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00:09
My work to this point is as follows: If $x > 0$, then [a theorem from my book] tells us that

\begin{align*}
\int_\mathbb{R} F(x) \, d\mu_F(x) &= \int_\mathbb{R} \mu_F \bigl((0, x]\bigr) \, d\mu_F(x)\\
&= \int_{\mathbb{R}} \int_{\mathbb{R}} \boldsymbol \chi_{\{ y \in \mathbb{R} \colon y \in (0, x] \}} \, d\mu_F(y) \, d\mu_F(x)\\
&=
\end{align*}
So at this point I figure I'm supposed to apply Fubini's Theorem, or something like that.
And I guess I need to do the $x < 0$ case afterward as well. I got a reply to the question that I posted here but I'm struggling to make sense of it.
 
1 hour later…
01:36
I have confirmation from my professor on a couple of points now: 1) $F$ is in fact a CDF, not just a distribution function; 2) I'm supposed to apply Fubini here, which will let me evaluate an integral; 3) the other two parts are supposed to follow relatively quickly after completing this.
I am struggling to accept this inequality. Shouldn't the inequality sign be reversed?
If we think of $ab$ as the area of rectangle, then surely this area will always be greater than the sum of the two integrals
i can't read that pic too well, but try the picture in en.wikipedia.org/wiki/Young%27s_inequality_for_products
It's also called Young's inequality. :)
the key to the pic might be to not separately graph f and f^{-1} on the same axis
but just graph of f and think of one integral as 'area under the curve' and the other as 'area left of the curve'
this pic right?
but i don't see how the yellow area represents the area under the inverse function
01:50
@robjohn yes :). A comment to my post says otherwise however. :(
yeah, that one. i thought about pasting directly, but did not want to deprive you of the numerous benefits of even more exciting information about young's theorem
@robjohn a minor typo in the later half: 'when k is a perfect fourth power' should be treated as 'when n is a perfect fourth power'.
02:35
discovered I'm going to have to add a good book on Group theory and Halmos set theory to my ever growing "to do list".........every time I feel a step closer I get pulled back in chico.....😪
03:09
The book by Halmos that’s titled ‘Naive set theory’?
books are dangerous
@Koro ...Yes.
@copper.hat and if you read too many you'll go mad...........I could always wonder what it would've been like to remain ignorant.....but now I have the Moral obligation to try and be intelligent...
@dc3rd that's the problem, the more you read the more you realise just how little one knows. i still have my pauling's general chemistry taunting me in my dining room.
03:26
maaaannn....jesus it really becomes more daunting and daunting having that concept wander in your head. How do you manage it?, because I feel I need to "know it all" before I could pursue any future advanced work and knowing it all really is not viable...
04:08
Haven't you noticed that the world is full of people who think they know it all?
Especially in politics.
05:02
I apologize if that question was meant only for @copper.hat
there's always the dumming-beretta syndrome
what if you want leaves from the company you work at for a certain duration which will be say 1 month from now and the company promotes you and your new job location is far from the place you were posted at earlier, but you have a fear that at new location they won't allow you leaves for that certain duration?
in such a case, will you resign from the company or wait to see whether they will give you the leaves you want? The latter has almost zero chances .
@LearningCHelpMeV2 Because you're doing $\int_0^b x\,dy$ with $x=f^{-1}(y)$.
05:22
koro your job really sounds like it sucks right now. that's not an answer to your predicament, just a comment
Most people would ask directly before accepting the promotion.
leslie: no doubt that the company sucks
Ted: not-accepting the promotion is not an option they give.
it doesn't sound like you have much leverage here to do anything other than either exactly what they want, or leave
but there are a million variables i don't know
05:39
Are you being compensated as if it's a promotion? or is it a "promotion" in the sense of we give you a new "title", pile on more responsibilities, call it a "promotion", but pay you the same.
they pay well. They'll pay even more after this promotion and benefits have also increased.
well then it seems it's a matter of how much you value taking your leave and if it will be worth it in the long run for you personally. Beyond the financial side of things, but that should be considered as well.
koro: you could use the new money to pay for cool new clothes or a new car or something and introduce the world to 'new koro'
you'd have to change your chat icon too
I took one exam and based on its results now I'm able to get admission in some colleges (called IITs) across the country for masters program but there is one college that I have always desired to go to since my under-graduation days. And that college has entrance exam soon. I want leaves for that.
not getting leave in that period would mean staying in the company for another year (and being richer financially) and not taking admission this year.
kinda sounds like you know what you want to do :)
05:48
Well are you in a rush to go somewhere?....
cause you could do it next year with less worries financially...
@leslietownes but this seems the case too....
the company pays well. I'll say that. I want to join college now as the job in that company is not what I want to do. I have been in the company since I (under)graduated.
that is, for almost 3 years now. I'm already 25.
far away hills are greener
to be 25 again and know what I know....😥😥
:-) i suspect i would make the same mistakes
I had joined the company because of financial problems at that time.
05:55
oh, i can't give advice on this anymore, because i'm only 21. sucks to be old i guess
Or I'm just being paranoid that I won't get leave in that period
@leslietownes no, I don't think that. I mean I feel that now I should be at college because I have worked in the company enough and I mentioned the number because I targeted to resign from the company in one year of joining but that didn't happen. It's been 3 years since I joined the company and I wanted this year to be different. That's my bad and I'm referring to that.
i was trying to change the subject to a world where everyone acknowledges that i'm 21
it would help if someone could say something like: "leslie, it is entirely plausible that you are 21"
it doesn't have to be any specific person
@leslietownes How about the grand ol cliche "you're as old as you want to feel"?
so you are therefore 21 Q.E.D
Thank you all! Your advices/suggestion are always helpful to me. I'll consider each one of them before making the decision :).
dc3rd: they only say that to old people [cry emoji that i don't know how to input on this device]
😥😥 found it
06:05
hahaha
😭
this is true.... 😭😭😭
i'm not sure my device supports that much fluid
😢😢
this one is amazing 😌😌😌😌
💦🌊🚢
📵
06:19
@TedShifrin Which problem set is this question "D" on that you're talking about here? https://youtu.be/l8rqK4sJuP8?t=2249

