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12:25 AM
Hello!!! :)
 
12:49 AM
@evinda Hi
 
How are you? @IWantToRemainAnonymous
 
@evinda Fine enough for the moment
What about you?
 
I am fine, thanks. :) @IWantToRemainAnonymous
How was your day? What did you do?
 
@evinda Sleeping for quiet a time, then some hours in the beach :))) What about you?
 
Nice.. Where are you from? I went to a baptism. @IWantToRemainAnonymous
 
12:55 AM
@evinda Nice. BTW read my username before asking that question X)
 
@IWantToRemainAnonymous I read it. Although I asked, because I thought that there are a lot of users from each country.
Anyway. @IWantToRemainAnonymous
I will go to sleep now. Good night!!!
 
@evinda Good night :)))
 
1:53 AM
Hi, maybe you guys can help me. I am trying to "solve" some single player game, bruteforce is fine but maybe you have some observations.
Say you make a deck of 30 cards by giving each card a cost from 1$ to 10$ (integer values).
Then you start playing by drawing 1 card each turn and after drawing you can spend the cards. Turn 1 you can spend 1$, turn 2 you can spend up to 2$, turn 3 3$ etc. and you stop playing after say n<=10 turns.
The goal is the spend the most amount of money on average. The question is how do you choose the card cost distribution and also how do you play.
ah should have used € instead of \$ lol
I copy paste again the problem with good formatting I hope:
Say you make a deck of 30 cards by giving each card a cost from 1€ to 10€ (integer values).
Then you start playing by drawing 1 card each turn and after drawing you can spend the cards. Turn 1 you can spend 1€, turn 2 you can spend up to 2€, turn 3 3€ etc. and you stop playing after say n<=10 turns.
The goal is the spend the most amount of money on average. The question is how do you choose the card cost distribution and also how do you play.

Say for example n=2 turns. So turn 1 is 1€ and turn 2 is 2€ and then the game stops. You don't want cards >2€ since you can't spend them. Putting all
 
I'm sorry, but I don't expect anybody is going to read all that.
 
its np
its not really some math problem so i hesitate to post it in the forum
can I edit chat messages? the first one I want to delete because of bad formatting
 
I think you can, but nobody will be too upset about the formatting.
 
2:43 AM
Mike do you know Eric Auld
 
Yeah, he's a friend of mine.
 
Ohh cool
 
Why do you ask?
 
I browse some math blogs and saw he had an account on here and it says he goes to UCLA
 
oh, I didn't know he had a math blog
quelle coincidence. he was talking about the thing in his most recent post last night
thanks for letting me know
 
2:59 AM
Yea, he doesn't update that frequently but he made one recently about a proof for Sylow's theorem
Interesting because the proof he posted is the first one I ever saw when I was taught it
 
i learned the one he mentioned as the 'standard' proof. i like reducing it to checking it for $GL_n(\Bbb F_p)$
 
 
1 hour later…
4:05 AM
Heyy
I changed my name
I used to be columbus8myhw
2
But now I have my real name up
 
5:05 AM
is there an inner rapport between this puzzle and this post , i guess so
the solution is given by optimising a linear inequality system right? i suck at algebra .
because bruteforcing is the last thing i resort to
 
r9m
5:55 AM
@Chris'ssistheartist Hey! That was too easy :P .. shall I spill the beans here? :) We all know how $x,y,z \in (0,1) \implies (1-x)(1-y)(1-z) > 0$ right :) Rearrange appropriately and integrate :D
 
Hello. I'm having trouble figuring out whether the set of all points $(x, y, 0)$ such that $x^2 + y^2 \le 1$ is a regular surface or not.
 
r9m
@AkivaWeinbergercolumbus Wow!! Nice to know .. :) (sneaks off to google up)
 
@Fermésomme look at boundary points
the z coordinate is irrelevant, just think of it as the closed unit disk in R^2
 
@Stan: You van think of the t partial as vertical acceleration and the x derivative, with appropriate simplifying assumptions, as the curvature of the vibrating string.
 
