« first day (2153 days earlier)      last day (2887 days later) » 

12:00 AM
$\Bbb Z/n$ for all $n$...
 
I said non-planar, @MikeM.
From the picture, I can't tell. I doubt I see $A_5$ or $S_5$.
or the double-cover of $A_5$?
 
I'm thinking of all the elliptic 3-manifolds. Many of them are quotients of $SO(3)$ by a finite subgroup.
Double cover lives in SU(2); it's the f.g. of the Poincare sphere. A_5 is the icosahedron group.
 
@tern? :)
 
what i'm thinking is to consider what permutations are allowed of $(x,y,z,-x,-y,-z)$
 
@TedShifrin all of the symmetries I described for the previous equation seem to apply to that one just as well
 
12:03 AM
it doesn't look like a tetrahedron
 
I agree, tern, but are we sure there aren't any others? I bet with you.
@Ali: There are various semi-regular polyhedra that don't look at all like the standard ones but have only those symmetry groups.
Let me find a picture to upload (from my algebra book).
 
I'll repost this here - just in case somebody has a suggestion about that:
in Linear & Abstract algebra, 3 hours ago, by TheGreatDuck
@MartinSleziak is there a good place i can go to read on how tensors are multiplied?
 
Here's a fun one, @Ali.
 
Its hard to unsee the star
 
Or this.
 
12:09 AM
So what symmetries do they have?
Or do I find this out later
Do you go through them in your abstract algebra geometry book?
 
You can figure it out for yourself. I'm telling you it has to be the symmetries of either a tetrahedron or (cube/octahedron) or (icosahedron/dodecahedron).
 
What annoys me slightly is that not all finite subgroups of $SO(4)$ acting freely on $S^3$ are subgroups of $SU(2)$
 
Well, why should everything be spin, @MikeM?
 
Hm? $SO(4)$, not $SO(3)$
 
Oh, right.
But why should a real action come from a complex action?
 
12:12 AM
Would make my life easier.
 
Oh, well, let's change all of math for you, then.
 
Sure, that'd be great.
 
And all smooth functions are holomorphic, too.
 
There's a paper I like (depending on the day) whose results only work for $SU(2)/G$. Sad...
Don't be silly, I know the difference between random functions and those that satisfy an analytic elliptic PDE. :)
 
Well, I wouldn't quite say smooth = random, but I take your point.
@Ali: Have you decided?
 
12:15 AM
Nobody ever likes my answer of "why is complex analysis so nice?" with "Linear elliptic PDE have extremely nice solutions"
 
I prefer the Cauchy kernel, myself. :P
 
Same answer, though.
 
Not entirely obviously, though.
That Bryant (Chern) paper @PVAL posted about $S^6$ has some cool stuff in it.
 
@TedShifrin That first one seems to have icosahedron symmetry
I guess thats the easy bit
 
You want to teach it to me?
 
12:17 AM
Yup @Ali
What about the second one?
I certainly don't know it all, @MikeM.
I'm just a lot better at structure equations and the Cartan game than you are, @MikeM.
I've never worked with $G_2$, however.
 
I don't know much about $G_2$. I know the tiniest iota about $G_2$ manifolds. :P
Obviously my perspective on them is different than Bryant's and Chern's.
 
Well, I know nothing (to quote Sgt. Schultz).
 
@TedShifrin The second one has tetrahedral?
 
I don't think so, @Ali.
 
Donaldson has an idea (program? maybe not quite so developed) on how to develop invariants of deformation classes of $G_2$ structures. There should be a Floer-type theory of manifolds and cobordism maps with CY 3-folds, $G_2$-manifolds and Spin(7)-manifolds.
 
12:20 AM
NO elements of order $4$ in the tetrahedron group, @ali.
I still know nothing, @MikeM. Besides, I'm old and retired.
 
So far there are some issues. One guy is making progress on defining a G_2 Casson invariant. You count instantons. Unfortunately a naive count might be 6 for one G_2 structure and 7 for another.
That's no good.
 
Why? If you have different structures?
 
Sorry, in the same deformation class.
 
Oh.
 
Bubbling-type problems. I don't know if that means anything to you.
 
12:22 AM
Yes, I know about Sachs-Uhlenbeck bubbling.
Or I once did.
 
It's my damn life.
 
Lawson's CBMS lectures were fabulous. We did a year-long seminar on that.
I think I told you that before. Sadly, I threw all those notes away too.
Lots of cool, cool stuff in your math life, @MikeM.
 
