« first day (1480 days earlier)      last day (3550 days later) » 

2:00 PM
between different areas of math
 
@skullpatrol where did i ever said that there were no interconnections?
 
17 mins ago, by Balarka Sen
these two are the least connected branch among all of mathematics.
 
i said least connected.
you misunderstood me
 
who is to judge what is the "least"?
 
the ones who studied both branches a little bit.
but you are now drifting away from the original point you made
=P
 
Huy
2:04 PM
@BalarkaSen: That is a misconception. You would need to study at least all branches a little bit to make such a statement - not just those two.
 
@Huy my statement was about number theory and integral-series. where are all the branches coming from?
 
Hmm ...
 
Huy
@BalarkaSen: Your statement was that they were the two least connected branches among all. So you need to know all other branches as well.
 
@BalarkaSen the person who enjoys both, should make that decision
 
@skullpatrol that's a fair point
 
2:06 PM
not others
 
@Huy that is a fair point too.
 
Huy
It's like saying "dogs and cats are the most violent animals among all - and I know so because I studied both" whilst not having studied crocodiles and other animals.
 
ok, i give up. let's not dogpile me anymore =p
 
Huy
I WIN!
flex
 
@BalarkaSen you make a fair point also
 
2:09 PM
about what?
 
@N3buchadnezzar pirate eyes
ooh
 
the connectedness of different areas
 
@Khallil: Very clever of you :)
hi @skull
@Huy: In response to your question, you need to make them readable and usable by people other than yourself in classes. I've had to be sure to add more examples (which I might myself do in class separate from the text/notes) and clear explanations. And, to me, the most important part of any math text is the exercises. That's what I take the greatest pride in.
hi @Balarka, @DanielF
 
2:23 PM
hi
 
Hi @Ted. How many of your students didn't survive the first week?
 
Huy
@DanielFischer: I can't find anything useful about the more general version of Riesz' representation theorem. In a proof, it is being used for $L^p$ spaces where $p \neq 2$ but it is stated that it does not work for $p = \infty$. Why is that? What conditions are there for the more general theorem?
 
LOL ... Both classes put together, about a dozen. A number of the business majors bailed out of my probability. I think they were given bad advice by the actuarial guy: Because the math stat course had been full for months, they decided that people wanting the actuarial certificate (indeed, preparing for the first exam) could take probability instead. I think that's a poor decision, as the statistics is crucial ... and the probability class in the math department is much more mature/demanding.
 
Huy
@TedShifrin: I would take probability over statistics any time. Why is the former supposed to be more demanding? Or did I misunderstand?
 
@Huy Which do you mean? The $(L^p)^\ast \cong L^q$ for conjugate exponents $p,q$? That works for $p < \infty$, though it might be that for $p = 1$ you need a $\sigma$-finite measure space, I don't remember exactly.
 
Huy
2:27 PM
@DanielFischer: The one I know only works for Hilbert spaces, and only $p=2$ yields a Hilbert space, no? So how can I extend it to $p \neq 2$?
@DanielFischer: What inner product would I even use to formulate the theorem?
 
But you're a math guy, @Huy. These are business majors wanting to be actuaries ... for that statistics is essential, along with the rudiments of probability. But, yes, we teach much more challenging classes in the math department than the stat folks do.
You don't need an inner product to have a reflexive Banach space, @Huy :D
 
Huy
@TedShifrin: I only know Riesz' representation theorem with an inner product. I don't know how it would work on a Banach space?
 
@Huy It's theorem 6.16 in Rudin's Real and Complex Analysis. Every book on measure theory should have it too.
 
It's in Folland, Stein, and all the standard integration books ... And, for sure, in Dieudonné's infinitely-many volume treatise on analyse.
 
Huy
@DanielFischer: Thank you. So it is what under wikipedia is listed as Riesz–Markov–Kakutani representation theorem I think.
 
2:32 PM
Possible.
 
@DanielF: When I'm deciding what small number of books to keep when I retire, I don't think Rudin's Real & Complex will be amongst them.
 
@TedShifrin I know. You insist on pictures.
 
Huy
@TedShifrin: Why is that?
 
LOL ... well, not in graduate real analysis ... :)
I can't keep everything, @Huy. I don't have room in my house for many more books, and I'll be downsizing to a much smaller apartment.
 
Ted doesn't like Rudin's style, @Huy.
 
2:34 PM
And, yes, that's true.
 
Huy
@TedShifrin: I meant why you won't keep Rudin's book.
 