@anon I'm guessing that neighborhoods of boundary points are not Homeomorphic to open subset of $\mathbb R^2$. But I can't prove it.
 
5:59 AM
hello @TedShifrin.
 
Heya @anon!
Good morning Balarka.
 
I have found a notion of tangent spaces, but now I have to compute one for the variety drawn out by $X^2 - Y^2 = 0$ :(
What's more, I don't particularly like the notion I have found.
 
@TedShifrin the t 2nd derivative makes sense. What do you mean by curvature? So like, if the string was completely straight, then the second x derivative would be zero?
@TedShifrin hi btw! How are u?
 
Tangent cone and tangent space are different.
 
What's a tangent cone?
 
6:05 AM
Yup. Curvature in the sense of calculus or beginning diff geo.
It is given by the lowest degree terms in the equation (so here, the cone is its own tangent cone at the origin).
 
Yay! Okay I will add that to my notes.
 
ok, sure, but the notion I have is this
you may or may not care if you want, I know it's hard to do this on phone.
meanwhile, let me try to compute the tangent space,
 
@Ted: His notion is correct.
@Ted: We have some Bombay sapphire here now. I don't know if that's to your taste.
 
LOL, it's more expensive than needed for my taste, @Mike, but thanks.
 
It was for my taste.
 
6:10 AM
If I make it ... My brakes were smelling burnt after lots of hills today ...
 
I had an oran-gin-a earlier which I was pleased with.
I actually went to get whiskey before I remembered how much an Islay costs
 
I don't waste Sapphire on crummy mixers :)
 
@Ted: I prefer just a splash of citrus of some sort.
 
@Balarka: for your example, the usual tangent space is the whole plane.
 
The people I worked with on my problem sets loved to put oranges in their drinks while working.
 
6:12 AM
I am computing it!
oh, usual tangent space. I would expect it to.
 
@Ted: Spoilers...
 
@Mike: I met Anthony yesterday and we talked for hours about grad school options ...
 
I just got distracted by this amazing discussion about curious substances.
 
Balarka still has plenty of exercises in my.book ...
 
@Ted: Our mutual acquaintance tried to freeze it. I objected.
Mm. He's doing good?
 
6:15 AM
It's not a bad idea, Mike. Less ice dilution. Lots of people do it.
 
OK, so $A = k[x, y]/(x^2 - y^2)$. Localize at $(0, 0)$ : the ring $A_{(0, 0)}$ is the ring of all fractions with whatever numerator from $A$ and denominator having constant terms sticking out. I want to find out all the linear maps $A_{(0, 0)} \to k$ satisfying Leibniz.
As it's linear, and satisfies Leibniz, it's completely determined by what it does to fractions with numerator $x$ or $y$ modulo $(x^2 - y^2)$
 
@Ted: I think rather stringy thT if dulls the flavors.
 
They come back when it warms up a bit, but without ice dilution.
What is the local ring at the origin, @Balarka?
 
oh, fractions with nonlinear terms in the numerator cancels out. for example, consider a fraction with $xy$ modulo $(x^2 - y^2)$ term. apply Leibniz, a fraction with $x$ above pops out, and when you evaluate that at $0$, everything vanishes.
OK, so it's just a linear combination of the images of $x$ modulo $(x^2 - y^2)$ and $y$ modulo $(x^2 - y^2)$. in other words, just $x$ modulo $(x^2 - y^2)$. tangent space is a one-dimensional $k$-vector space.
 
Don't believe you.
 
6:25 AM
You're wrong. Sorry.
 
A veto from Ted and Mike is never a good sign.
 
@Ted: I don't take it with ice anyway, so I guess I don't mind so much. But fair enough
 
I think there's something wrong with the descending from $x$ modulo $(x^2 - y^2)$ and $y$ modulo $(x^2 - y^2)$ to $x$ modulo $(x^2 - y^2)$?
 
I don't like it warm, at all, Mike!
You have Ted veto experience, Stan!
 
6:27 AM
@Balarka: Be more explicit about what your functional $v$ does. This is not a complicated ring so tell me what it does to literally every element.
 