"Bubbling occurs starting in dimension 8. In this work we use only manifolds of dimension at most 6."
Yeah. If only I could write it down ...
 
@Ali: If my picture isn't clear enough, google "truncated cube."
 
that gave it away :P
 
12:24 AM
nods
 
I was trying to see if it was cut from something
Tetrahedron works if you ignore most of the shape
 
It's way past your bedtime, anyhow :)
Ignoring most of the shape isn't cool for symmetry.
 
but its a saaaatturrrddaayyy
 
ohhhh
 
or it was
 
12:26 AM
yeah, it ain't no more ...
 
ok then good night @TedShifrin
 
Isn't it like 2:30 AM?
 
Hello!
 
1:30
 
g'night, @Ali. Always a pleasure to chat with you.
 
12:27 AM
Not that bad
 
hi @MathWanderer.
 
@TedShifrin Are you the professor who wrote
 
Good night everybody else
 
books in the linear algebra?
 
Hope you all sleep as well as I am about to
 
12:27 AM
A book, yes, @MathWanderer.
Do you wish to complain?
 
What makes you think I am about to complain?
 
Most of the comments on Amazon and Barnes & Noble are complaints that the book is too hard. :)
 
Well, they're terrible books, so it's only natural.
:D
 
I have nothing to complain at this point
 
@MikeM: God knows Balarka still doesn't know any multivariable calculus/analysis.
 
12:29 AM
Depends on the day.
 
LOL, ok, @MathWanderer.
If you can do the medium and hard exercises in that book, @MathWanderer, you're doing very well.
 
@TedShifrin Your book does contain insane problems but I thought the book is not difficult
 
Oh, you have no idea what insane problems are :D
 
Although I read Hoffman/Kunze's Linear Algebra first, which I thought was a good prep.
 
That's a more advanced book than ours.
It's meant for junior-level math majors. Ours is meant for sophomores who know no linear algebra.
 
12:31 AM
Well, different approaches help me
 
Ordinarily, I would say to do their book after. But, whatever ...
 
I need to review linear algebra before going to Lang's Algebra and Hungerford's Algebra in-depth
I started with H/K first since I got it for gift
 
Lang is absurd unless you're a very sophisticated graduate student. Do Artin.
 
So here is my story
my current research in the machine learning and topology, and their applications to biology introduced me to the algebra
And I got those books as a farewell gift from emeritus professor
 
Gifts don't necessarily mean it's the right thing to do.
 
12:33 AM
Of course
that was a reason why I have been searching for elementary books to complement them
Artin, Pinter, Herstein, etc.
 
Look at Michael Artin's Algebra. You will know most of the linear algebra in there.
Forget Herstein. I don't know Pinter.
 
Do you not like Herstein?
 
No.
It treats algebra as symbol-pushing.
And tricks.
Artin truly shows how algebra fits inside unified mathematics.
 
Ohhhh....I see...because I have his Topics in Algebra, which I never read.
Artin, I see.
I am probably going to sell Topics in Algebra and buy Artin
 
(I have strong opinions. Many people do not agree with me.)
 
12:36 AM
How do you think about Rudin's PMA, but the way?
by the way, I mean.
 
Lang is a bad book with good exercises.
 
I think it's a fine book for a strong student who can draw lots of his own pictures. But it's very hard for most students.
Rudin has excellent exercises, but professors who teach out of it and never draw pictures or provide motivation/illumination make it hopeless.
 
That is why I like Pugh
But many of my friends swore by Rudin as a gateway to truth
 
Well, I'm not a strong enough student to get anything out of it. The exercises were good qual prep.
 
I don't know Pugh's book, but I know him very well. I took graduate courses from him. I expect his book is quite good.
smacks @MikeM
 
12:37 AM
His book has many pictures. \
 
Yes, he's very geometric in approach.
As you noted, I like pictures, too. Even my abstract algebra book has lots of pictures. :P
But I'm not recommending it to you.
Too low level.
 
What is the name?
It might be helpful too
 
Something like Abstract Algebra: A Geometric Approach.
I like "A Geometric Approach." Although I skipped it in my differential geometry text. :P
 
It is written by you too!
 
I said so, didn't I?
 
12:40 AM
When you said "Even by abstract...", I thought you mean the books you have
 
Oh, sorry about that.
 
I should take a reading at that book too
On this Summer, I am exploring many branches
like algebra, representation theory, complex analysis, etc.
I could not resist my temptation and curiosity to get my brain to them
 
Well, representation theory is linear algebra with a lot of group theory, so you need group theory first.
 
