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10:01
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249438622653341445697854845796856525153425658975845842132023
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@Secret Just stop that already.
Mathematics is a human construct and thus it is strongly tied to culture and politics
It is interesting how everything is related to politics now
10:19
Hi @Tobias
@AlessandroCodenotti Hi
finally, activity, this is so boring
I have an algebraic number theory question you probably know the answer to
You will be amazed at the many things I don't know or don't remember about algebraic number theory
10:22
Hi, i got a question:
suppose $\Bbb F \subset \Bbb K_1 , \Bbb K_2$ all fields.
denote by $\Bbb K_1\Bbb K_2$ the minimal field containing both $\Bbb K_1$ and $\Bbb K_2$ i need to prove that if $[\Bbb K_1:\Bbb F]$ and $[\Bbb K_2:\Bbb F]$ both finite then $[\Bbb K_1 \Bbb K_2 : \Bbb F]\le [\Bbb K_1:\Bbb F][\Bbb K_2:\Bbb F]$
I have a number field $K$ and $h_K$ its class number. If $h_k=1$ then $\mathcal{O}_K$ is an UFD, if $h_K>1$ I can conclude that it isn't a PID. Are there examples with $h_K>1$ and thd ring of integers is an UFD nonetheless?
someone can help?
@AlessandroCodenotti Hmm, I think UFD implies PID for Dedekind domains
le soneon dies not exist (Gib)
right, this was mentioned in the answer to math.stackexchange.com/questions/1051366/…
@AlessandroCodenotti Anyway, feeling slightly Italian, as I have a bolognese on the stove simmering away. But also very Danish, as my lunch was potatoes on rye bread.
10:30
Suppose I have a continuous path in a hausdorff space, is it possible to construct an injective version of it.
@quallenjäger What would it mean to be a version of a path? You mean up to homotopy?
This Lemma 3.2, I can't understand.
I don't see what the mapping $q$ does.
(Gib and the recent instability will be explained in next downtime (except that the search is terribly not helpful as neither "question" nor "hello" capture the dozens of them)
And I can't find the reference to it because the book is so old.
@TobiasKildetoft ah right, that's actually not hard to prove. Pick $a\in P$ with $P$ a prime ideal, factor $a$ as $p_1\cdots p_k$, there must be a $p_i$ with $p_i\in P$, hence $(p_i)\subseteq P$ so $(p_i)=P$ because primes are maximal and $(p_i)$ is also prime, so every prime ideal is principal
@TobiasKildetoft Nice, I made it with my flatmates during the weekend too! (We call it ragù rather than bolognese in Italy usually though)
10:37
@AlessandroCodenotti Interesting. We usually call it meatsauce
Also, most Danish people don't actually let it simmer at all, so it tends to be somewhat gritty in texture.
The longer it simmers the better it tastes, there's no limit
agreed
I was a little lazy today though, so I skipped a few ingredients. It is just onions, meat, carrots and (canned) tomatoes.
Fact #1: Liad dies not exist
Fact #2: Marystar dies not exist
Fact #3: (I forgot)
11:24
@AlessandroCodenotti no, like you make a function that does things at the points 3.4 and 3.6
I know, I was joking
12:06
in The Factory Floor, 8 mins ago, by Secret
A "world" reachable only via "zooming"
Because space is too boring and restricted, why not make things more interesting by creating a space like thing with relations instead
In the context of maths, "zooming out" from various algebraic structure will end up in category theory
Of course, "zooming" too far out and then there will be nothing in common and thus you end up in the empty set
$$\text{Zoomout}(A,B) = \min (X : A \cap B \subseteq X )$$
gives more drugs to Secret
12:27
@AlessandroCodenotti oh woooooooooooow the guy means $\frac{d}{5-r}$ instead of $d/5-r$...how did this get published
12:46
Feels like those 2-2/2 questions on facebook
Let $(a_n)_{n\in \mathbb{N}}$ be a sequence of numbers. I want to show the equivalence of the follwoing statements:

(1) There exists a real number $q$ ($q\neq 0$) with $\frac{a_{n+1}}{a_n}=q$ for all $n\in \mathbb{N}$.
(2) There exists a real number $q$ ($q\neq 0$) with $a_{n+1}=a_1\cdot q^n$ for all $n\in \mathbb{N}$.