I have all the p-sets so just need a number.
 
2 hours later…
08:44
@WilliamSun I feel like the one-point compactification of rationals is a good candidate, but I haven't proved it holds for this space yet.
Other examples I considered for which it doesn't hold:
1. Infinite set with cofinite topology
2. R with compact complement topology
The first one fails since we can subtract a point, that's compact but not closed. The second fails because as a subspace, [0, infinity) is compact but not closed
Equivalently your question asks for a non-Hausdorff compact space such that all compact subsets are closed
 
1 hour later…
10:09
Is U(n) properly contained in SO(2n)? n>1
10:43
are all unitary matrices diagonalizable??
Why not?
Not just diagonalizable, it's unitarily diagonalizable with all eigenvalues norm 1.
Someone drank 3 bottles of alcohol in the public lounge last night and didn't clean up. I saw vomitus in the public restroom on the same floor.
11:23
Let $X$ be the one point compactification of the rational numbers and $Y\subseteq X$ be compact. If $Y\subseteq \mathbb{Q}$ then $Y$ is closed by definition of $X$. Otherwise $\infty\in Y$. We'll prove that $Y_0 = Y\cap \mathbb{Q}$ is closed in $\mathbb{Q}$, so that $Y$ is closed in $X$. Since $Y$ is compact in $X$, for any compact $K$ in $\mathbb{Q}$, $Y\cap K$ is compact. Let $y_n\in Y_0$ be such that $y\in \mathbb{Q}$.
Then $K = \{y_n: n\in\mathbb{Q}\}\cup \{y\}$ is compact and so $Y_0\cap K $ must be compact, and from this it follows that $y\in Y_0$. So $Y_0$ is closed.
@WilliamSun This is a space which isn't Hausdorff such that a subset of it is compact iff it's closed
This seems to work for any metric space which isn't locally compact
11:45
0
A: Subspace closed iff compact implies Hausdorff?

JakobianHere's a counter-example. Let $X = \hat{\mathbb{Q}}$ be the one-point compactification of $\mathbb{Q}$. It's well-known that such compactification is Hausdorff iff the underlying space is locally compact and Hausdorff, so $X$ is not Hausdorff. If $Y\subseteq \mathbb{X}$ is closed then it's compac...

@AlessandroCodenotti is my proof correct?
12:03
@Jakobian Maybe a dumb question, but why isn't $\mathbb Q$ locally compact and Hausdorff?
It's Hausdorff but not locally compact
Oh yeah my bad
If it were locally compact at any point then you would obtain that set of the form $[a, b]\cap \mathbb{Q}$ is compact for some $a<b$, but this isn't closed in $\mathbb{R}$ so can't be compact
So rationals are in fact nowhere locally compact, which is an absolute failure of local compactness
seems fine to me
12:21
Thanks for checking it :)
13:11
@TedShifrin understood, thanks sir.
 