That is also one of reason why I started to read Hungerford
 
Complex analysis is beautiful stuff, but you shouldn't get too far removed from the material you need for your work.
 
12:42 AM
Well, I will be doing a reading course in analytic number theory on August, so I need them
 
Hmm, @Ali said he was asleep, but he is still here.
Yes, you do for that, @MathWanderer.
 
But I found elementary books that I can finish within a month.
 
LOL
 
Jameson and Needham, I especially like first
 
I don't know that.
 
12:43 AM
It is very concise book, elementary exposition, and treat only few topics
 
If you've been through Pugh or Rudin, you should do complex more seriously, but you can't devote that much time to everything.
 
How "serious"? I won't be taking any course in complex until next Spring
I can only budget small amount of time for learning complex, so Jameson fits me
 
Seems ridiculous to self-study and then take a course. Why not reorganize?
 
Could you provide me advice?
 
Just saying you should save complex for the course and wait for analytic number theory. You seem to be trying to learn everything.
 
12:46 AM
Should I not pursue ANT then?
 
Postpone until you've actually taken the complex analysis course. No need to do everything at once!
You should also prioritize a little bit according to your research area?
 
My research area involves a lot of mathematics
 
I would have expected more applied things.
Like dynamical systems/differential equations.
 
Not really....
We try to apply topology to figure out better ML algorithms for data analysis
 
I just can't see all the algebra, number theory. Topology, sure.
 
12:48 AM
And algebra is needed since my collaborated work is on algorithm efficiency too
 
I never knew that the theory of algorithms was so much abstract algebra.
 
Me neither until my professor and collaborator told me
 
LOL, ok, I'll trust you.
 
Algebra though is used extensively in topological data analysis
as TDA is heavily depended on cohomological structures
 
More homological algebra and algebraic topology. Not so much algebra per se.
 
12:50 AM
Isn't algebraic topology need algebra?
My grammatical mistake, I am sorry about it.
 
I'll let @MikeM answer that one. Don't go crazy with abstract algebra. But learn basics about groups, rings, modules.
 
You should know some algebra.
 
Thanks for the advice.
 
You won't need the Sylow theorems, say.
 
I see.
 
12:52 AM
Or Galois theory, really.
 
I see!
 
The material in Artin on finitely generated modules over a Euclidean domain is important.
 
Thanks for the information!
Right now, I am reviewing Rudin and reading Dugundji and Hungerford
but I should adjust my plan then
 
Yikes @Dugundji.
 
???
I love that book!
 
12:53 AM
Although I sort of liked that book. It has mistakes.
It's way too much point-set topology.
 
Yes, but I thought it makes an excellent intro. to topology.
Much better than Munkres or Willard books
 
Overkill. If you know half of Munkres, you're fine.
 
I did not read it
 
Yet you know it's "better"?
 
When I took gen. topology course on the last semester, which Munkreas was required, I read Dugundji
 
12:54 AM
You are unlikely to need that much general topology, then. If you took a course, move on to other stuff.
 
Thanks for the information. I was just doing some problems I was not able to figure out on the last semester.
I feel like if I cannot do every problem, I am done for
 
No, relax!
 
I'm sure all the stuff about non-metrizable spaces is deeply relevant to topological data analysis :p
 
@MikeM: Sarcasm may be lost on this audience.
 
12:56 AM
But I feel stupid and incomplete when I could not finish the book from page one to last pne.
 
@MathWanderer: Seriously, you do not need to be an expert on every single thing. Concentrate on basics that are truly relevant.
 
@TedShifrin Thank you for the advice. How should I resist my temptation?
Whenever I tried to move on after not able to solve problems, I cannot think of nothing else...
 
LOL. There's only so much time and so much mental energy.
 
I also feel like everything in the books is important
 
If a particular problem truly fascinates you, work on it.
I'm puzzled that after all this advanced stuff you find my book that challenging. Very few problems should be challenging compared to Dugundji, etc.
 
12:58 AM
Your book is not challenging like others, but some problems do
 
@MathWanderer: THis is why teachers provide some guidance to what's important and what's not. There's too much stuff in books.
 
I see.
 
OK, I'm leaving. Good luck, @MathWanderer. I know you'll do fine.
 
Thanks, you too!
I am going back to my stuidy
 
 
3 hours later…
3:47 AM
2k rep!
 
r9m
well done @LeakyNun ! :-) .. btw what exactly is a leaky nun? XD
 
@r9m anagram of my name
 
r9m
@LeakyNun I see :)
 
 
3 hours later…
6:28 AM
I love math!
 
