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14:00
Haha people wrote "see Munkres" in the analysis homework
Is Ramanujan here
?
You can ping people by writing @nameofperson
Hi @MikeMiller!
@Faqruddin hi
Welcome @Faqruddin on this chat room
@DHMO hi
@Ramanujan yes
@DHMO I have a question can I?
14:07
@Ramanujan yes
I already said yes
How to solve it? @DHMO
@DHMO are u here?
@DHMO what?!?!?!?!?
Let me think, will you?
I will,
14:25
Hint: $\displaystyle \tan^{-1} \left( \frac{1}{1+n+n^2} \right) = \tan^{-1} \left( \frac{1}{1+(n)(n+1)} \right) = \tan^{-1} \left( \frac{(n+1)-(n)}{1+(n+1)(n)} \right)$
@Ramanujan here
You did how my math sir do , great
OK , so it will be 4 th option @DHMO
@DHMO lol your hint from the other day
a bunch of people tried to solve that problem using that stuff and got destroyed by the prof
@0celo7 what?
@0celo7 oh, that one
sorry :p
@DHMO iam totally destroy at this problem
(uploading)
14:33
@Ramanujan yes
@DHMO The actual proof uses Baire category.
The Baire category theorem (BCT) is an important tool in general topology and functional analysis. The theorem has two forms, each of which gives sufficient conditions for a topological space to be a Baire space. The theorem was proved by René-Louis Baire in his 1899 doctoral thesis. == Statement of the theorem == A Baire space is a topological space with the following property: for each countable collection of open dense sets , their intersection is dense. (BCT1) Every complete metric space is a Baire space. More generally, every topological space which is homeomorphic to an open subset of a...
you should have posted the question here instead
so I could be flamed by the gods such as @BalarkaSen and @MikeMiller before I misdirect you
You didn't misdirect me, I had thought of those "naive" proofs already and concluded the prof was trolling.
And he was.
What's the question.
That was to 0celo, not Ramanujan.
14:37
There is no sequence of continuous functions converging to the Dirichlet function on $[0,1]$.
Hi @Danu.
also, what norm? you mean pointwise, of course, yes?
Right, forgot to say that.
@Ramanujan hint: the end term of each bracket (1,3,6,10,...) are the sum from 1 to n
14:39
I don't know how to do it off the top of my head, to be honest.
@BalarkaSen It's hard, I don't blame you.
I believe ya.
But a bunch of grad students thought they had it, but they ended up showing they didn't understand how $\epsilon$-$\delta$ proofs work.
I.e. that you FIX $x,\epsilon,N$, whatever.
14:40
I don't think this can be done by epsilon delta tweakings. Pointwise norm doesn't have good tools to fiddle.
@BalarkaSen Yeah. Turns out that a pointwise limit of cont. functions has a meager set of discontinuities.
Nice result.
but the Dirichlet function is everywhere discontinuous
@DHMO I didn't get what it mean by sum from 1 to n
@Ramanujan for example, 10 is the sum from 1 to 4
> The Dirichlet function can be written analytically as $$D(x) = \lim_{n \to \infty} \lim_{n \to \infty} \cos^{2n}(m! \pi x)$$
Mind = blown
14:43
So 10 is last term of 4th bracket , right?
@DHMO That's not a counterexample btw.
Is there a category whose objects are topological manifolds and those automorphisms are the self-homeomorphisms homotopic to the identity?
@Ramanujan yes
@0celo7 what are you referring to?
@DHMO It does not contradict what I told Balarka.
But maybe you weren't thinking that.
@DHMO let me do it on my own from now,thanks
14:44
@0celo7 What does not contradict what you told Balarka?
(also, sorry about saying the adjective "norm" after pointwise when I really meant topology)
Can't you just do that by determining the baire class of discontinuities?
@DHMO your $D(x)$ expression.
@BalarkaSen I knew what you meant.
@0celo7 Didn't say it does. I just mean that I am fascinated by the cleverness of whoever came up with that thing
Like, Dirichlet's discontinuities are baire class 2, but the discontinuities of the pointwise limit of continuous functions is class 1?
14:45
@SteamyRoot Yes, but that's nontrivial.
oops, I just spotted a typo in my equation
Is there a trivial proof?
Yes.
@abenthy The only obvious one is where there are no maps other than isomorphisms.
Oh
Hmmm
14:47
@MikeMiller But not every self-homeomorphism is homotopic to the identity, so this doesn't work, does it?