Oh, looks like I will be in Chicago nect May or June, Stan.
 
@MikeMiller Every element goes to (via $v$) a linear combination of the images of fractions with numerator $x$ or $y$, otherwise my brain fell out.
ok, I guess it's 2-dimensional.
right, my bad, i was sleepy and assumed every ring is an integral domain.
 
Hm? What's $v(x/(2+x))$? I am really unsatisfied by your answer.
 
$v(x/(2+x))$ is just what it is. I mean, if you have $v((x^3 + xy^2 + 3xy + y^2+x)/(2+x))$, then that is the same as $v(x/(2+x))$, by linearity and Leibniz.
 
Ok, I give up.
 
6:38 AM
:(
 
6:49 AM
@MikeMiller I tried this with the smooth variety $k[x, y]/(y - x^2)$ and it seems to work fine. Elements in the stalk at $(0, 0)$ are fractions with denominator having constant terms. Any linear functional $v$ applied to a fraction must be a linear combination of the images of fractions with numerator $x$ and $y$. However, $y = x^2$ modulo that ideal, so $v(y/(c + g(x, y))) = 0$. So that means the tangent space is one-dimensional.
Dunno what goes amiss for the 1 dimensional cone you gave me.
Actually, the tangent space of $X^2 - Y^2 = 0$ should be $0$-dimensional.
Hrm.
 
@r9m do you mean facebook ?
 
@Balarka Hello
 
hi.
 
Amr
7:27 AM
hello everyone
Can someone suggest for me an undergraduate probability theory book that covers Markov chains. I would like the book to be rigorous
Thank you :)
 
7:42 AM
@MikeMiller Actually, fuhget about the stalk. Just define the tangent space of a variety $V$ at $x \in V$ to be the vector space of linear maps $v : k[V] \to k$ such that the Leibniz rule $v(fg) = f(x)v(g) + v(f)g(x)$ is satisfied. The stalk unnecessarily complicates things.
 
r9m
@Agawa001 well whatever google search returns ..
 
@r9m hey !
 
r9m
@TheArtist hello :)
 
@r9m do you know how we can $\int \frac{dx}{1+\sin^4 x}$?
 
r9m
@TheArtist wolfram-alpha? :P
 
7:48 AM
@r9m :P method lol
@r9m btw which year in uni are you in ?
 
r9m
@TheArtist 3rd
 
@r9m last year?
 
r9m
@TheArtist yep
 
@r9m how is it ? easy? whats the plan after that?
 
r9m
@TheArtist easy? certainly not easy for me .. and plans? There are a few but none worth mentioning
 
7:53 AM
0
Q: Prove that the group $G$ is abelian.

Subhadeep DeyIn a Group $G$, $a^2b^2=b^2a^2$ and $a^3b^3=b^3a^3$ holds, $\forall a,b\in G$.Prove that the group $G$ is abelian. My approach was following: Let $a,b\in G$ Then, $a^2b^2=b^2a^2$ and $a^3b^3=b^3a^3$ holds. Now, $$a^3b^3=b^3a^3 \\ \implies aaabbb=bbbaaa\\ \implies a\cdot a^2\cdot b^2\cdot b=b...

Well shouldn't it be $(ab)^3= a^3b^3$. I cannot seem to get anything with the conditions given in the question
@Balarka ^^ Can you given me some hints on the above question
 
I'm busy right now.
 
Okay fine.
 
I don't think the question is wrong.
I have seen things like this before.
 
@r9m oh i see.
 
Even I have but there seem to be a dead end anything I do
 
7:59 AM
@Remember This is just basic algebra manipulation.
 
8:10 AM
@r9m hehe :-)
 
r9m
@Chris'ssistheartist So I was right! right? :D
 
@r9m If you have in mind what I have, yes. I supposed that yes. :-)
Not sure 100% though. :-)
 
@morphic I guess so that in your question about the map between $GL_n(F)$ to $D_2n$ you have to show each map is homomorphic and injective . I remember doing this question in DF
@morphic Also a map between $f: D_2n\to GL_2(\Bbb{R})$ extends to a homomorphism if $f(r)^n=1$ and $f(s)^2=1$
 
@Balarka: Go do the tangent space of the come again at different points using your old definition and your new one.
 