To show the equivalence we have to show that $(1)\rightarrow (2)$ and $(2)\rightarrow (1)$, or not?
But how? Using induction?
@AlessandroCodenotti this is such a strange paper
there's a random 3 now that I don't know what's with it
that's not english but you get the point
Has the result been published somewhere else or are you stuck with this paper?
In some notes but they're worse
completely different proof too
this lemma is completely ridiculous too
I can assure you that kind of stuff doesn't happen outside analysis
(Blatant lies)
13:03
the proof has like 4 typosd
$lies^{lies^{lies}}$
(that makes a good echo)
What is the boundary asymptotic of a surface?
13:30
Yay I got a B in the class I was worried about, complex ii :D
congrats
It was funny, when my lin alg ii prof walked into the room holding the exams he was visibly embarrassed, he explained that
He had so much fun designing problems that when he finished making the exam he realised it was too long
So he said just work on it for the entire alloted time and he'll grade what we do, we don't need to try to do all of them lol
-_-
I was so happy one of the problems was to prove the Riesz r
Representation thm because i love that thm and its proof
 
1 hour later…
14:47
Can somebody explain what Im doing wrong here?
Trying to prove that if A and B are orthogonal then AB is also orthogonal
I know the conventional proof
But here is my attempt at one
Nevermind I figured it out
15:14
Any hints for this? I'm really stuck.
15:27
In how many possible ways we can Rearrange the word "" MATHEMATICS """" such that all the vowels stay separately?
15:43
@SwapnilDas I didn't work out the whole thing but it may help to note that (1+x)^1/x converges to e as x goes to zero from the right
Because (1+1/n)^n converges to e as n goes to infinity
16:12
Taking help from math.stackexchange.com/questions/1974159/… and applying L' Hospital rule, the answer comes = e/2
16:38
I'd say that (1+1/n)^n goes to exp(1) is common knowledge and you'd just have to justify changing the limit from 1/x to 0 to x to +infinity
17:04
Hi, if I look at the solution set $M$ of the set of non-linear equations

$$\begin{cases} y_1&=x^2 \\ y_2&=x^3\end{cases}$$

this is a manifold. I can check this using the implicit function theorem by defining $f: \Bbb R^3 \to \Bbb R^2$ and

$$f(x,y_1,y_2) = \begin{bmatrix} y_1-x^2 \\ y_2-x^3\end{bmatrix}$$

and checking that the $2 \times 2$ matrix

$$\partial_{y_1,y_2}f = \begin{bmatrix} 1&0 \\ 0&1 \end{bmatrix}$$

is indeed non-singular. My question is if I don't use the suggestive notation $y_1,y_2$, but for example $y$ and $z$, how do I know which part of the total derivative of $f$ I
Hello Phimcole, if you are interested in this question ->

https://math.stackexchange.com/questions/135766/average-distance-between-two-points-in-a-circular-disk?utm_medium=organic&utm_source=google_rich_qa&utm_campaign=google_rich_qa,

then please help in verifying whether on applying the result of the thread
https://math.stackexchange.com/questions/2386926/average-distance-to-a-non-central-point-in-a-circle