Because I have to pick books I really love and know I want to be able to look at.
 
Huy
@TedShifrin: What would be some books you already know for sure you will keep?
 
Most won't be standard introductory textbooks.
 
Huy
@TedShifrin: I'd still love to know some examples.
 
2:36 PM
I would be happy to take the ones you don't want :D
 
Definitely Griffiths and Harris Algebraic Geometry, the collected works of Chern and Griffiths (but not of Cartan or Weil ... much as I've loved having them on my shelves) ... a few advanced diff geo texts. I'll certainly keep Artin and Dummit and Foote.
@skull: It's first come first serve. @Mike wants dibs on some ... I don't know if he expects me to schlep them to shipping.
 
how big is your collection?
 
Huy
@TedShifrin: Do you have any advice on how to think about reflexive spaces? The definition is so abstract to me and I can't really imagine anything if I'm given a reflexive space.
 
I haven't counted. Not including calculus, linear alg, etc., textbooks, probably somewhere around 500.
 
:-O
 
2:39 PM
@Huy: No, I'm no analyst and I haven't thought about this stuff in years. I always defer to @DanielF on such matters.
 
I really like this quotation about General Relativity when Einstein first published it.
 
@TedShifrin Mind correcting one of my (it would seem) wrong short proof ?
 
I must admit that I've never thought about how to think of reflexive spaces. I always just considered either the definition, or the useful fact that all bounded subsets are relatively weakly compact in reflexive spaces.
@skullpatrol Namely?
 
“Professor Eddington, you must be one of the three persons in the world who understand general relativity.” To which Eddington, unruffled, replies, “On the contrary, I am trying to think who the third person is!”
 
Salut @Hippa, le méchant. Tu m'as manqué :)
 
2:45 PM
@TedShifrin J'étais en vacance :/ pauvre de moi
 
Where, @Hippa?
 
Right, so what're we supposed to criticize?
 
@DanielFischer Corsica
 
Ah, pauvre petit, véritablement ...
 
2:46 PM
I hear Corsica has nice parts, @Hippa. Can't judge for myself, never been there.
 
@DanielFischer It has nice beaches and a hot weather
I hate those :c
 
Beaches can't be nice. Although not particularly high, it is said that la montagne est joli.
 
Offhand, @Hippa, I don't see your first claim. Why should $\ker(f)\subset Co$?
 
@TedShifrin Let's say there exists a function $g$ not in $Co$ such that $g\in\ker f$. Then $f(f(g))=0$ therefore $g$ is a constant
 
Why?
 
2:50 PM
Because only constants have zero derivatives on $\mathbb{R}$
 
Ah, ok, duh.
You have $f(a)\subset Co$ rather than $f(a)\in Co$, but your argument seems right.
 
@TedShifrin Where do you see that ? i see $\subset$
Oh wait
Nvm i read it the wrong way
 
Right, it should be $\in$.
So I think your argument is right. I don't believe that fractional derivatives are linear maps from $C^\infty$ to $C^\infty$.
But I've not thought about this carefully.
 
Idk anything about fractional derivatives :/
Is there a fractional derivative for every function on $C^\infty$ ?
Is the square root always defined ?
 
Look at the wiki page the guy linked to. The half-derivative of $x$ won't be smooth.
 
3:03 PM
I've got a quick question on something I heard a while back. A friend told me that he heard of finding the $\pi$th derivative of a function. How is that even possible?
 
I don't find it on the wiki :c @TedShifrin
 
Fourier transforms and pseudodifferential operators, @Khallil.
"Fractional derivative of a basic power" @hippa
 
@TedShifrin Isn't that smooth ? prntscr.com/4fnpc7
 
First of all, it's only defined for $x>0$, and, no, it's not smooth at $0$, so if you try to glue to get a globally defined function, you're going to have bad news at $0$.
P.S. @hippa: $\sqrt x$ is not differentiable, let alone smooth, at $0$.
 
Oh true facepalm
 
3:06 PM
fractional calculus gets me wet
 
T'as tout oublié en vacances? :)
don't differentiate in the shower, then, @Mick :)
hi @Jasper
 
@TedShifrin Hi. Did you get my question in your inbox?
 
I dunno. What question?
 
@TedShifrin Pas tout, mais au moins je pense à m'enrichir :P chemistry.stackexchange.com/questions/15541/…
 
gets me wet with sweat, sweating bullets about the fractional calculus questions on my exam
 
3:08 PM
@TedShifrin Oh, I asked in chat whether it is important to study infinite-dimensional manifolds in for example Lang's Fundamentals of Differential Geometry where he defines Banach and Hilbert manifolds.
 
ah, @Hippa is training to be an alchemist. MIT had an office door labeled Department of Alchemy when I was an undergrad :)
Not until you've mastered finite dimensions, @Jasper.
 