8:25 AM
and $f(r)f(s)=f(s)f(r)^{-1}$@morphic
 
@MikeMiller Alright.
 
 
1 hour later…
9:43 AM
@r9m how about the double integral with the golden ratio? Did you work on it?
 
10:03 AM
'ello, @Alex
 
user147690
Ello @BalarkaSen
 
user147690
I'm just doing some Lie algebra stuffs atm
 
good. didn't fall ill, then?
 
user147690
Yep, I feel good :)
 
excellent.
 
user147690
10:05 AM
I hadn't seen lattice of subgroups before today(just saw it in D&F and read it)
 
it's not something that's recommended to know to study advanced stuff in algebra, but it's a good geometric depiction of a lot of things
 
user147690
D&F covers it pretty nicely, I just found it some light fun. He says there are uses for infinite groups, anything interesting you know of?
 
user147690
(Sublattices in that case I mean)
 
really? I don't know what he is referring to.
i just use to "see" the isomorphism theorems. also, it's useful to "see" the galois correspondence in galois theory
 
user147690
Nvm he says 'these partial lattices for groups will also be used when we are dealing with infinite groups' and I guess I took it to mean it has interesting uses
 
10:10 AM
i think he just means we can draw these lattices for infinite groups too
 
Or $$\int _0^1\int _0^1\int _0^1\frac{1}{1+x^2 y^2+x^2 z^2+y^2 z^2} \ dx \ dy \ dz \le \frac{\pi^3}{32}$$
 
user147690
Yeah that makes sense, it would be very painful to not skip intermediate groups when you are looking at something of high order in an infinite group against something of low order
 
user147690
Not that I have ever really done anything at all with infinite groups explicitly
 
"wait, aren't all groups finite?"
 
user147690
What's that a quote from :P
 
10:13 AM
that's what a finite group theorist says when talking to a geometric group theorist.
to which the geometric group theorist replies
 
user147690
Oh hahaha
 
"nah, the only finite group is the trivial group"
 
user147690
I've heard something along the lines of that from a student in my class
 
user147690
But I think he was serious
 
lol
my right hand's hurting from last night, dunno what happened to it
 
user147690
10:16 AM
Pressing the pen too hard? Do you do pen tricks when you are bored or type with a book in front of the keyboard?
 
r9m
@Chris'ssistheartist which one?
 
user147690
They're the three things that always get me
 
I indeed do the last thing you mentioned. In fact, my pain's going away when I am not typing, but it's coming back each time I try to type something.
 
user147690
Book in front of the keyboard is the worst. I always really quickly get RSI
 
what's an rsi
 
user147690
10:18 AM
Repetitive strain injury, although it might not be that, I have always assumed it was that
 
@r9m Check the other room.
 
user147690
A subspace $U$ is invariant under $T$ if $u\in U\implies Tu\in U$ right?
 
mhm
 
10:37 AM
Hello@AlexC
 
user147690
Hey @Rememberme
 
user147690
I like your current avatar
 
Doing algebra ?
 
user147690
Lie algebra atm yes
 
@AlexClark God knows when it might change again
 
10:38 AM
did you figure that group theory problem out, @Rememberme
if not, I am free right now.
 
You mean the abelian one. If yes no i did not .
 
can you tell me what the problem was again?
 
Sure.
1
Q: Prove that the group $G$ is abelian.

Subhadeep DeyIn a Group $G$, $a^2b^2=b^2a^2$ and $a^3b^3=b^3a^3$ holds, $\forall a,b\in G$.Prove that the group $G$ is abelian. My approach was following: Let $a,b\in G$ Then, $a^2b^2=b^2a^2$ and $a^3b^3=b^3a^3$ holds. Now, $$a^3b^3=b^3a^3 \\ \implies aaabbb=bbbaaa\\ \implies a\cdot a^2\cdot b^2\cdot b=b...