to the former problem, the answer comes 0.82*r as against 145*r/28*pi
17:19
@philmcole: You write down the entire derivative matrix and see which minor (if any) has nonzero determinant. ... But, again, this is back to a graph $y=\phi(x)$, and graphs of smooth functions are always manifolds ($x\in\Bbb R^k$, $y\in\Bbb R^\ell$).
@Ted This would be a lot of minors to check in general. Is there no way to see which variables to name $y_1,y_2,y_3$ and so on from a plot?
This was in fact the first of the examples you did in the lecture on YT :) You said something like "I'm gonna name the variables according to the theorem to help you apply it" and my question is how would we have figured out which variables are which if you hadn't named them like that?
If it's really a graph, you should see an identity matrix in the derivative matrix. But if it's not, you actually do have to hunt to find a nonsingular submatrix (locally).
I'm examining a sequence of maps on an infinite-dimensional Hilbert spaces that converges pointwise but not uniformly, I don't know how to prove this step and would like a hint
17:35
Okay thanks!
It all depends on the maps, @GFauxPas. I don't think I've ever thought about this in an infinite-dimensional setting.
right well it gives an example but the exercise is to prove it converges pointwise and jnot uniformly
i'm stuck on this part
What's the pointwise limit?
Let $\{e_n\}_{n=0}^\infty$ be an orthonormal basis. then for $x \in H$, $||\langle x,e_n\rangle e_n - \langle x,e_m\rangle e_m || = 1$ for $m \ne n$
That's not true for all $x$.
Try $x=0$.
17:37
the sequence is $T_n x = x + \langle x, e_n \rangle e_n$
Try $x=e_1+e_2$.
oh oops this isn't a pointwise limit I'm sorry it's saying that
I don't know where you got that norm equality.
$||T_n - T_m|| = 1$ in the operator norm
You understand the difference between that and what you said?
17:38
yes yes definitely
$\sup_{h \in H, ||h|| = 1} ||(T_n-T_m)h||$
Right.
I guess it's not obvious to me instantaneously that that difference has operator norm $1$. At least $1$ is clear.
okay well let's start from the beginning of the problem if you're willing to work on this together with me Ted
I didn't have to do every problem on the final but I want to know how to do this one
So you're trying to show that $T_n$ can't converge pointwise?
show $T_n \to I$ pointwise
hint: pythagorean theorem
@GFauxPas That converges pointwise to $x$, right?
17:41
that's the first part of the exercise
the latter term is is the projection of $x$ onto an orthogonal basis so
OK, so that operator norm condition certainly shows you can't have uniform convergence. Uniform convergence would imply uniform Cauchy-ness.
$||\langle x,e_n\rangle||$ must go to $0$ just because $e_n$ is a basis
sup nerdoids
and hi @Alessandro
heya nerd Eric.
Hi @Eric @Ted @GFauxPas
17:43
$||\sum \langle x,e_n \rangle e_n ||^2 = \sum ||x_n||^2$
Too many bars flying around, @GFauxPas.
wait no
And no.
too many - yeah lol
i like to write $||| \cdot |||$
17:45
fires Eric
for the sake of transparency i am lying
I don't believe that operator norm 1 thing.
You're not even translucent, Eric.
I didn't get up to it yet!
mayhbe i didnt write out all hypotheses
@GFauxPas: You haven't told us what $x_n$ is. You mean $\langle x,e_n\rangle = x_n$? That's a scalar, so no double bars.
let's start from the beginning because I lost track of whats a scalar
let $H$ be a hilbert space, infinite dimensional, with orthonormal basis $\{e_n\}$
countable
17:48
Sure.
$T_nx := x + \langle x,e_n\rangle e_n$
$T_n(x)$.
claim: $T_n \to I$ pointwise. Hint: pythageorean theorem
Or Parseval's identity or ...
or CS
?
17:50
A DIN A4 sheet is divided into thirds. A rectangle is the root of the tree, the other two rectangles are each divided into thirds again. Two rectangles form the branches - one to the left, one to the right - the others are again divided into thirds and so on.
Cauchy-Scwarz
Could you give me a hint how we could calculate the area of that tree?
(some notes)
GFauxPas and philmcole is not *dies not exist*. Do not include them in the tally
which schwarz is cauchy-schwarz
17:52
I don't see how it's Cauchy-Schwarz, @GFauxPas, but maybe I'm missing something obvious.
(hopefully SE chat is smart enough to pick out "dies not exist" when I finally search for it some time later)
The functional analyst is here!
@MaryStar: I don't understand enough to say anything.
no it's Parseval you're right, I got them confused
17:53
Yup, 0celo renders me redundant. Yea!
looks for another R
$||T_nx -Ix|| = ||x + \langle x, e_n \rangle e_n - x|| = ||\langle x, e_n\rangle e_n||$
Henlo
