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1:34 AM
I think I don't get what P(W<=w) means.
And it also doesn't makes sense what that cdf means.
I get the P(W<=w) means but I don't get what P(W>w) means
 
1:52 AM
I think I am confused about that CDF means here for w>=0.
looks like geometric distribution
;_;
and it keeps saying Poisson distribution count no of occurrences I don't see that I see it counts probability or is it talking about that n choose k stuff where we do count them then multiply with their probability
i think it is talking about the input of that pmf
and is $P(no occurrences in [0, w] )=e^{−λw}$ from Poisson I get it that you plug 0 into Poisson but I don't get why take exponent to $w$.
 
 
2 hours later…
3:57 AM
$P[W \le w]$ is the probability that $W$ is $\le w$.
 
4:27 AM
It's surprising that derivative is a covering map between germ spaces
 
 
4 hours later…
8:37 AM
Is there any good way to understand the book, "deep learning" by Ian Goodfellow ? The book is quite math heavy, and I don't understand several things. Like how one equation leads to other. Is there a guide book of sorts to help me?
Or is there any other book which can hold my hand while going through math?
Any math book I could read first which will help me through this?
 
 
4 hours later…
12:42 PM
math.stackexchange.com/a/1999735/668308 How does the reduced l.e.s. of pair $(X,A)$ ensure $\varphi(X) = \varphi(A)+\varphi(X/A)$?
 
1:31 PM
Are there any books on CG one could prescribe me?
Co-ordinate Geometry*
 
1:43 PM
what I don't get is why is $e^{-t\lambda}=P(T>t)$ and not $P(T\let)$
I can only interpret it as 0 success until t but I don't see why it is same as the probability of waiting time until the first success greater than t time unit...
I see it as $P(T=t)$
 
@NotTfue $P(T\gt t)$ is the probability that there are no occurrences in $[0,t]$. That is $\frac{(\lambda t)^0}{0!}e^{-\lambda t}$.
 
I see what you did there :)
Thanks
 
2:17 PM
I would word it as $P[T \ge t+1]$
 
@onepotatotwopotato $H_n(X/A)\cong\tilde{H}_n(X,A)$
 
@Thorgott I know what you're trying to say. But the additivity of rank is for s.e.s. not l.e.s.
 
One more question I don't get how you get this formula for probability until k th success.
I tried thinking from 2 success but I don't seem to grasp the idea by just looking the formula.
 
I like the linear algebra proof of the algebraic numbers being a field
 
2:38 PM
@NotTfue How is that consistent with your formula above $P[T>t] = e^{- \lambda t}$?
 
@onepotatotwopotato the alternating sum of ranks in any exact sequence starting and ending with $0$ is $0$
the slightly more general fact is that if you have a finite complex $C_{\bullet}$ of f.g. abelian groups (or f.g. vector spaces over some field), then $\sum_i(-1)^i\mathrm{rk}(C_i)=\sum_i(-1)^i\mathrm{rk}H^i(C_{\bullet})$
it's a slightly more elaborate version of rank-nullity
hi @Ted
 
9
Q: closed unit ball in a Banach space is closed in the weak topology

user112564Let $V$ be a Banach space. Show that the closed unit ball in $V$ is also closed in the weak topology. I know this is a consequence of the statement any closed convex subset in $V$ is closed in the weak topology, which the proof used the geometric Hahn-Banach theorem. My question is: does this ...

 
Hi @Thor
 
In this answer the person uses sequences to show the ball is weakly closed, but one should use nets. I'm surprised no one has catched the error
 
2:56 PM
Hi professor Ted, I watched implicit function theorem/inverse function theorem videos completely today. After watching the videos, I was able to solve a question on implicit function theorem asked in one of my exams. In particular, I also like the analogy with linear algebra suggested in the video (for example, in implicit function theorem video: D_2 F (video uses different notation for this) is surjective means that we can solve for the corresponding pivots in terms of free variables).
Thanks a lot for the videos :).
Now, I'm trying to understand the manifolds.
some lectures seem to be repeating though (3500 and 3510).
 
