books :(
Here's the theorem, @Semiclassical: If you have a $C^2$ flow on a $C^2$ $2$-manifold. Then a nonempty compact minimal set is either a stationary point, a periodic orbit, or the whole manifold.
The proof takes a number of pages.
A comment: A flow on a Klein bottle with no stationary points must have a periodic orbit :P
 
that's a nice conclusion. i'd been wondering if there could be non-closed orbits whose image was dense only in some subset of the 2-manifold
it would seem the answer is "no---if it doesn't close, then the trajectory gets arbitrarily close to every point of the manifold"
 
6:43 PM
The standard example of that is the irrational slope line on a torus, @Semiclassical.
 
right. it's incommensurate with the torus in that case
 
I don't know what that means.
 
not rational, heh. i'm just being wordy for no good reason
 
Oh, in the old Greek sense ...
 
right.
and i know about the KAM theorem (or knew of it at one point, anyways) so that's somewhat familiar to me
 
6:47 PM
@Ted: what's that theorem of Thurston's, that gives three possibilities for what a map on a hyperbolic manifold might behave like under repeated composition?
 
i guess it's a bit surprising to me that there's no cases which 'fill' only some portion of the 2-manifold
 
I don't know that stuff, @Mike. One of my colleagues does for sure, but I don't.
You can have Reeb components, @Semiclassical, which is sort of like that. But the boundary of that region is periodic orbits.
 
ah, hmm
is that applicable for the 2-manifold case? i have a hard time envisioning how that could work
 
I guess Reeb components aren't relatively dense. I was thinking of something else.
 
6:51 PM
reeb vector fields live on contact manifolds, which are odd-dimensional, but maybe this is just something unrelated by reeb?
 
You don't need contact structures to fill up an annulus, @Mike.
 
the definition of a reeb vector field that i know is something associated to a contact structure. that's all I was saying
 
Ah. Same Reeb.
 
but unrelated work. alright
I can't read any of this stuff on the definition
know zip about foliation theory
 
I know how to unzip my pants.
 
Do you guys know zip and unzip too?
 
My pants have buttons
 
A foliation consists in writing a manifold as a union of disjoint submanifolds, such that there is a local product structure to it @Mike.
See Frobenius Theorem :)
 
Do you know how to exfoliate @JasperLoy?
 
6:57 PM
I know what it is, @Ted.
 
I thought Ted would smack me.
 
Well, how was I supposed to know what "zip" meant, @Mike?
 
@ʙᴀᴅᴀᴛᴍᴀᴛʜ I don't wash my face except with water.
 
I don't know anything highly nontrivial. The Frobenius theorem - and their applications to proving the Lie correspondence - is about it
@TedShifrin I said about the theory, not about the definition :D
 

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