that's not 0, I did something wrong
Hi Demonark.
It is $0$ in the limit, @GFauxPas. That's what Parseval tells you.
ooh otherwise it wouldnt be square summable
17:59
Right.
Okay ^^ so $T_n \to I$ pointwise
thanks
next
$||T_n - T_m||_{\text{op}} = 1$ for $n \ne 1$ , which you're skeptical of
Right, I'm skeptical. I say it's at least $\sqrt2$.
You mean $n\ne m$.
yes I do
$\sup_{||x||=1}||T_n x - T_m x|| = \sup_{||x||=1}||\langle x,e_n \rangle e_n - \langle x, e_m \rangle e_m||$
Oh, never mind. I say it's at least $1$, not at least $\sqrt2$.
How do we know it can't be bigger?
it could be a typo, let's see
18:06
I think it's correct that is $1$
@TedShifrin immediate, no? or are you prodding GFauxPas
$||(T_n-T_m)(x)||$ is maximized when $x$ has components only along $e_n$ and $e_m$, right @GFauxPas?
that's kinda just giving it away I think
twiddles thumbs
okay I did it using parallelogram law
which I'm allowed to do because Hilbert
hides from Ted for not using geometry
18:09
LOL
sounds like overkill to me but ok
::rocket launcher::
what's an easier way
dinking
Oh that's a new one
$x_{n}^{2} + x_{m}^{2} \leq \sum_{i = 1}^{\infty} x_{i}^{2} = \|x\|^{2}$
But $|x_n|+|x_m|$ isn't the sum of squares :P
Do we get to use CS now?
18:12
So $T_n \to I$ pointwise but $T_n \not \to I$ uniformly
the norm of $T_{n}(x) - T_{m}(x)$ is $x_{n}^{2} + x_{m}^{2}$ @Ted
last part: assume now that$H$ is finite dimensional. Prove $T_n \to I$ uniformly
Um, really?
yes? Am I doing something stupid
18:13
but that's equal to $2$ and I have it's $1$
lol
or square root of that i mean
I don't see that, either. I see an inequality.
$T_{n}(x) - T_{m}(x) = (0, \cdots, x_{n}, \cdots, x_{m}, 0, \cdots)$?
if we're using the fact that ONB gives iso to $\ell^{2}$
whoa acronyms
OK, so I had $\|T_n(x)-T_m(x)\|\le |x_n|+|x_m|$.
18:15
orthnormal basis i smart
all $0$s except $n$-th coordinate and $m$-th coordinate
norm of that guy is definitely the root of $x_{n}^{2} + x_{m}^{2}$.
I can get to yours by CS.
We're all ending up at the same place.
Well, @GFauxPas, you should get to that easily.
sure, but im like 99% sure im not talking crazy tho
I only said you were crazy when you said norm was norm squared.
oh ya that was just misspeaking
mistyping
18:17
And I actually never called you crazy. :P
the inequality i first wrote was still right and gave us the keys to the kingdom
i called myself crazy
trying to find the point in my work where I assumed infinite dimensionality
Actually, I don't understand the question. There are only finitely many operators if the dimension is finite. So WTF?
trashes the examiner
oh it's asking generically
prove pointwise convergence in a finite dim Hilbert space is uniform convergence
pointwise convergence of a sequence of operators
well you can just use a covering argument, can't you
Oh ...
Covering? Huh?
18:25
wait nope that didnt make sense
You mean compactness of the unit ball?
yeah but that proves it the other way
I'm not following.
that's how you can prove that uniform convergence implies pointwise convergence on compact sets
but I want the harder implication, that pointwise implies uniform
Uniform convergence always implies pointwise convergence. What you talking about?
You're confuzled.
18:27
I'm saying I was confused as to which implication direction I was doing
I still say it's what I said.
anyway I have to go on to my take home assignment
thanks guys, especially Ted
OK, bubye.
:) I'll still be around, I just have to do more myself now because this isn't review it's actually something I have to do
18:46
hi @TedShifrin
I missed talking to you
@AlessandroCodenotti lmao I just noticed the proof also involves 3.1 and 3.9
@TedShifrin I just came to update you with news :) I won a 15 k scholarship. My Thesis also won best thesis award.
I have been doing nothing but math lately. Today since I have been working hard. I will invite my wife out for a dinner outside and thank her for all the support.
Awesome Adeek
congratsw
make sure to bring your thesis to the dinner so you can talk about it with your wife
yeah.
I will cya guys later.
19:02
@0celo7 they're just messing with you at this point, are you sure it's not a prank from your collegues :P?
@AlessandroCodenotti the claim is actually so elementary that it's embarrassing that I'm having so much trouble with it
19:14
@AlessandroCodenotti the guy is working on the ball of radius 6...maybe I should go to the ball of radius 10 so I can work with all integers
That's why people usually say "pick a big enough ball" and skip the numbers
That's why people usually say "pick a big enough ball" and skip the numbers