@TedShifrin By the way, have you seen that book? It seems terribly difficult.
 
Yes, I own some version of it.
 
Ah, one needs the calculus of Banach spaces first.
 
3:10 PM
Of course.
But the proof of the inverse function theorem that I prefer (and have in my text) works fine in Banach spaces. One of many reasons I prefer it to the proofs in Spivak's Calculus on Manifolds and Munkres's Analysis on Manifolds.
@Hippa: Did you get your laptop repaired?
 
Lang treats Banach space calculus in his Real and Functional Analysis.
 
@TedShifrin Nope but i cleaned it a bit with an air spray, it's less hot now
 
One of my friends/former students almost killed his car driving it without the fan working and it just died, overheated. Luckily, he didn't blow the head gasket. But close. Similar issues with computers, I fear ...
 
@TedShifrin On a related note, is it important to study integrals of Banach-valued functions instead of just real or complex valued ones? Lang uses this approach in his real analysis text.
 
It's no big deal, @Jasper, once you understand what an integral is. You can add vectors in a vector space just as you can add real numbers. You just don't have inequalities. But, again, I think that's not worth worrying about at the outset.
 
Huy
3:14 PM
@DanielFischer: Is $W^{1,p}(\mathbb{R}) = W_0^{1,p}(\mathbb{R})$?
 
Hey all
 
Hey, @nsanger.
 
Heya, @nsanger. Long time no see.
 
Yeah, I was away for about 6 weeks, stuck in the depths of number theory....
 
3:17 PM
Ah, at Ohio State or at BU? :)
 
Funny to run into you though. I tried to buy your textbook yesterday, but I'd waited too long and the $50 used copy was gone D:
BU
 
Geometry is ultimately a question of moral. But the question is, are we here to learn moral or geometry?
2
 
Huy
@MatsGranvik: Why is geometry a question of moral?
 
I've known Glenn Stevens for years. PROMYS is a great program.
 
@Huy Yes. For $\mathbb{R}^n$, the spaces are equal.
 
Huy
3:18 PM
@DanielFischer: Also for $p = \infty$?
 
@TedShifrin, yeah it's really well designed and has great people (and Glenn Stevens is the man!)
 
@Huy After reading Spinoza's Ethices, 10 years ago, I came to this conclusion.
But I can't justify it.
 
@Huy I wouldn't think so. Who considers $W^{k,\infty}$ anyway?
 
Huy
@DanielFischer: I don't know, maybe my professor in my upcoming exam.
 
Though the program's designed so that Glenn's lectures are about 3-4 days behind whatever you're doing on the problem sets, so sometimes you can get a little bored :o
 
3:19 PM
Okay, @Huy. Then, how is $W^{k,\infty}$ defined?
 
oh, I didn't know that part, @nsanger. Seems crazy.
 
Huy
@DanielFischer: $W^{k,\infty}(\Omega) = \{u \in L^\infty(\Omega)| \, D^\alpha u \in L^\infty(\Omega) \text{ for all } \alpha \in \mathbb{N}_0^n, |\alpha| \leq k\}$?
 
@Khallil we don't define through dance in math
 
Okay, then we have $W^{k,\infty} \neq W_0^{k,\infty}$.
 
@TedShifrin, Well, the problem sets still give you a lot of guidance and ask clever questions so that you "self-discover." It's really difficult, but you're collaborating with other people, so it's doable.
 
Huy
3:23 PM
@DanielFischer: How do you immediately see that?
 
I mean, they practically give away the entire proof of Minkowski's Theorem (and sort of the Four Squares Theorem) :/
 
So the lectures are just to organize your thoughts and understanding once you've discovered stuff? Excellent. I love it.
Sounds like you had a wonderful time, @nsanger. Awesome!
 
@Huy Constants are in $W^{k,\infty}$, but not in $W_0^{k,\infty}$, since uniform limits of $C_c$ functions vanish at infinity (where "infinity" may be the boundary of $\Omega$).
 
Huy
Ah, thanks.
 
@Huy @DanielF: Compact support is a dastardly thing :P
 
3:25 PM
Yeah they're basically there to help you out if you miss really important things, and to present proofs and ideas in their "polished" form.
 