Couldn't you like, pick some rational $x_1$ and irrational $x_2$
and then use the intermediate value theorem between them
@SteamyRoot If you can write a proof I'll be very happy
But no one could think of one that didn't use the Baire machinery.
> This function is continuous at irrational $x$ and discontinuous at rational $x$.
(Not the Dirichlet function)
that's too amazing
@BalarkaSen Ok, should I study for QM or read Hatcher?
Toss a coin.
14:50
Or play Fallout?
Play Fallout.
Why would you say that?
Right... Let $[a,b] \subset [0,1]$ and $(f_n)$ a series of continous functions converging pointwise to Dirichlet. Pick $x_1$ rational and $x_2 > x_1$ irrational in $[a,b]$
There must exist some $N$ such that $f_n(x_1) > 2/3$ and $f_n(x_2) < 1/3$.
@0celo7 @BalarkaSen why is the function below continuous at irrational x?
$$\begin{cases}0&\mbox{for x irrational}\\1/b&\mbox{for x = a / b a reduced fraction}\end{cases}$$
when $n \geq N$
14:54
@SteamyRoot Yes, that's how everyone begins the "proof"
Why is that starred?
@0celo7 1) It's obviously the more tempting choice for you 2) you wouldn't blame for making that decision, unlike if I told you to read QM 3) I can work in peace without getting asked a barrage of Hatcher questions from you, and moreover complaints about how completely nonrigorous and totally outrageous Hatcher's book is.
@BalarkaSen 1) That's why I wanted you to tell me something else 2) I really need to study QM and that's what I'm actually doing 3) Outrageous?
I quit.
Lol what?
Hence there is some nonempty interval $I \subset [x_1,x_2]$ such that $f_N(I) = [1/3,2/3]$.
(using intermediate value theorem)
And $I \subset [x_1,x_2] \subset [a,b]$. So repeat this with the interval $I$ instead of $[a,b]$
You get a sequence of indices $N$ together with intervals $I_N$, and the sequence of intervals is (non-strictly) decreasing
So if you take the intersection of all those intervals, you get something nonempty.
15:01
@abenthy You could say that the objects are topological manifolds and the maps are self-homeomorphisms which are homotopic to the identity. That's a perfectly good category!
But what is a map "M -> N homotopic to the identity"?
@0celo7 could you help me?
Right, that's the problem. All you have are maps M -> M.
@SteamyRoot Interesting.
That's a category. Just a lame one.
15:02
Then take a point $p$ in that intersection, and consider the subsequence of functions $f_N$ of the sequence of functions $f_n$. $\lim f_N(p) \in [1/3,2/3]$.
Ah okay, thanks Mike!
but this limit must be the Dirichlet function, which doesn't take values there. Contradiction.
@SteamyRoot How should I credit you?
I'm writing a note to my prof.
"some random dude on Math.stackexchange.com" or so, I don't really care.
I can't remember if I meet with my advisor at 10 or 11.
15:05
On a side note, I forget: isn't there a conjecture about all K(G, 1) manifolds having that property?
Alzheimers
@BalarkaSen No.
The property that every homeomorphism is homotopic to the identity? :P
Obviously, there's a counterexample for the torus (switch meridian and longitude). Duh
I think that might be for homotopy equivalence.
Again, no.
You can tell me what the set of self-homotopy equivalences are.
Isomorphisms G --> G, of course.
15:07
The set of homotopy classes of homotopy equivalences of a K(G,1) is Out(G) btw.
Ah, right, I was thinking of basepoint preserving ones
Hmm, I can't remember the right and not obviously stupid statement of the conjecture
The conjecture you're thinking of interpolates between the homotopy-theoretic information, which is easy, and the topological info, which is hard. If $M, N$ are aspherical topological manifolds, the conjecture is that any map $f: M \to N$ that induces an isomorphism on fundamental groups is homotopic to a homeomorphism, unique up to isotopy.
Ah, right.
-unique, that's unreasonable.
@DHMO iam getting 1275×638 - 613×1227
15:10
It's still not proven, right?
@Ramanujan seems good
@abenthy Not in dimensions greater than three (and when the dimension is that small, I think uniqueness does hold, which is what confused me). There is some progress. It's now basically a K-theory question I think.
@DHMO will I get answer now?
@Ramanujan isn't that the answer?