Small errors are fine
 
Not when they completely change the conclusion
 
I didn't mean errors. It seems that 3500 was covered in an earlier semester and 3510 was covered after that semester.
 
@Jakobian *caught
 
@Thorgott and it seems it follows from s.e.s. version
 
3:10 PM
Here || || represent L2 norm, how does 2.56 follow from 2.55?
 
What's surprising is that even on sites like mathonline.wikidot.com/goldstine-s-theorem they use the same argument
With the same error...
 
Math online’s article on cantors diagonal argument also has an error.
 
Okay, that's not the only error on that page, it's even worse
 
Once someone performed a diagonalization by sending each digit to n+1 and 9 to 0, so wrong!
and unfortunately this argument is quite common
that people don’t see the blunder
@Jakobian
It’s so shocking
 
3:33 PM
well sure, there could be problem with non-uniqueness of decimal expansion that they don't mention
 
Yes correct!
Another error people think that the cantors proof of the existence of transcendental numbers is non constaructive
of course not !
 
3:49 PM
@user541396 Do you know what the $2$-norm of a vector is?
 
4:11 PM
@TedShifrin It is the square root of sum of squares of each component of a vector.
$$ \sqrt((a^2 + b^ 2)) $$
 
@geocalc33 that would not be an embedding. Also I thought the domain was the open 2-ball?
 
4:53 PM
@user541396 $\|x\|^2_2 = x^T x$.
 
5:05 PM
@user541396 so (the square root of) the dot product of the vector with itself.
 
@TedShifrin that make sense, sum of squares of each component of a vector is equal to the dot product.
 
@Alessandro Well, now it looks like your country is joining the far-right lunacy. 🤦
 
5:33 PM
I had someone downvote and then ask about the thing they didn't understand. I answered their query, but there is no way for them to retract their downvote after 5 minutes, unless the post is edited. I am glad they explained the downvote, but a downvote for something one doesn't understand is a bit confounding.
 
5:59 PM
Unfortunately it does look like it
Maybe I'll stay away for a few more years
 
6:13 PM
@Alessandro Democracy in the US is hanging by a thread. Crazy times.
@robjohn I got a downvote on an old post for no good reason. I can only think that's it's someone who's going back 5+ years and punishing answers to what they perceive are PSQs.
Oh yeah, and I got a downvote on this new one. Probably because it has answers in 10 different places.
 
@TedShifrin the downvoter edited some of my explanation in my comment into the answer, and then removed the downvote and upvoted. If they had simply asked and not downvoted, it would not have been
 
I downvote only rarely, and after the OP has not been responsive to my comments/suggestions/important corrections.
@User1865345 Indeed.
 
@TedShifrin hope for the best in midterms.
 
@TedShifrin That is what I see as a more responsible way to act, but it is not the standard.
 
Very little about me is "the standard," @robjohn :D
 
6:36 PM
i downvote old posts from people who disagree with me on this chat. mostly ted.
 
@TedShifrin this is extremely distressing. There seems to be an amnesia of 75 years ago.
 
We know, leslie.
 
@leslietownes I know how you feel, all of my downvotes are against Ted's posts.
 
I feel so popular.
 
I speak in vacuous truths.
 
6:40 PM
How is that different from vacuous untruths?
 
not much; takes fewer letters.
 
I'll keep that in mind.
 
"none" has more letters than "all"
 
In the anti-universe?
Oh, you edited.
 
I did, sorry
 
6:46 PM
Leslie was witness to my exchanges with a person who shall go nameless who is apparently taking an abstract measure-theory based probability course without having the least feeling for or knowledge of basic discrete probability and what things mean. Such courses truly distress me without bound.
Sorta like UC's honors sophomore math which leaves it to the students to learn multivariable calculus and its computations and subtleties on their own and instead does math in Banach spaces and functional analysis. shakes head
 
How can one understand anything about the course without a basic understanding of discrete probability?
 
you just symbol push and say 'sigma algebra' from time to time
 
Understand? Is that an Orwellian term?
 
yeah good point
 
symbol pusher: Hey little girl, wanna free variable?
we'll talk about bound expressions tomorrow...
 