8 hours later…
21:03
Hello
Someone have an idea about this limit math.stackexchange.com/questions/4413414/…
you can handle the case n = 1 using math.stackexchange.com/questions/433135/…
if you write f_n(x) = (x/n)^(nx), then f_n(x) = [f_1(x/n)]^{n^2} is a polynomial in the case n = 1
even with polynomials, handling the derivative via the limit definition gets ugly if you don't want to break the problem into simpler pieces
Is anybody comfortable with the meaning of the sentence containing the words "Borel measurable" in this picture? It has to do with the construction of a general product measure on two spaces.
for fixed A_2, the function sending x_1 to mu_{x_1}(A_2) is a real valued function on X_1
so they probably want this function to be measurable as a map from (X_1, M_1, mu_1) to (R, Borel sigma-algebra, lebesgue measure)
i.e. "Borel" signals the sigma algebra they are using on the reals
21:20
I wrote the following question in my notes: "do I think of $\mu_{x_1}(A_2)$ as a function $\mu_{x_1}(A_2) \colon \mathbb{X}_1 \to [0, \infty]$ so I need this function to be ($\mathcal{M}_1, \mathscr{B}_{[0, \infty]})$- measurable? "
Sounds like we might have the same idea
Or a similar idea.
the notation kind of sucks here because they're not giving that function a name, and they're calling its variable "x_1"
Thanks. I have a specific implementation of this theorem and I'm trying to figure out why that sentence applies. I'll try to put up a picture of the exercise.
for fixed $A_2$, they're thinking of the function $f: X_1 \to \mathbb{R}$ given by $f(t) = \mu_t(A_2)$ and they want it to be measurable when you give the codomain the borel sigma-algebra
thinking of the codomain as R, or [0, infty), would make no difference for the resulting concept
does anyone know whether the set of arithmetic functions form an integrally closed domain?
First I thought obviously not, but now I'm thinking maybe
21:24
My $\mu_{x_1}$ is a gamma probability measure.
I can show more of my work but I'm trying to avoid cluttering things unnecessarily
this notation is really going to get confusing. "$\mu_{x_1}$" is not one measure but a family of measures indexed by $x_1$
at least in the context of trying to apply the theorem above
Right, the idea is that you have a measure space and a measurable space, and in order to construct the product measure we need a whole suite of measures $\mu_{x_1}$ on $\mathcal M_2$.
all i was trying to do is preserve the distinction between a single measure and a suite of them, if you've got that, then good
i do wonder if the subscripts are making this more confusing, but maybe that's between me and the author of the text
In the first part of the question I'm asked for a product measure $\mu$. This is my product measure:
\begin{align}
\mu(A) &= \int_{\mathbb{X}_1} \mu_{x_1}(A_{x_1}) \, d\mu_1(x_1) \notag \\
&= \int_{\mathbb{X}_1} \left(\int_{A_{x_1}} \frac{1}{\Gamma(2)x_1^2}x_2^{2 - 1} e^{-\frac{x_2}{x_1}} \, d\mu_1(x_2)\right) \, d\mu_1(x_1)
\end{align}
And to verify that this formula is valid, I am working my way through the assumptions in the first picture I posted and trying to explain why they hold here.
I think I've got everything down except arguing that $\mu_{x_1}(A_2)$ is Borel measurable... as the picture describes. Maybe it's obvious, but I'm not seeing it
i recommend decreasing the number of subscripts. what is the gamma distribution with k = 2, as a function of theta
what formula do you get when you measure a set subset A of [0,infty) with that measure (it will be some expression involving A and theta)
21:32
So unless I'm mistaken, I need the preimage of a measurable set from $\mathscr B_{[0, \infty]}$ to be in $\mathcal M_1$
you want that formula to be measurable as a function of theta
that at least reduces the amount of symbols involved. then the question might be, can i reduce the problem for arbitrary A to simpler A, etc.
Re: subscripts, I'm just trying to use the statement of the theorem as a template for how to solve this problem
"what formula do you get" Isn't that just the inner integral above? Or am I missing something?
It sort of seems like we're agreeing, although I don't really know what I'm talking about so I could be wrong
i was suggesting it might make sense to reduce the number of subscripts so you can see if the formulas make more sense
for example 1 is an element of X_1, so "mu_1" means two different things (it's the ambient measure on X_1, and it's the measure on X_2 in the family corresponding to the element 1 of X_1)
Oh, you mean replacing $x_1$ with $\theta$?
yes, plug things that don't have subscripts into the theorem, instead of trying to add subscripts to your problem so that your problem looks more like the theorem
i'm gonna be idle for a bit but i'll be back later
21:39
Thanks for your help
\begin{align*}
\mu_{\theta}(A_{\theta}) &= \int_{A_2} \frac{1}{\Gamma(2)\theta^2}x_2^{2 - 1} e^{-\frac{x_2}{\theta}} \, d\mu_1(x_2)
\end{align*}
Not sure there are any other things that make sense to replace.
That should be the inner integral from above
So... this is a probability measure defined on $\mathcal M_1$. I need the preimage of a measurable set to be in $\mathcal M_1$. Unless I'm confused
But it's weird because they define $\mathbb X_1 = [1, \infty)$, so is what I wrote above even a probability measure on that space? I.e. isn't the gamma distribution defined on the positive reals?
Wait. It's a probability measure defined on $\mathcal M_2$, I think
So I need the preimage under $\mu_{\theta}$ of a set from $\mathscr B_{[0, 1]}$ to be in $\mathcal M_2$. Maybe.
I think that's probably correct. So the question is... why is that true.
Maybe there is some result stating that probability measures are measurable functions. I will look on Google
I know that "measures are continuous" in some sense, and that continuous functions are measurable (given certain assumptions), but I would need to think about how to fill in those assumptions, if I wanted to use that route to argue that the probability measure is a measurable function.
I wonder if I need to come back to this later. There are just way too many moving pieces - it makes my head spin
So unless I'm mistaken, in the second part of the question I need to use my product measure (above) to compute the measure of the area where these two shaded regions overlap:
Hopefully this will just be a simple (Riemann) double integral, but I need to think about it

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