@Ramanujan it should be 1275x1274/2 - 1225x1224/2 instead, i think
Answer is in number
15:13
since 1275-50=1225
I did 1275 - 49
By seeing above terms
(brackets)
should be 1275 - 50 instead
If you look 4th (any ) bracket first term in 10-3 not 10-4 @DHMO
@Ramanujan yes, but you need to minus an extra 1
Why?!?!?!?
15:21
OK, a manifold has a spectrum $\mathbf{L}(X)$ coming from surgery theory. Apparently the Borel conjecture is equivalent to saying that the map (whatever it is, I don't know) $M_+ \wedge \mathbf{L}(\Bbb Z) \to \mathbf{L}(M)$ is a homotopy equivalence when $M$ is aspherical. This is some flavor of K-theory.
@MikeMiller: Does the case for dimension 3 use the full machinery of Geometrization, or is it easier? For hyperbolic manifolds Mostow gives this, certainly. I don't know many other examples of aspherical 3-manifolds (take the torus into account and I'm out of examples).
@BalarkaSen You need geometrization in general. For Haken manifolds, Waldhausen proved it.
Oh, I do know
Knot complements :P
I'm talking closed. If you want to allow boundary, you have to be more careful.
@MikeMiller Interesting. Thanks.
Hmm, ok
15:23
The Borel conjecture doesn't allow boundary, but it's still true for 3-manifolds with boundary. Now it's just any map that induces an iso on $\pi_1$ and sends the "peripheral subgroup" (conjugacy class of image of the pi_1 of the boundary) of the first to that of the second, then it's homotopic to a homeomorphism.
I see, nice.
Otherwise a knot complement would be determined by its $\pi_1$.
@DHMO I need pure way to solve this problem (uploading)
@Ramanujan what is your formula?
so the start term is 1226 and your end term is 1275
what is your next step?
Right, good point
15:26
I got same terms
And now I am adding by using sum of n natural numbers formula
@SteamyRoot I wrote "a Belgian grad student showed me this"
Last term I got from that it is sum from 1 to 50
Fair enough :)
And first term by subtracting 49 from last term@DHMO
@Ramanujan yes, you're just repeating what i just said
what is your next step?
15:29
1275×638 - 613×1227
But it's too large
how do you derive this from the fact that the start term is 1226 and your end term is 1275?
First I get nth term of 50th bracket to get 1275(from your given hint) and then I came to know that first term will be 1275 - n(of bracket ) -1 from observation
And there is only common difference of 1
So we can apply sum of n natural numbers formula
@DHMO
and what is the formula?
n(n+1)/2
@DHMO one more problem,please I want to get it by pure way ( not by substitute)
@Ramanujan yes, n(n+1)/2, but why is it 1275x1276/2 - 1226x1227/2 ?
15:41
@DHMO sorry, formula is for sum of n natural numbers from 1,but we are having from middle so we are adding from 1 to total and subtracting from 1 to middle to get our required sum
yes, that is what i wanted you to say
actually, you only need to subtract from 1 to middle-1
consider summing from 7 to 10
7+8+9+10
it is (1+2+3+4+5+6+7+8+9+10) - (1+2+3+4+5+6)
notice that the end term of the second bracket is just 6
instead of 7
Oh,igot it now,iwas not concentrating more on that term
@Ramanujan q12 or q13?
Thanks
13
You can do , I will do dinner and come now
let that expression be f(n)
f(1) = sqrt(2) right
the problem is not clear on how the ... ends
15:46
@DHMO continue,
the problem is not clear
What do you mean by problem is not clear?@DHMO
is f(1) 2 or sqrt(2)?
Sar(2)
√2
ok
and as we can see, $f(n+1) = \sqrt{2+f(n)}$
15:48
@DHMO why did you think f(1) =2?
@Ramanujan never mind
sorry, I have no idea how to do it without substitution
OMG! It's again
???
15:53
@DHMO why there is C_1 as only 1 in area of triangle,remembered?
I don't remember
Area of triangle in detail form
Why c_1 is 1 1 1?
oh
because it evaluates to the right answer?
just expand the determinant
to check that the two formulas are identical
I can't see any way how mathematics can expamd any matrix(before knowing it)
what do you mean?
16:03
I know we get same,but my question was to know how it came in formula and in any mathematicians mind while making that formula
ask the mathematician who came up with that formula
He might not revealed it for us to know the beauty of mathematics
I think so
@DHMO. Will I get answer for that if I post it on MSE?
Is $\Bbb R^{\omega}$ countable or uncountable?