6:50 PM
I think a lot of schools have their prerequisites and course syllabi totally out of whack ... and uncaring faculty don't help.
 
Seems like reminiscent of New Maths Movement...
 
Let's do Rudin-abstract courses for people who can't do the most elementary of proofs.
I did a huge amount of the math major advising for many years at UGA, but I was astounded by the faculty who would not discourage a student who was struggling/failing from signing up for 3 or 4 major-level courses — despite the guaranteed failure — because the student insisted they needed that to graduate.
Yeah, failing another 3-4 courses will help you graduate on time.
I was told that it was unprofessional of me to take into account the person(s) who happened to be teaching courses, too, when advising. Au contraire — I think that was ultimately professional.
/end rant
 
In fact, that's the state of many places. That happens on a regular basis. You feel powerless. You can't change the system. It happens right before your eyes.
 
Is the "you" in your sentences the student or the faculty?
 
ted: 3-4 at the same time? yikes.
 
6:57 PM
I feel sorry both for the students and the faculty. The system didn't change for long.
 
the responsible thing would be to discourage even many non-struggling students from trying that.
 
@leslietownes Yes. I advised responsibly and tried to push hard on the best students but give something manageable to the struggling ones. I occasionally took over advising them completely if they were going to listen to me.
@leslie Yes, of course. Depends on the courses, of course. We had some fluffy courses and some fluffier teachers/graders.
 
when i was in undergrad they would generally warn students about taking more than two, and only back off (or encourage it) if you already had track record of doing OK with it. it seems almost sadistic to do that with people who are struggling to graduate.
 
My point was that often the students do it to themselves and won't listen to good advice. There were a few occasions where I refused to sign off on a student's ridiculous proposal and said they'd have to find someone else.
But there are also faculty who just don't really care and want to spend as little time as possible (both on teaching and on advising).
 
@leslietownes there were many accounts that we heard; that could have ended in a good note but didn't happen. Point is the system prevailing is not robust. How it handles seems to be highly variable and not in a meaningful way.
 
7:07 PM
yeah, a lot of it was at the level of recommendations that a student could ignore. some advisors (for declared majors, people who hadn't declared didn't have this level of gatekeeping) did refuse to sign off on stuff. i don't know what happened then. maybe you get a different advisor.
 
Btw, does Leopold Vietoris still hold the record of writing a paper at the most advanced age? Someone needs to break this record. We need to keep our mathematicians healthy and fit.
 
How old?
 
Wikipedia says: Vietoris remained scientifically active in his later years, even writing one paper on trigonometric sums at the age of 103.
 
Hmm, that's pretty old. Chern stayed active pretty much until his death, but that was a far cry from 103.
 
Someone should have interviewed him in 60 minutes. We needed to know his mystery.
 
7:12 PM
ehh, he wrote it at 95 and kept it in his desk. #vietoristruth
 
@TedShifrin He went for another seven years. Austria's oldest man, maybe still now.
Anyone running for Congress and pledging anything to safeguard our mathematicians and making them work till their 100th birthday would get a vote of mine.
 
I think most math papers probably aren't worth publishing. Let the people with something meaningful and impactful to say keep going, and the rest of us can get old quietly.
I would rather have Congress put a statute of limitations on their own membership and change the rules for the Supreme Court.
I know you're joking, but Congress has no business meddling in this.
 
@leslietownes might be. We need to investigate this angle.
 
Speaking of trig sums, there's some cute things going on in this problem. physics.stackexchange.com/q/729352/123208 It's easy enough to solve using tangents, but I suspect there's an elegant synthetic geometry solution.
FWIW, that sort of construction arises in sundial design, where you're projecting equal angles in the equatorial plane to the corresponding angles to some other plane, eg the local horizontal plane.
 
7:33 PM
Is the symbol for isomorphism, $\cong$, universal?
Or maybe, widely accepted is a better way to put it. Like if I used this symbol with a random mathematician on the street, they'd know it?
 
it's very common, but the expected follow-up question would likely be, isomorphism in what sense?
 
