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00:22
I don't think I ever saw a theorem like this before
00:43
did any of people here saw a theorem like this before?
Looks like properness, but the statement has undefined notation.
You mean $\overline{B}_r^n$?
its just a closed ball of radius $r$ and center $0$
I bet that will imply properness, whence closed embedding.
yep. It does
Maybe its just that I haven't seen a whole lot of theorems involving proper maps, every time I've been amazed
have you been properly amazed
00:56
perfectly even
01:15
Yeah, proper is the automatic replacement for compact that makes everything groovy.
01:41
groovy?
@copper.hat Like, totally, man.
:-)
apparently the term 'ratioed' has appeared in Twitter, seems to mean negative commentary
Twitter, never a source of prime information, seems to have really gone down the tubes
In keeping with its owner ….
How’s recovery going?
@copper.hat Did they give you the Good Drugs™?
they gave me percocet, an opiod. i am nervous of drugs in general (alcohol is bad enough for me :-)), so i am very parsimonious. i used 4 tabs total.
can't say i noticed any modulation of pain not any mood changes.
not that there was much pain to be honest
01:50
@copper.hat Good plan.
My father, two weeks before he died, while he was in palliative care, refused morphine.
"I don't want to be a part of the opiod epidemic."
...jackass. :/
very principled of him, i admire that, i am sure it was frustrating for family
when i had an appendectomy i was offered (and refused) morphine.
my dr came by and said that he understood, but some might help me relax and heal quicker, so i relented and took a half dose.
for the next few hours i would drop off, have intensely vivid dreams, lurch awake sending my stomach into spasms. so that was the last time i took morphine.
part of me was hoping the percocet would bring back the dreams but no dice
After my major surgeries I was told to use some pain meds to help recovery. But I didn’t use that much.
During the last semester of my masters program, my ex-wife woke me up at about 2 in the morning, abut 8 hours before my very last final. Her gall bladder had... failed, and she was writhing around on the bathroom floor in tremendous pain. I took her to the hospital, they gave her morphine, and I got a C on that final, and a B+ in the class. It is the only non-A I got in the masters program.
But she was super happy with the drugs.
01:59
It’s not like she did it on purpose …
Gall bladder surgery can be dangerous.
@TedShifrin In the hospital after my surgery, they tried to give me morphine, but it gave me extreme nausea. I took Tylenol and was fine.
@TedShifrin I don't know... in retrospect, it might be the kind of thing she would do...
(Honestly, I don't actually care all that much about the grades---after two years as an undergraduate, my GPA was 1.95; I managed to finish a bachelors with a 3.5 overall average; and I was kind of over "classes" by the time I started my phd---I still managed a good GPA overall, but didn't give no sh*ts. It is just ever so slightly annoying to me that almost managed a 4.0 over two and a half years, only to not quite cross the line).
Anywho, I need to go make food.
Have the hot flashes decreased in severity and duration @robjohn
@user85795 No, and I have another dose of the medicine that causes them this week.
02:06
:(
@robjohn I don’t think I’ve had morphine, but definitely a bot of codeine.
Sorry to hear that, @robjohn. Thinking good thoughts for you.
@TedShifrin Codeine doesn't cause any problem, but morphine is a no-go for me.
@TedShifrin At least I only get these injections once every 3 months.
@TedShifrin thanks.
How many coV shots have you taken, if you don't mind me asking? @robjohn
I think I’m at 5, plus flu and RSV.
@user85795 I've had 3, but I'll probably get covid, flu, and RSV before going to TX for the eclipse in April.
02:19
This is the bad time of year now.
TX? Can't wait for the flicker pictures :-D
Some hospitals here have just restarted masking requirements.
Some places here never stopped.
I never stopped.
@robjohn Man, I'm trying to figure out how to swing a trip somewhere for the eclipse. I don't think I'm going to be able to manage it. :(
02:27
@XanderHenderson this will be the first total eclipse I've seen.
@robjohn Didn’t know that!
This eclipse may eclipse most others.
It is the last to be visible across the US for a while.
That would make it a total eclipse of the heart. @TedShifrin
02:46
Not for those of us who are heartless.
03:18
The heart of a heartless world.
03:46
Hard to beat!
 