@Ramanujan I don't think so.
@DHMO I will try but, if not I can get upvote too
@Ramanujan I don't think you will get upvotes
16:10
@DHMO why?!?!?
it is not really a mathematical question
asking how a formula is derived
it would be closed as too broad
So you want to say it can be used as formula?
16:13
When we get something like √2+√2+√2+√2+… instead of adding up all , we can write n=4 in formula?
yes?
Haha(lol)
Asking or saying?
both
I think yes
Till we know why cos( π/n) will be
@DHMO one more question?
Can I ask one more question?
yes
16:20
Again without substitute,becoz I got it by substituting n=1
4 or 5?
are you familiar with geometric series?
Yes (series which has common ratio between each successive terms)
can you express 777....7 n times in terms of such series?
16:24
Only last term 777777…………ntimes?
yes
r=n^0
???
7 7 7 7 7 has common ratio of 1
no...
77777 does not mean 7+7+7+7+7
16:27
Then there is no common ratio as one term doesn't create any series
77777 means 7+70+700+7000+70000
OMG,how could I miss this (school boy error)
OK so common ratio is 10 @DHMO
yes
OK wait ,let me do it on my own now
no....
second line is wrong
$n7$ is right
$n70$ is wrong
16:33
@DHMO I got till here , also I don't know properly how to apply summation
i gave you a hint
you didnt use it
@BalarkaSen I sent you the Taubes exercises.
Thanks a bunch!
@DHMO oh,i got ,it is (n-1 )70 and so on
yes
i said express the n-th term as a geometric series
use the formula on each term
16:36
....
@DHMO why can't I go in this way?
@Ramanujan you can if you know how to
for me, i don't know how to
OMG!!it's again
?!?!
what is this supposed to mean
47 mins ago, by Ramanujan
OMG! It's again
16:38
A mathematician don't know how to get it
How to solve
$7+77+777+\cdots+777...7 = \dfrac{7(10^1-1)}{10-1} + \dfrac{7(10^2-1)}{10-1} + \dfrac{7(10^3-1)}{10-1} + \cdots + \dfrac{7(10^n-1)}{10-1}$
start from here
But I think nth term will become (n-n)7×10^n =0
@Ramanujan yes, if you use the method in your notes
@DHMO iam not getting how to do it😥 @DHMO
$\dfrac{7(10^1-1)}{10-1} + \dfrac{7(10^2-1)}{10-1} + \dfrac{7(10^3-1)}{10-1} + \cdots + \dfrac{7(10^n-1)}{10-1} = \dfrac{7\times10^1-7}{9} + \dfrac{7\times10^2-7}{9} + \dfrac{7\times10^3-7}{9} + \cdots \dfrac{7\times10^n-7}{9}$
16:43
First term is 7/9?
@Ramanujan I think I know how to do it from here now
@Ramanujan who told you that?
@DHMO wait,dinner is ready (chicken 65) 🍢
Let $S=7n+70(n-1)+700(n-2)+\cdots+7\times10^{n-1}(1)+7\times10^{n}(0)$.
$10S=70n+700(n-1)+7000(n-2)+\cdots+7\times10^{n}(1)+7\times10^{n+1}(0)$
Subtracting the first equation from the second:
$9S=-7n+70[n-(n-1)]+700[(n-1)-(n-2)]+7000[(n-2)-(n-3)]+\cdots+7\times10^{n}(1-0)+7\times10^{n+1}(0)$
$9S=-7n+70+700+7000+\cdots+7\times10^{n}$
$9S=-7n+\dfrac{70(10^n-1)}{10-1}$
$S=-\dfrac79n+\dfrac{70(10^n-1)}{81}$
$S=-7\left(9n-\dfrac{10(10^n-1)}{81}\right)$
$S=-\dfrac7{81}(9n-10(10^n-1))$
$S=-\dfrac7{81}(9n-10^{n+1}+10)$
bye, see you later
Hello could someone help me with the lim as x goes to 2 of (2^x - x^2)/(x-2)
@DHMO wait
@DHMO I don't know how to properly apply summation
Did I did it in right way
?