Huy
ok
 
@TedShifrin Eh, I just spent a year on that :(
 
Hey guys, I know this is a weird question to be asking here. But do you guys wash your new clothes before wearing?
 
@TheArtist Yes
 
I don't have new clothes
 
6:41 AM
@LeakyNun what if it's winter time (no sun) and you don't have a dryer. Do you think I should wait days to dry off my clothes?
 
@TheArtist depends on whether you need the clothes quickly.
 
@LeakyNun Hmmmm.
@BalarkaSen hehe
 
Huy
7:06 AM
@TheArtist wait days.
also, buy a dryer
you can also put them in your oven
(kids shouldn't try this)
 
great idea
 
@Huy you can put in your oven, are u serious?
 
@Huy well, i don't know the point of this disclaimer in many places. when they tell you "kids, don't try this", don't they realize they're literally telling him to try this?
 
Huy
@BalarkaSen: maybe that's the point ;)
@TheArtist: if you're experienced, sure. or you can hang them up right next to it and open the oven and use the warm air to dry your clothes
you just shouldn't set your oven to like 550°F and then leave the flat for 2 hours
 
@Huy devil: "even i couldn't have put it better"
 
Huy
7:09 AM
=)
 
Hmm, I just had an idea about something I wanted to prove. Now let's hope it works.
 
@BalarkaSen what is it you want to prove?
 
Huy
that an oven is a safe environment for clothes
 
Something about self-intersection and normal bundles.
 
Huy
maths, really? boring.
 
7:16 AM
@BalarkaSen never mind
 
7:33 AM
hi guys!
I asked this question a while ago. math.stackexchange.com/q/1819207/200649
If possible please have a look. :)
 
@Huy Does it work with microwave ovens?
 
@AbhishekBhatia The PDF of the $Uniform(0, 1)$ is equal to $1$ everywhere between $0$ and $1$.
This is the definition of the Uniform Distribution.
 
@Axoren how?
 
$f_U(u) = 1$ always, it doesn't matter what kind of expected value you're calculating.
This is the definition of the Uniform.
Every point in the interval $[0, 1]$ has equal probability, but there are so many points that each of them would have probability of "essentially" $0$.
So, instead, we talk about them as a probability density around the point.
The probability density is $f_U(u) = 1$
 
That makes sense, but how can it be 1.
 
7:43 AM
You seem to be having an issue with the definition of the $Uniform(0, 1)$ distribution. Why do you think can't it be $1$?
 
That means each very small interval around x has probability 1.
 
You're still confusing topics.
Once you're in a continuous distribution, you can't talk about probability in the same way anymore.
 
Huy
@TheArtist do you understand what a microwave does?
 
Every point has a probability of $0$, there are more than $\omega$ points on any real number interval.
There are just too many points to ever assign all the points in an interval with a non-zero probability.
So, instead, we work with densities.
Consider $g(u) = P(U < [0, u])$
 
@Huy boils water in the food?
 
7:47 AM
$g(u)$ is the cumulative distribution function of $U$, where $U$ is the $Uniform(0, 1)$.
$g(0.5) = 50% = 0.5$, because we know that half of the points are in the interval $[0, 0.5]$, so there's a 50% chance of being in that subinterval.
@AbhishekBhatia Do you agree with that?
What you'll notice is that for $u \in [0, 1]$, $g(u) = u$.
We know that the probability function is the derivative of the cumulative distribution function.
So what happens when you take the derivative of $g(u)$ with respect to $u$?
$f_U(u) = g'(u) = 1$
 
Just watching the lecture again, will get back and read your comments in 10 mins. :)
 
@AbhishekBhatia I may not be here in 10 minutes, so I'll leave a little bit extra for you when you return.
Good luck with this stuff, it's not trivial to the uninitiated or even the initiated.
So, the remarkable thing is that if we take infinitesimal steps along the interval, we notice a non-zero shift in the cumulative distribution function.
And that the shift is of slope $1$.
What we've actually done is shown that for every $dx$-wide neighborhood about a point $x$, we have probability $dx$ of hitting it.
So, we can say that it's density is $\frac {dx}{dx} = 1$.
 
8:13 AM
@Axoren Thanks!!, makes sense now. The probability is $dx$, I thinking it of be 1. Also you're earlier points made it clear. If you can post an answer I will accept it.
 
Does the center of mass of a rod divide the rod into to 2 pieces of equal mass?
 