3 hours later…
07:03
To beat or not to beat.
Is there some efficient way to count number of partitions of $k$ into $n$ parts where each part $\le n$ where $k >n$? More specifically I'm interested in $k=193$ and $n=60$.
07:17
see math.stackexchange.com/questions/3392798/… (relating partitions of A into B parts all C or less to partitions of X into at most Y parts all Z or less), math.stackexchange.com/questions/2237261/… (generating functions for the latter), en.wikipedia.org/wiki/Gaussian_binomial_coefficient ("balls into bins" interpretation of q-binomial coefficients)
none of that seems exactly on point but its all pretty close
there are some recursion relations satisfied by that as a function of k and n (for recognizing the recursion it maybe even helps to let the bound on each part's size vary separately, independently of n)
I'll work with that thanks👍
 
2 hours later…
09:41
if $G_1,G_2$ are open (bounded) simply connected disjoint neighbourhoods of $z_1,z_2 \in \mathbb{C}$, then is it necessarily the case that the sets $A(G_i - z_i) + \frac{1}{A} z_i ,i=1,2$ are also disjoint? Here $A$ is some non-zero complex number.
10:11
nvm, ignore my last question
 
5 hours later…
15:09
If $f:\Bbb R\to\Bbb R$ is continuous, then $f(a) = \lim_{k\to\infty}{f(b+ka)-f(b)\over k}$ for any $a,b$?
@onepotatotwopotato continuity is a local property
this limit depends on behaviour of $f$ at $\infty$
at $a$ you can pick any value
positive $a$ to make it simpler
thank you for the comment but please ignore. I found that $f$ is a norm, not just a continuous function.
I see
Mad
Mad
15:47
can someone help me understand how he gets the last inequality
https://math.stackexchange.com/a/3125952/695930
if i plug in his condition i am getting the $ 2 \lVert R(\lambda) \rVert * \sum_j \lVert R(\lambda) \rVert^{2j}$
16:20
Let $I=(-a,a)$ be an interval (finite or infinite). Suppose that $(K_n)_{n=1}^\infty$ is a sequence of real-valued, Riemann-integrable functions defined on $I$, with the following properties: 1) $K_n(s)\geq 0$, 2)$\int_{-a}^a K_n(s)ds=1$, and 3) if $\delta>0$, then $\lim\limits_{n\to\infty}\int_{\delta<|s|<a} K_n(s)ds=0.$
$(K_n)_{n=1}^\infty$ is called a positive summability kernel. I wonder the following; are these kind of kernels even functions? It looks to me as if $K_n(s)$ is a kernel, then $K_n(-s)$ is a kernel too, but I'm not sure if this defines it as an even function.
why the last equality hold?
could you take a look @Thorgott ? is it a consequence of some known fact?
@psie You mean a particular entry?
they don't have to be even from the definition
@onepotatotwopotato I am experiencing an absolute lack of context
ok. I meant probably a whole sequence, i.e. if $K_n(s)$ is a kernel for all $n$, then so is $K_n(-s)$ for all $n$, right? From the definition, I agree, they don't have to be even.
$M$ is a closed, connected, oriented 3-manifold (and irreducible). Nonsingular means nonvanishing.
16:32
what I meant to imply was that I have no clue what the notation is supposed to mean
$\chi$ is an euler characteristic and $\chi_{F}$ and $\chi_{F'}$ are euler classes of plane bundles $F$ and $F'$ on $M$
16:47
@psie whole sequence would be $(K_n)_n$
$K_n(s)$ is a particular entry of this sequence, at a particular value $s$
function $f$, function $f(x)$, the latter is abuse of notation
here we have two functions, $n\mapsto K_n$ and fixing $n$, $s\mapsto K_n(s)$
if your sequence $(L_n)_n$ is defined by $L_n(s) := K_n(-s)$, then yes, $(L_n)_n$ is a positive summability kernel according to definition above
ah, Poincaré duals of plane bundles, perhaps
then we're almost making sense
no clue what it means to evaluate those classes on $K^{\prime}$ tho
or what the fibration "associated to $q\omega^{\prime}$ is supposed to be in the first place
but whatever they may mean, $\chi_{F^{\prime}}(K^{\prime})=\chi_F(K^{\prime})$ should be immediate from $\chi_{F^{\prime}}=\chi_F$...
if not, the notation is repugnant
@Jakobian ok, the reason for the question is that, in Stein and Shakarchi, they claim that $$\lim_{n\to\infty}\int_{-\pi}^\pi K_n(s)f(s_0-s)ds=f(s_0),$$ for $f$ being continuous at $s_0$. In my textbook, they claim that $$\lim_{n\to\infty}\int_{-a}^a K_n(s)f(s)ds=f(0),$$ if $f$ is continuous at $0$.
I'm not sure how I obtain the former limit from the latter, or vice versa? Making the change of variables $u=-s$ in the former integral, I get $$\int_{-\pi}^\pi K_n(-u)f(s_0+u)du.$$ Setting $s_0=0$, we should obtain the latter limit of the integral provided the entry $K_n(-u)$ is a kernel.
the main question is why it's equal to $\chi(K')$ and the proof doesn't explain anything about it (no lemmas or argument given before)
17:12
@psie No, that's the wrong approach
You don't substitute into an integral, instead you consider a new function $g(s) = f(s_0-s)$ if anything
The difference being in the integral limits ($a$ vs. $\pi$
ok
as you see, its more about going for what makes sense than trying out everything
17:38
53
A: What are the open subsets of $\mathbb{R}^n$ that are diffeomorphic to $\mathbb{R}^n$