17:06
hello
17:17
@Ramanujan If you're going to put a summation sign, use indices and start/end values
I'm guessing what you want is $\sum_{i=1}^n (i+1) = \sum_{i=1}^n i + \sum_{i=1}^n 1 = \frac{n(n+1)}{2} + n = n\left(\frac{n+1+2n}{2}\right) = \frac{n(3n+1)}{2}$
(note that it's wrong, though. When you place $n$ in front of the brackets, you get $n\left(\frac{n+1+2}{2}\right)$ instead of $n\left(\frac{n+1+2n}{2}\right)$
@MikeMiller I'm learning about cohomology with bundle-coefficients now. In the standard case (so trivial bundle), there are isomorphisms $*:\cal{H}^{p,q}(X)\to \cal{H}^{n-q,n-p}(X)$ where $\cal H$ are harmonic forms. This, at least in Huybrechts book, follows from the analog for the real case (which is assumed), plus the fact that $*$ maps $p,q$ into $n-q,n-p$. Now, for bundle valued forms, is there also an analogous real statement, making the complex one easy?
@BalarkaSen what exercises?
17:43
@Danu: It's better in the holomorphic category, because $\bar\partial$ is well-defined on holomorphic sections, so you don't even need a connection to make sense of differentiating sections. Check it for yourself.
Hi Ted!
Oh, I sort of missed the point of your question.
Your comment doesn't make much sense to me
math.stackexchange.com/questions/1978908/… can any one convert it into matjax pls.
It's an important comment. In the real category, you cannot differentiate sections of a bundle unless you have a connection on the bundle (or unless the bundle is globally trivial).
17:46
@TedShifrin I believe that. I think I know that :P
I took a course on "Riemannian geometry" after all
The generalization of the isomorphism you're thinking of is called Serre duality.
But it was about constructing the Chern-Weil homomorphism (?!) instead
Oh, I forgot about that. Well, I love Chern-Weil.
I know, Ted:) That's what I'm studying now :D
Anyways
My question was more practical
Anyhow, I answered your question. Wait for Serre duality.
There is no real analogue.
17:47
Sorry, let me reiterate.
For $*: \mathcal H^{p,q}(X)\to \mathcal H^{n-q,n-p}(X)$, Huybrechts claims it is an isomorphism because (i) on Riemannian manifolds $*:\mathcal H^k(X) \to \mathcal H^{n-k}(X)$ is an iso, plus the fac that $*$ maps $(p,q)$-forms into $(n-q,n-p)$ forms.
That's sorta not satisfying of course
But never mind that.
I'm fine with it.
You can check it directly from the identities you've done for the various Laplacians in the complex setting.
Now, I'm onto bundle-valued forms and he claims (without comment) that $\bar *_E$ induces an analogous isomorphism
@TedShifrin Yeah but we're not assuming Kahler!
I'm just wondering if the $\bar *_E$ case is exactly the same story as the $*$ case (probably, since every other aspect of it seems to be...)
Oh, right. OK. No, it's a bit more subtle. The bundle has to get dualized.
Does he not do Serre duality?
You need a plain ol' $(n,n)$-form to integrate, so you need to pair $E$ and $E^*$ ...
So look carefully at what his $\bar\star_E$ is doing.
hi @PVAL
17:52
@ted hiya
Ah, the topology crew has arrived. Just in time for me to leave :P
@Danu Ask me again at around 1 if you haven't figured stuff out.
I have to meet with Ciprian in five minutes.
LOL @ "around 1" ... we're on so many different time zones. :)
Have a good meeting.
@TedShifrin Yes, I'm sorry, of course!
He'll figure it out.
17:54
He does do Serre duality
Serre duality is good,
So the meaning of harmonic forms has to be clarified, @Danu. Are we talking $\bar\partial$ Laplacian?
Arn't we on Kahler
He says we're not.
@TedShifrin Yes
And we're on Hermitian only
17:56
OK, so it's obviously not possibly going to follow from anything in the real category.
Well, see my earlier statement
for $*$ it makes a lot of sense
But then again yeah...
The $\bar\partial$ of course doen'st coincide with the $d$ Laplacian
That's actually a bit weird
Uh huh.
Perhaps he's just being slightly inaccurate
Time to go. We'll see how it goes.
Even in the Kähler case, you have trouble once the coefficients are in bundles.
Have a good meeting, @MikeM.
17:57
Right, so @Ted in the discussion of $*$ he does it for the $d$-Laplacian
Oh, so he is actually using hermitian structure and putting on the canonical connection compatible with the holomorphic structure and the hermitian structure. Then you get $\mathscr H^{p,q}(E) \cong \mathscr H^{n-p,n-q}(E^*)$, the iso being conjugate linear.

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