Oh boy. Well, I currently only know it in relation to rings
 
80
Q: Difference between "≈", "≃", and "≅"

GOTO 0In mathematical notation, what are the usage differences between the various approximately-equal signs "≈", "≃", and "≅"? The Unicode standard lists all of them inside the Mathematical Operators Block. ≈ : ALMOST EQUAL TO (U+2248) ≃ : ASYMPTOTICALLY EQUAL TO (U+2243) ≅ : APPROXIMATELY EQUAL TO...

 
@UnderMathUate almost surely.
 
@PM2Ring Thanks. I'll take a look at this.
@User1865345 Ok, cool. That seems to be the consensus so far lol
 
7:40 PM
"Are A & B isomorphic?"
"A is, but B isn't."
 
Isomorphic to each other...?
 
:)
 
An isomorphic version of that joke, for non-mathematicians:
"Oh look! Twins! How lovely! Are they identical?"
"John is, but Mary isn't."
 
My goal is now to understand this joke by the end of this semester 😅
 
7:52 PM
sounds like you do understand it
 
Oooh, I wasn't sure if there something more to it than the fact that the statement doesn't make any sense based on what an isomorphism actually is.
 
yes, it's just confusion of a word describing a relation on pairs of things with an adjective describing a property of a single thing
 
Dang, I ruined the joke
 
a galaxy brain non-joke lurking in the background is that it's generally not that informative to focus on whether things are "isomorphic", but instead it is better to think of maps between things as either being or not being isomorphisms
but now we've exited the realm of joke entirely
 
That sounds interesting, though. If you feel like ELI'm-an-undergraduate-with-only-one-day-of-experience-with-isomorphisms, then I'm wondering why that's the case.
 
8:03 PM
@robjohn “-30 User was removed.”
 
@leslietownes I disagree
both are equally important
 
would your opinion change if i limited it to the first 1-2 years in which a student meets the concept
 
No, I think we have two (perhaps more) outlooks on things, we sometimes look at maps, sometimes at object themselves
 
under: the very low level vibe here is, if you're in an intro class and asked to prove that things "are isomorphic" it is likely that you will only be able to do so by constructing a map between them and proving it is an isomorphism. if you try to center one of the two things and just ask what it "is" (e.g. in terms of some grand classification theorem which you probably don't have yet) you won't get very far
thorgott and i just disagree about this (i do limit my statement as indicated above)
 
@leslietownes Oh, ok. Well, that makes sense for the most part then.
 
8:24 PM
why am I dragged into this
 
oh, jakobian
i think of you as the same person
hahahahaha that's really funny i have no idea why i did that
or will continue to do that
 
Both Germanic names?
 
i think that's it
that's really the only thing
i'm going to be a lot of fun in my 70s if this is how i am now
 
So Thorgott is (in some Germanic sense) isomorphic to Jakobian. ;)
 
8:47 PM
hey, I'm no functional analyst
 
it is your destiny
 
9:05 PM
@anak yes it is, I typed that incorrectly. I don't quite understand how the 3-vector field in the ball, is pulled back along the embedding to get the 2-vector field on the disk.
 
Hi! Does it holds ${\rm rank}\, H_{f}(b)={\rm rank}\, H_{f\circ g}(a)$, where $H_{\cdot}(\cdot)$ is the Hessian matrix and $g(a)=b$ and $b$ and critical point of $f$?
If $b$ is a critical point of $f$, then $\nabla f(b)=0$, then $H_{f}(b)=J(\nabla f (b))=0$, then ${\rm rank}\, H_{f}(b)\leqslant 0$.
 
@user1027216 You need $g$ to be a diffeomorphism (i.e., rank $Dg(a)=n$). Where are you getting rank $\le 0$? Of course not.
 
Sorry, I didn't write the hypothesis: $U,V\subset \mathbb{R}^{n}$ open set, $f: V\to \mathbb{R}$, $f\in C^{2}$, $g: U \to V$ a diffeomorphism with $g\in C^{2}$.
 