Usually people don't say that twice though
Rekt
People with an awful internet connection do
Hi @Balarka
19:22
now he randomly wrote a $C^3$ estimate
even though everything is second order
I think this is an April Fool's paper
@BalarkaSen dude are your exams finally over
I need you to read some dumbass topology I wrote
im physically and mentally way too exhausted to look at math at this moment
19:23
Say I have a surface (smooth, in $\Bbb R^3$) that contains a line. I see that this line must be a geodesic. Must the gaussian curvature on points of the line be 0? I think so because the gauss map is constant in the direction of the line @Balarka
Both statements are false
Why do you think it is a geodesic
Hyperboloid of one sheet, a line in it's standard pencillization or whatever its called
Because the covariant derivative of the tangent field looks zero to me
One of those lines
Neither geodesic nor K = 0 along them
19:25
Weird, I asked this question like two weeks ago
What you need is the angle between the surface and some plane to be constant along the line
then K=0
Ahhh that was an exercise in Ted's book
Hmmm, I'm missing something, how can it not be a geodesic?
it's also a claim in a Yau paper, Balarka
But I doubt Yau read ted's book
19:27
Both are Chern students ;)
If $\gamma$ is a straight line don't I have $\gamma''=0$ and so the covariant derivative of $\gamma'$ is also zero?
Yau doesn't even read his own books
@AlessandroCodenotti $\gamma''=0$ in R^3, but not relative to the surface.
You need to project $\gamma'$ to the tangent plane of the surface and then take it's derivative as you move
That's nontrivial because of curvy stuff my man
@Semiclassical So I found the ultimate procrastination tool
Hmmm I'm confused
You shouldn't nest \text{} and dollar signs or so they say
19:36
who
are you talking about me
why are you hating
it works perfectly
I use \mathsf{Theorem}: instead of a proper theorem environment
does that trigger you
you're a maniac
A lot of stuff which people knowledgable about latex suggest avoid work perfectly in most cases
19:37
I nest \text and dollar signs lmao
my preamble could probably kill a small child because it's so messy
@Balarka I don't understand. Say I have a vector field $X$ along a curve $\gamma$ contained in a surface $S$, with $X(t)\in T_{\gamma(t)}S$ for all $t$. Then the covariant derivative of $X$ along $\gamma$ is the projection of $X'(t)$ on $T_{\gamma(t)}S$, right?
Yes
The projection in the case of a lines in the hyperboloid would not be zero. Think, eg, about what happens when you project the direction vector of the line at a point on the waist of the hyperboloid (where the tangent plane lies vertically)
For a geodesic in particular I use $X=\gamma'$
19:44
@0celo7 In my general quest to be the anti-0celo7
Isn't $gamma''$ zero before projecting? And so zero afterward as well?
Oh maybe you're right
Like that's it, after that it's just my name, \maketitle, and then it starts the document
I've just forgotten everything. All the more reason to not ask me math rn
On that note I should go to sleep and rest
According to MSE, the line is a geodesic
But that does not mean it's a line of curvature
That was a conjecture of mine, sad to see it's wrong
and then more
19:47
I'll summon @Ted
@0celo7 that's disappointing
Well, for a paper I wrote it was a bit longer
here it is for a note I wrote to my advisor yesterday
Good night @Balarka
19:50
You guys are aware that you can just put everything into a preamble.tex file and then include it in every document and forget about the horror you created?
wot
oh but I have different settings for notes, homeworks, and my thesis
keeping track of that would be hard
I use an online compiler and also my stuff is usually very short but I've heard of this
my thesis was supposed to be short
it didn't work out that way
Well, my stuff = my preamble
I even put chapter and stuff into different .tex files
too organised
It's literally the default plus like, at most 5 packages
19:53
My .tex documents are the only well organised thing in my life
My phone really likes sending messages twice tonight
@ÍgjøgnumMeg that's so algebraic it's not even funny
at least I have Hom though...I'm not a total n00b
@ÍgjøgnumMeg why \roi instead of \O?
then again I probably only use that for the Hom bundle...
@Alessandro I can't remember, I think I tried that and it didn't work for some reason
that, or I'm just a moron
@0celo7 I think there was a hom somewhere in there for a dual basis
but I only used it once so I didn't put it in the preamble

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