Fallen ApartI sent an e-mail to Erwann Aubry (see Georges Elencwajg's answer) and asked for a scan of this proof from the book "Calcul Différentiel". He replied and send me the proof translated into English. Here is his original paper. And this is how the proof goes: Theorem. Every open star-shaped set $...

Non-empty open star-shaped subset is diffeomorphic to $\mathbb{R}^n$
Mad
Mad
17:52
https://chat.stackexchange.com/transcript/message/64949278#64949278
anyone?
it looks like the geometric series.
 
2 hours later…
20:08
@Jakobian Yes, this is a famously challenging "obvious" result. The issue is that distance to the boundary may not even be continuous.
@TedShifrin can you give an example of that?
Take a disk centered at the origin and remove the (relatively closed) outer portion of one ray. So the distance goes from $1$ for all other radial directions to, say, $1/2$ for that particular one.
is this usual euclidean distance, or are we looking at distances in a given direction?
Usual Euclidean distance from the origin to the boundary as a function of direction from the star point.
If you try to write down a homeomorphism to the disk you naturally want to work with this function ... Or so it seems.
I haven't thought about this in ages, but I remember coming upon something like the proof Jakobian linked years ago.
Okay, I am thinking of something different; the distance function to each $e\in E$ is a continuous function and the distance to $E$ is often considered to be $\inf\limits_{e\in E} d(x,e)$
and the infimum of continuous functions is continuous. But that is not what you are thinking about.
perhaps that is not correct...
Okay, the infimum of infinitely many continuous functions is not necessarily continuous.
Sorry, I'm in the middle of cleaning today. Not thinking clearly.
20:27
Something like Minkowski's functional rather than distance function
20:55
@TedShifrin This is a problem which is near and dear to my heart. What do the neighborhoods of the boundary look like? How messed up can they be? How does that change diffusion on the set near the boundary? etc.
It doesn’t have to be a set of positive reach.
21:20
$$\sqrt[\large2]{2\sqrt[\large3]{3\sqrt[\large4]{4\ldots \sqrt[\large n]{n}}}}\lt\left(\frac{e-1}{e-2}\right)^{e-2}\lt2$$
@robjohn What is the limit?
$n^{1/n!}$ appears to appear.
21:37
What is $\sum (\log n)/n!$?
@TedShifrin About "3".
Do we even know what $\sum (\log n)/n^2$ is?
@TedShifrin That one is also about "3".
What is “3”
One more than "2".
But with bigger error bars.
21:43
You mean "1" more?
Is it normal to feel that all the people in your life are insane
Um, no.
@TedShifrin Nope. One more.
It suggests that perhaps you're the insane one.
2
Am I?
21:44
@Jakobian Probably.
And both Ted and I are doctors, so we probably know what we are talking about.
We've had plenty of insane students ... well, some, anyhow.
I'm trying to plot the function $f(t)=t^4-2\pi t^2, \ |t|<\pi, f(t+2\pi)=f(t)$ in python, to verify to myself it is continuous at $t=\pi$. The reason; I'm trying to compute $\zeta(4)$ and for this you require continuity at $t=\pi$, for the Fourier series to equal the function. I get a pretty continuous graph, but I'm wondering, why did they specify it like this...$|t|<\pi$? Why not $|t|\leq\pi$?
Actually, @Xander, the $/n^2$ sum is already less than $1$ (about $.938$).
I think I've shown on multiple occasions that I'm not unreasonable
@TedShifrin That's about "3".
Just don't forget the error bars.
21:46
And the $/n!$ sum is about $.604$.
Surely those are not known in closed form? We do have people on this site who know such things.
@TedShifrin Yup. Also about "3".