What I am trying to do is to manipulate
$a^3b^3=b^3a^3$
 
OK, got it. Like I said, this is tricky algebraic manipulation.
 
So how you did you start
 
10:43 AM
The trick is to try to prove that $a^2b^3 = b^3a^2$.
To do this, start with $(a^2b^3)^2$. This is $a^2b^3a^2b^3 = a^2(b^3a^2)(b^3a^2)a^{-2} = a^2(b^3a^2)^2 a^{-2}$. Now, as square elements commute, this is $(b^3 a^2)^2a^2 a^{-2} =(b^3a^2)^2$.
Hence, $(a^2b^3)^2 = (b^3a^2)^2$.
Similarly, you can show that $(a^2b^3)^3 = (b^3a^2)^3$ by using the fact that cubes commute (I haven't done this explicitly, check this)
so this is done. now show that $a^2b^3 = b^3a^2$ implies $G$ is abelian. this is standard.
 
Okay got that .
You can write an answer there @Balarka
Thanks a bunch!! @Balarka
 
@Rememberme nah, you should fill in the details and write an answer.
 
@Balarka No I won't write an answer . I would given a hint as you always give me :P
 
@r9m google results dont really point to same guy.
 
10:54 AM
Okay If you are free I want some motivation on some stuff
 
ok, ask away
 
It will be a bit big so it might take some time in writing since I am on my phone
 
r9m
@Agawa001 ya ,. I saw that
 
We say that $f$ and $f'$ are homotopic if $F: X\times I \to Y$ is a continuous map if $F(x,0)= f(x) , F(x,1)=f'(x)$ for each x. Here f and f' are continuous maps from $X\to Y$ . My questions are:
1) Why does one want to define homotopy in such a weird manner. Taking products of spaces then maps. Why?
2) Here $I=[0,1]$ , Why is that $X\times I$ only . why not any other set . Or better why not any other compact set?@Balarka
 
Right, you won't understand the general definition. Let's look at special cases for paths, called path-homotopies.
 
11:01 AM
Okay.
 
$X$ be a top. space. A continuous map $\gamma : [0, 1] \to X$ with $\gamma(0) = x_0$ and $\gamma(1) = x_1$ is called a path between $x_0$ and $x_1$. Makes sense?
 
Yes
 
Let $\gamma, \gamma' : [0, 1] \to X$ be two paths of equal basepoint $\gamma(0) = x_0 = \gamma'(0)$ and $\gamma(1) = x_1 = \gamma'(1)$.
 
Okay
 
A path-homotopy (or less commonly called a continuous deformation) between $\gamma$ and $\gamma'$ is a "continuous" sequence of paths $f_t : [0, 1] \to X$ for each $t \in [0, 1]$ with the same endpoints $f_t(0) = x_0$, $f_t(1) = x_1$ such that $f_0 = \gamma$ and $f_1 = \gamma'$.
Visualization : you have two paths $\gamma, \gamma'$ inside $X$ with endpoint $x_0, x_1$. You "slide" the path $\gamma$ to the path $\gamma'$ continuously.
 
11:07 AM
Ahh, Get that
 
Well, your $t \in [0, 1]$ is a measure of time. At time $t = 0$, your path is in $\gamma$. At time $t = 1/2$, you have slided your path $\gamma$ to something in the middle. At time $t = 1$, you have slided your path to $\gamma'$.
You have done this fixing the endpoint all the time.
 
Okay , This seems very intuitive
 
Now, I have said something vague in my definition of path-homotopy above. Can you identify what?
 
You brought the idea of time ?
 
I said the definition of path-homotopy, not the intuition I gave you.
6 mins ago, by Balarka Sen
A path-homotopy (or less commonly called a continuous deformation) between $\gamma$ and $\gamma'$ is a "continuous" sequence of paths $f_t : [0, 1] \to X$ for each $t \in [0, 1]$ with the same endpoints $f_t(0) = x_0$, $f_t(1) = x_1$ such that $f_0 = \gamma$ and $f_1 = \gamma'$.
I have said something vague/philosophical in the definition above.
 