Huy
@TedShifrin: What do you mean by that? :D
 
Yeah it's awesome though! Number Theory is definitely the branch of math I like most at this point
 
Well, @nsanger, it's never been my favorite, but it's common for people in your situation to feel that way. And some go on to make it permanent, but not all :)
 
Haha, what do you mean by "people in your situation"?
 
I mean pre-college where you're exposed to so much more mathematics
 
3:26 PM
Oh gotcha.
 
I think my favorite course in college was differential topology ... and it turned me into a geometer, despite all the other things I took.
Sadly, that course is taught in relatively few (undergraduate) mathematics programs.
 
Yeah, I don't know if it'll stay my favorite either. I really want to learn some algebraic geometry, and also some algebraic topology. I know next to nothing about them right now, but talking to some people this summer I'd like to eventually
 
But, I'm thrilled to note that almost every undergrad at UGA who's taken that class from me who's gone on to grad school has ended up in some geometric area. Not all, but most.
Keep it up, @nsanger. Lots to learn :)
 
yeah...
roadmap for this year is to finally learn friggin' multivar and lin al, and then do some algebra, probably from herstein
 
@DanielF: I can see you've been an evil influence on me. One of my students just emailed me asking if I could give incite on one of his homework problems. I told him it was a good thing he wasn't an English major insighting a riot. :D
Do Artin instead of Herstein. Much better global understanding and far more interesting.
 
3:31 PM
:)
 
And it's ok to learn multivariable/lin alg your first year in college, seriously.
 
Huy
Colleges don't usually teach multivariable/lin alg in the first year?
 
For the advanced first-year students we do, @Huy.
 
Huy
You seperate advanced from non-advanced students?
 
Over half my multivariable math class is first-year students. And I can already tell a few of them will be among the stars of the class.
Yes, @Huy.
 
Huy
3:32 PM
I see. I've never heard of that before.
 
@TedShifrin, yeah I don't mean to give off the impression I'm freaking out and trying to learn everything before college. It's actually more because my top college choice doesn't really offer it, as far as I know :O
 
We have plenty of college students starting in beginning calculus (or precalculus), and lots who know a year or two of calculus.
What choice is that, @nsanger?
 
Uchicago
 
Oh, they sure do.
They offer a totally-off-the-deep-end course, assuming Spivak + linear algebra. I don't like the course. It's chased people out of math.
It's more like MATH 55 at Harvard.
So if that's your goal, then, yeah, you should learn my book first.
But I think Chicago's curriculum is nuts.
 
Yeah I have mixed feelings about the course too
 
Huy
3:34 PM
@TedShifrin: We just have one standard curriculum for the first two years. Not a special one for advanced students or a special one for not so advanced students.
 
Oh, and they do offer a medium-level multivariable course, too, not just the engineering style.
Totally different in the US, @Huy.
 
In one sense it sounds challenging and interesting, even though I'm not that into analysis
 
Plus, remember, that we have plenty of science and engineering majors, too, who aren't interested in proofs.
 
but it also does seem a little crazy, and might be problematic your first year of college when you're trying to settle in and make friends and whatnot
 
I would recommend the middle-track course, @nsanger, unless you do learn my book first.
Yeah, I think it's definite overkill.
 
Huy
3:35 PM
@TedShifrin: We have seperate lectures for engineers or CS majors but usually they are still proof-heavy, depending on the professor.
 
A few kids who've taken our Spivak course here while in high school have gone to Chicago and taken that course. One continued to finish a math degree, but is now doing a Ph.D. in philosophy. The other gave up on math and is now doing ecology/biology.
 
Honestly, if I get in there, I'd probably just do whatever the department recommends
 
yeah, @Huy, the US since the 60s has had non-proof-based math courses pretty much.
 
And do they say they left math because of that course, lol?
 
Definitely.
 
3:36 PM
Wow, lol...
 
@nsanger: Are you saying that all the stuff you learned in Spivak (which is a lot of analysis) you dislike?
 
Huy
@TedShifrin: I have mixed feelings about non-proof based math courses. Even in high school we proved everything properly, except maybe the fundamental theorem of algebra.
 
No.
 
So how do you know you dislike analysis, @nsanger?
 
No working through Spivak has been a lot of fun.
I don't dislike it
 
3:37 PM
oh, ok, I misunderstood.
 
hello @TedShifrin
 
Well, if you're seriously going to work through my book and learn both theory and computation, you'll do fine in that Chicago course. But expect to spend 15-20 hours a week on math. They'll tell you that.
hi @Balarka
 
Okay.
 
hi again @BalarkaSen
 
If you go to Stanford or Yale, @nsanger, the standard hard freshman course uses my book.
 