@TedShifrin I would be surprised if either had a nice closed form.
And $e^2$ is also "about '3'"?
@TedShifrin Yup.
By induction, all positive real numbers are "about '3'" :)
Honestly, I'm not sure if I can think of a number which isn't about "3". Maybe TREE(3)?
21:47
Maybe for you $0$ is also about "3." shrug
I think Jakobian may be right, then, to think you're insane.
My brother thinks that the police is following him, my mom is an alcoholic. I know I have my own issues but they're not on that level
"Three" is my goto nonsense answer.
I can't think of any reason to assume off the bat that either $\sum \log(n)/n^2$ or $\sum \log(n)/n!$ would have a nice analytic closed form. I'd be pretty surprised if either did.
My sympathies, @Jakobian. That is no fun, although I don't know that those render them insane.
Although there is probably some gnarly expression involving hypergeometric functions or some such silliness.
21:50
@Xander Where is @chris'ssis when we need him/her/them?
@TedShifrin Insane was a hyperbole, it'd be better to say unreasonable.
Thank you for sympathizing, I was just wondering if its normal to have things like this in your family
@Jakobian Tolstoy had something or another to say on the topic.
You'd be surprised at the craziness/f****-upedness in many families. Dealing with my mom's Alzheimers and the stress that put me under was no walk on the beach. That was a 15-year ordeal or so.
@TedShifrin Ugh. That sucks.
@XanderHenderson any particular books you'd recommend?
21:56
For all that I dislike Tolstoy, the opening line of Anna Karenina is one of the best in literature.
This book is huge
Все счастливые семьи похожи друг на друга, каждая несчастливая семья несчастлива по-своему.
@Jakobian Not compared to some of his other works.
It is just a wee little novel.
If it were a math book it'd be a lot
@TedShifrin I am not sure the limit has a nice closed form. The log is computable as $\sum\limits_{n=2}^\infty\frac{\log(n)}{n!}$. This came from extending a duplicate question that I saw yesterday. Following up on Approach0, the best upper bound is $2$.
I just posted the numerical value from Mathematica above.
22:01
The only book I seriously read was "The Stanger" by Albert Camus
@Jakobian I mean, that is a good book, but you haven't read any others?
I've tried to, but reading a book is very painful for me
non-mathematical book
I read that book in high school (in French).
@TedShifrin My mother had the same experience. I, unfortunately, have only read it in English.
@TedShifrin $1.8290246795635718644$ is the approximate limit of $\sqrt[\large2]{2\sqrt[\large3]{3\sqrt[\large4]{4\ldots \sqrt[\large n]{n}}}}$.
22:05
I was very much into Existentialism in high school. I read all the Camus and Sartre I could.
I'm not too sure why, but I can't focus at all when reading a book, I continue to re-read the same page over and over, only beginning to understand something after a while
I've read Война и Мир in Russian, however.
@TedShifrin Hah! Me too.
@robjohn I assume our two numerical results are consistent :)
22:07
Yes, the log is $0.60378286279$
It just shows to go: Almost any integral/series we write down cannot be evaluated other than numerically.
@TedShifrin It was really disappointing when I figured that out.
First, you tell me that I can't divide by zero, which is fine. Then you tell me that $i$ is a thing... okay. But then you tell me that you can't actually integrate or sum anything in a nice way? *flips desk*
That's why I became a geometer :)
Ha!
I embraced the suck, and became a fractal geometer, doing analysis on fractals.
Just out of spite, perhaps.

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