11:13 AM
"continuous sequence" of paths ?
 
precisely.
Well, a sequence of paths $\{f_t : [0, 1] \to X\}_{t \in [0, 1]}$ makes sense. But what does it mean to say a "continuous" sequence of paths?
OK, so what we need to do is that for "very close" $t$ and $t'$, the image of $f_t$ and $f_t'$ are "very close" too.
 
Okay, So that is what you mean when you say continuous
 
nope, note that I haven't been rigorous here either.
 
Yes "very close"
 
to be rigorous, consider the map $F: [0, 1] \times [0, 1] \to X$ defined by $F(t, x) = f_t(x)$. Imposing that $F$ is continuous neatly formalizes the idea that $\{f_t\}$ is a continuous sequence of paths.
so, improved definition : Given paths $\gamma, \gamma' : [0, 1] \to X$ with the same endpoints $x_0 = \gamma(0) = \gamma'(0)$ and $x_1 = \gamma(1) = \gamma'(1)$, a path homotopy between them is a collection of paths $\{f_t : [0, 1] \to X\}$ for $t \in [0, 1]$ such that $f_t(0) = x_0$ and $f_t(1) = x_1$ for each $t$ and the collection is "continuous" in the sense that the map $F : [0, 1] \times [0, 1] \to X$ defined by $F(t, x) = f_t(x)$ is continuous.
 
11:20 AM
Okay.
 
user147690
I never can tell if Sayan is actually following or not, since he says he understands it at all points when I have no idea what's going on half the time
 
Well what is not there to understand here @AlexC
 
@AlexClark really? am I that bad at this? :P
more seriously, where have you got lost?
 
Nope.
 
user147690
Haha I'm not really paying too much attention admittedly
 
11:22 AM
fair enough
 
user147690
I'll try at it properly after I get some more exercises done
 
To test that I was listening .
Let me think of an example
 
nah, no need to give examples, @Remember. tell me if you understood how that $F$ came up or not
coming up with examples here is a nontrivial task, and Munkres will lead you through that
 
Yes F came up in order to make the sequence of paths "continuous"
 
right.
ok, equivalently, define a path homotopy between $\gamma, \gamma'$ to be a continuous map $F : [0, 1] \times [0, 1] \to X$ such that $F(0, t) = x_0$, $F(1, t) = x_1$, $F(s, 1) = \gamma(s)$ and $F(s, 0) = \gamma'(s)$
 
11:25 AM
Okay get it
 
that is, you map a square to $X$ whoose sides are sent to the points $x_0$ and $x_1$, and whoose top is sent to $\gamma$, bottom is sent to $\gamma'$.
@Rememberme did you really? ok, question : where did the sequence $\{f_t\}$ go in this new definition?
if you understand this (which i don't expect you will at the first go), then you should be able to answer that question.
 
Please how to apply the integration by parts on this integral $$\int_{\Omega} div(|\nabla u|^{p-2}\nabla u) v(x) dx$$
 
@r9m do you have an idea how to solve the puzzle i posted lastly in this chatroom?
lemme relink it
 
I will give you a picture using the square idea
 
2
Q: Maximum Side of a Square Dissected into Rectangles

user260674Suppose a $m \times m$ square can be dissected into $7$ rectangles such that no two rectangles have a common interior point and the side lengths of the rectangles form the set $\{1,2,3,4,5,6,7,8,9,10,11,12,13,14\}$. Find the maximum value of $m$. I have calculated that the value is at most $22...

 
11:28 AM
ok. but you have to tell me where the sequence of paths $\{f_t\}$ vanished. (it's hidden in the definition)
 
Is that fine @Balarka
 
Please how to apply the integration by parts on this integral $$\int_{\Omega} div(|\nabla u|^{p-2}\nabla u) v(x) dx$$
 
it is! well done.
 