OK, and where are you getting your rank assertion?
The whole point is that the signature of the Hessian tells you what sort of critical point you have. If $H$ is positive definite, then you know the point is a local minimum; and so on.
@leslietownes 70s? What about your 50s?
 
at some point i'll just have munchkin take over my internet presence. nobody will notice the difference personality wise and her memory will be better
 
9:20 PM
Her math might be a bit lacking.
 
she can count up to the mid-twenties now. at this rate...
 
She can take over arithmetic for your investment portfolio. It is quickly zooming to 0.
 
9:39 PM
But, if $\nabla f(b)=0$, then $\det \, \frac{\partial^{2}f}{\partial x_{i}\partial x_{j}}(b)=0$.
 
> I woke up this morning to a news alert that our @GirlsWhoCode middle-grade book series was banned by some school districts as part of the Mom for Liberty effort to ban books. To be honest, I am so angry I cannot breathe.
FWIW, GirlsWhoCode is one of the charitable organizations that Stack Overflow donates to, on behalf of the community moderators.
 
10:07 PM
27 mins ago, by user1027216
But, if $\nabla f(b)=0$, then $\det \, \frac{\partial^{2}f}{\partial x_{i}\partial x_{j}}(b)=0$.
Moreover $H_{f}(b)=\mathbf{0}_{n\times n}$ then ${\rm rank} \, H_{f}(b)=0$. What is the wrong here?
Of course if $\det H =0$ it does not implies the matrix is the null-matrix. But in this case the matrix (I think) is the null-matrix, then rank is zero(?) I'm a bit confused here.
Maybe my mistake is just in the part where I'm concluding that the matrix is bound to be the null matrix.
 
@user1027216 Very wrong.
Try the simplest example. $f(x) = \sum x_i^2$.
 
10:22 PM
yes, I agree, I am very wrong :-(
 
Examples are important.
 
The problem is that mentally once I calculated the first derivative, I evaluated at the critical point which is not right, as it must be when calculating the second derivative and there the example you provided can happen. That is: there is nothing that forces the matrix to be the null matrix.
 
@copper.hat sorry above was exponential but I am deriving gamma one
 
That is the wrong: $$\left(H_{f}(b)\right)_{i,j}=\left( \frac{\partial^{2}f}{\partial x_{i}\partial x_{j}}(b)\right)_{i,j}=\left(\frac{\partial }{\partial x_{i}}\underbrace{\left(\frac{\partial f}{\partial x_{j}}(b)\right)}_{=0} \right)_{i,j}=0 \implies H_{f}(b)=\mathbf{0}_{n\times n}$$
 
If I can understand the expansion for just k=1 and k=2 may be I will understand the whole gamma distribution because I just need to take derivative of cdf
 
10:28 PM
That is a mistake students make day 1 of calculus. All derivatives are always 0.
 
Ok, the correct is: if $\nabla f(b)=0$, then $\det H_{f}(b)=0$ or $\det H_{f}(b)\not=0$. For now, that is all that can be said.
 
probability of anything happening is 50%
 
@user1027216 Saying that $Q$ or $\lnot Q$ is not really saying much...
 
haha but it is valid reasoning :-)
It doesn't add much, but it works, unlike the previous one.
So, it seems to me that it is now time to analyse $H_{f\circ g}(b)$
 
10:50 PM
@TedShifrin it's not German, it's Polish word for Jacobian
 
Which might happen to be the same as the German word …
My one PhD student was Polish :)
@user1027216 It adds literally nothing.
 
@TedShifrin I know, I understand. I'll retire for now to continue thinking and I'll come back later. Thank you very much.
 
11:19 PM
@TedShifrin either all derivative is zero or it a mistake made by student which is it?
@leslietownes May be we are living in different universe :)
 
either we are (50%) or we aren't (50%)
 
@leslietownes This is a good chance for me to make money from you >:)
Sht I see it is poisson my bad
 
11:49 PM
@NotTfue Yes.
 

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