8:46 AM
@xpmrz not necessarily
Imagine a rod of the density of a blackhole is attached with another rod of the same mass. The center of mass would not be the point where they attach
 
Huy
@TheArtist: and do you know how a drying machine works?
 
9:55 AM
Where(like room) should I put the my review - bad post to close or delete ?
 
@MithleshUpadhyay I guess it is usually enough that you review the post, we would be swamped in chat. The room which is the most closely related to flagging is C-R-U-D-E. However, it is rather inactive lately - there are not many users who come there.
in C.R.U.D.E., 2 days ago, by Martin Sleziak
This room now has much less activity than it used to have. For example, if you look at January 2015, there were several users relatively active in this room.
BTW if you think that for some reason it is useful for other users to see the review, there is already complete history shown for users with access to the review queue. (One list per review type, link can be easily seen if you are the review queue.)
And history of review by a specific user is shown in the profile.
 
@MartinSleziak, Thanks.
 
10:11 AM
Is the Gamma function the only continuation of the factorial function that is smooth to every degree?
i.e. continuous at every derivative
Also, how is it derived?
 
10:32 AM
@LeakyNun No, it's not the unique extension of the factorial function which is complex analytic. However, with mild condition, it becomes unique such.
IIRC it's called Bohr-Mollerup's theorem.
 
@BalarkaSen Sorry I don't understand "mild condition"
 
You need to impose some extra conditions. Then $\Gamma$ does indeed become a unique complex analytic extension of factorial.
 
what are those conditions?
And for all intends and purposes, I mentioned the condition "smooth to every degree"
 
I don't remember it anymore: I just gave you the name of the theorem, you should google it.
 
is that a valid defining condition?
 
10:34 AM
the extra condition is log-convexity, i believe
 
I don't understand what "smooth to every degree means". Do you mean infinitely differentiable?
 
> Bohr-Mollerup Theorem. Γ(x) is the only function that satisfies f (x + 1) = x f (x) with log( f (x)) convex and also with f (1) = 1.
@BalarkaSen yes.
 
Being analytic is stronger than being infinitely differentiable.
Namely, you should also require it admits a Taylor series at every point.
 
I see
but the theorem does not include this condition.
is this condition alone uniquely defining?
 
@LeakyNun Mmm, I see. I forgot how surprising the theorem is: it doesn't even assume the function is differentiable :)
@LeakyNun What do you mean?
 
10:38 AM
My question is, can I define another function which is an extension of the factorial, and is analytic, uniquely?
Well... is gamma analytic? also, what is its taylor series?
 
@LeakyNun Yes, there are quite a few analytic functions which extend the factorial. Google may tell you some.
Yes of course the gamma function is complex analytic.
 
what is its taylor series?
 
There's a wikipedia page on the gamma function, you know :P You'll find all your answers there.
 
$f(z)=\Gamma(z)+\sin(\pi z)$ :o
 
Here is a function which is complex analytic and extends the factorial, but is clearly not the gamma function.
Or that, sure.
 
10:43 AM
nice.
@BalarkaSen I can only find the taylor series of ln(1+Gamma)
 
I can see Laurent expansions written out in many places.
You can't find a globally defined Taylor series because $\Gamma$ is not entire: it's complex analytic away from the negative numbers, iirc.
 
Oh, sorry
 
I am no expert on gamma function though, so I am not the ideal person to ask about what they do or don't.
I just know a bit of complex analysis.
 
I see
 
@TedShifrin I went to bed with my laptop open
 
10:59 AM
Hello again.
I want to jump into the next section in Guillemin-Pollack but I am not sure if I remember everything from the previous ones.
By remember I really mean understand
 
11:22 AM
athr4 qe3q
 
Huy
that's my password
 
how do you even remember it
 
Huy
I just ask Mike whenever I forget it
 
11:50 AM
spread the word
 
Hi guys @MikeMiller @BalarkaSen @Huy
and @Andrew
 
Hidee ho.
 
Hellu everyone.
 
How's life?
 
Is cool. Trying to learn math. And you?
 
Huy
11:55 AM
hi @Danu
 
@AndrewThompson Going through another round of doubts whether I want to switch to math or not :\
 
Toss a coin.
 
@Danu You study physics IIRC?
 
@AndrewThompson Yep
 
@Danu Okay, so there are pros and cons for staying with physics:

Pros: You can appeal to muggles by pretending being friends with you will be just like in The Big Bang Theory.

Cons: It's physics.
 

« first day (2153 days earlier)      last day (2887 days later) »