Thank you.
 
so $\{f_t\}$ here are just $F$ restricted to the straightlines in the square parallel to the sides that gets sent to $\gamma$ and $\gamma'$ resp.
ok, good. so we just defined path homotopy between two paths by a map from a square.
11 mins ago, by Balarka Sen
ok, equivalently, define a path homotopy between $\gamma, \gamma'$ to be a continuous map $F : [0, 1] \times [0, 1] \to X$ such that $F(0, t) = x_0$, $F(1, t) = x_1$, $F(s, 1) = \gamma(s)$ and $F(s, 0) = \gamma'(s)$
 
11:36 AM
Yes.
 
Generalizing this, $f, g : X \to Y$ be two maps between arbitrary topological spaces $X, Y$
A homotopy between $f, g$ is a continuous map $F : X \times [0, 1] \to Y$ such that $F(x, 0) = f$ and $F(x, 1) = g$.
Note that there is no condition like for the path-homotopy about sending sides to endpoints or something.
 
Get that. It is a way we have thought of path homotopy
Now the definition of path homotopy does not seem arbitrary
 
@Remember Can you give me a continuous sequence of maps definition for homotopies too?
 
Let me think
 
Just try to see what they should be in this case : generalize you picture above.
@Remember Just to know, you know how to visualize $X \times [0, 1]$ for arbitrary top. space $X$?
 
11:43 AM
Yes pasting a copy of X on [0,1] I guess
 
huh, no. what is $S^1 \times [0, 1]$?
you'll need product spaces and quotient spaces frequently if you want to learn algebraic topology. if you are not very comfortable with them, recommend you relearn all this.
 
I get that $S^1\times \Bbb{R}$ is a closed cylinder
 
no, it's not a closed cylinder
Do we recall that we have the discussion that product space $X \times Y$ can be thought off as a copy of $X$ stuck at each point of $Y$? So $S^1 \times [0, 1]$ should be a circle stuck at each point of $[0, 1]$. To do this coherently, you have to stack one circle over another. You get a closed cylinder.
For $S^1 \times \Bbb R$ it's a cylinder with ends extending to infinity in both sides. It's homeomorphic to an open cylinder $S^1 \times (0, 1)$
You should really learn all this thoroughly. I know you haven't learnt this well. Try learning it from Armstrong after your exams.
 
So my idea of sticking stuff was right just was not able to execute it
 
No, sticking a single copy would do nothing. That's called wedge sum.
 
11:49 AM
Okay will relearn it again.
 
Till then, no algebraic topology.
 
C'mon :p
 
I was serious.
 
I am also
Surely I will read it again @Balarka
 
Anyway, next time, use Armstrong. It has a lot of pictures.
 
11:50 AM
pdf available ?
 
probably not, but you can pirate it if you want
 
I have to no .. No choice. I have to many books at the moment
A question :
How do you pirate stuff. I have never done it
Okay.
 
I have to go.
 
Bye
 
@robjohn, How to prove this? : let $a,b\in \mathbb R$ and $a<x<1$, then for some $m\in \mathbb N$, $x^m\le a$.
@Rememberme, did he mean Basic Topology?
 
12:05 PM
Yes @Silent
 
Thanks. By the way which book you use? I use munkres @Rememberme
 
Same here. Though after looking at armstrong munkres has less pictures
 
yup
@Rememberme, did you get an ebook?
 
Yes
 
@Silent what happened to $b$ and where did $x$ come from?
 
12:09 PM
I was about to say the same question @robjohn
 
@Rememberme I think $b=x$
 
@robjohn, sorry! please pardon me, $b=x$
 
@Silent you can take cases when $0<x<1$ and $x<0$
 
@Silent can you show that $\lim\limits_{m\to\infty}b^m=0$?
@Silent do you also assume that $a\ge0$?
 
@robjohn, yes, sorry again
 
12:15 PM
sorry, bad joke
 
Sorry again :p
 
@Silent $b^m$ is a decreasing sequence, bounded below by $0$, so it has a limit.
 
@robjohn, thank you.
 
Then $\lim\limits_{m\to\infty}b^m=\lim\limits_{m\to\infty}b^{m+1}$ so subtracting, we get $\lim\limits_{m\to\infty}(1-b)b^m=0$. Since $1-b\ne0$, we get $\lim\limits_{m\to\infty}b^m=0$
 
r9m
12:20 PM
@Agawa001 I don't have a clue .. sorry
 
@robjohn thank you so much.
 