3:38 PM
If I can't find a cheap version of your book, I might use this PDF of Hubbard & Hubbard I found, though.
 
It's very idiosyncratic in its approach, but quite popular among bright folks.
 
It seems to cover the same material, roughly. Right?
 
It doesn't lay out the computational stuff as clearly as I do...
 
analysis
meh
 
Oh...
 
3:40 PM
Yes, @nsanger, roughly the same material. Just very idioscyncratically organized/presented.
 
Argh, it's frustrating how many books ignore computation...
 
If you want to email me, @nsanger, perhaps we can work something out ...
 
.... hmmm
I don't want to deprive you of money though....
 
@TedShifrin Hi
 
Don't be silly, @nsanger.
 
Huy
3:42 PM
@DanielFischer: If $u \in W^{1,p}(\mathbb{R})$ and $\rho \in C_c^\infty(]-1,1[)$, then $u * \rho \in C^\infty(]-1,1[)$, right?
 
I'm not in it for the money. But I want your word that you'll use it only for yourself, and not to share.
hi @Jayesh
 
I have a doubt, did you ever plot contours (for complex anlaysis or otherwise) for your books using some plotting software? If yes, what did you use?
 
Ahh, of course, I would never just freely distribute something like that
 
Yes, @Jayesh. I'm a big fan of Mathematica, but it isn't cheap.
 
@Huy $C^\infty(\mathbb{R})$. That holds for every locally integrable $u$.
 
Huy
3:44 PM
@DanielFischer: But not necessarily $C_c^\infty$ yet, right?
 
@nsanger: This is why, ultimately, I decided to leave my diff geo text in .pdf form for everyone to use free around the world :P
 
When someone writes a 500 page textbook I'm not just going to rudely disregard that
 
@TedShifrin Ahh, okay, we have mathematica on an institute license here, so I guess I'll try that out. I've been using matplotlib for a lot of stuff, but contour integrals, I'm having a hard time searching for it. :-)
 
Well, @nsanger, the world is full of folks who do ... Just sayin' :D
 
@Huy Unless $u$ has compact support, or $\rho = 0$, it's very rare that the convolution has compact support.
 
Huy
3:46 PM
@DanielFischer: I see. Thanks.
 
ParametricPlot or ParametricPlot3D are wonderful commands in Mathematica, @Jayesh.
 
@TedShifrin i happen to be one of them.
 
I'm not surprised, @Balarka.
 
Hehe, yeah. Again, my copy of Hubbard & Hubbard is probably not endorsed by the publisher hehe...
 
@TedShifrin why not?
 
3:47 PM
@TedShifrin Thank you.
 
Huy
@TedShifrin: What would you advise to a student who really wants to read a certain textbook but can't really afford it?
 
Hatcher (also at Cornell, along with Hubbard) made a big deal when he published his Algebraic TOpology text that Cambridge Press would allow him to continue to freely distribute the .pdf.
 
And also my copy of Finite Dimensional Vector Spaces, and baby Rudin, and Dummit & Foote, .... woops
 
@BalarkaSen You're 14.
 
In my day, @Huy, we used libraries :P
 
Huy
3:47 PM
@TedShifrin: I checked the biggest library in Switzerland, and they don't have it either.
 
@Huy Fernleihe?
 
Huy
@DanielFischer: Is that even possible internationally? O.o
 
@Huy I don't know, you can ask.
 
Huy
I'll try.
 
@DanielFischer erm. i don't see the implication.
 
3:49 PM
I'll right, I'm off. I'll send you an email later @TedShifrin. Thanks, very much :D
 
Well, @nsanger, you're not inspiring me with confidence :D
 
@BalarkaSen When you're grown up, you'll not be surprised by anything that 14-year-olds do.
 
politely applauds @DanielF
 
is 14 that special?
 
yes, enjoy it
:-)
 
3:53 PM
i though the only cool thing about it was that 14 is the unique number that can be written as a product of 2 and 7 in 2 ways.
 
rolls all fourteen eyes
 
@BalarkaSen just don't forget to have some fun (other than math)
 
Prof. @Ted! @Balarka!
 
Huy
@skullpatrol: I think he's still to young for that.
 
@Huy 14 year olds can have good clean fun
 
Huy
3:59 PM
@skullpatrol: Tell me more about it.
 

« first day (1480 days earlier)      last day (3550 days later) »