@Silent so you can apply this to get that there is an $m$ so that $b^m\lt a$ for any $a\gt0$?
 
@morphic If you want the proof you can come here
 
@robjohn if $b<1$?
 
@Silent well, we showed that if $0\lt b\lt1$, then $\lim\limits_{m\to\infty}b^m=0$
 
12:25 PM
@Rememberme Ok
 
Well Start by taking that the map is injective
 
@robjohn, yes so i can say that for any a>0 we have $b^m<a$ for some m.
 
So $\phi$ is injective . now let $g\in \text{ker}\phi$ . What does it mean to be in the kernel @morphic
 
@Silent You might want to state the relevant part of the definition of a limit that makes that true.
 
@Rememberme The image of the element is the identity in the group it's being mapped to
 
12:30 PM
Yes. Now since it is already an established fact that $\phi$ is an homomorphism, it should map the identity to the identity
Well I had forgotten to write this $\phi : G\to M$ is an homomorphism
 
@robjohn, let $\epsilon =a$, so there exists some N such that for all $n>N$ $|b^n-0|<a$
 
So we have $\phi(1)=1=\phi(g)$
Now since $\phi$ is injective what can we say about g now?@morphic
 
@Rememberme $g = 1$
 
So what does that tell you @morphic
 
@Rememberme Only the identity in $G$ is mapped to the identity in $M$
 
12:33 PM
if the map is injective
Now try the converse@morphic
Require a hint? @morphic
 
@Rememberme After a couple more tries
 
Ok.
I have to go
 
@Rememberme Oh then give the hint now
 
@robjohn hi, Please how to apply the integration by parts on this integral $$\int_{\Omega} div(|\nabla u|^{p-2}\nabla u) v(x) dx$$
 
@Rememberme nvm i got it
 
12:47 PM
@Vrouvrou can you compute the div?
 
1:01 PM
huh, tried this stupid nerd test. was classified as a supreme nerd god.
can't say i am disappointed.
 
0
Q: Calculating $\int_0^1 \frac{\log (x) \log \left(\frac{1}{2} \left(1+\sqrt{1-x^2}\right)\right)}{x} \, dx$

Chris's sis the artistHow would you like to calculate this one? Do you see a fast, neat way here? Ideas? $$\int_0^1 \frac{\log (x) \log \left(\frac{1}{2} \left(1+\sqrt{1-x^2}\right)\right)}{x} \, dx$$ Sharing solution is only optional.

 
Hello!! Does someone of you know a good book about first-oder logic and predicates?
 
Isn't it amazing? I'm the only one here that posts questions and says that the solutions are optional. I should be massively upvoted. ;)
 
user147690
1:21 PM
@BalarkaSen Nerd test 1 or nerd test 2?
 
user147690
Nvm too long to bother :P
 
i upvoted despite that i dont know how do the hell i tackle that
 
1:45 PM
@Chris'ssistheartist How do we $\int \frac{1}{1+\sin^4 x} dx$ ? ^_^
 
@TheArtist See my integral on main (in the meantime).
 
@Chris'ssistheartist i upvoted and commented ;)
 
lol @TheArtist
 
@TheArtist this might help $$\frac{1}{2} \left(\frac{1}{1-i \sin ^2(x)}+\frac{1}{1+i \sin ^2(x)}\right)=\frac{1}{1+\sin^4(x)}$$
 
Apparently I'm not the only person with my name
as Google shows
 
1:51 PM
@TheArtist And then use the well-known integral $$\int \frac{1}{1+a \sin^2(x)} \ dx=\frac{\arctan\left(\sqrt{1+a} \tan (x)\right)}{\sqrt{1+a}}$$
 
@AlexClark 1
 
@Chris'ssistheartist Thank You :D I see :D
@Agawa001 haha ;)
 
@TheArtist ;)
 
user image
2
@Chris'ssistheartist ^ here is a gift from me to you for that :D
 
@TheArtist :-)))))))))))))
 
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