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12:02 AM
@PedroTamaroff good point! ;p
 
@Gato So, do you know how to start with this?
 
not really @PedroTamaroff
 
Where did you get this problem from?
(Perhaps you can find some inspiration there, you see.)
 
r9m
@robjohn Nice!! :-)
 
In fact I was trying to prove that $exp(in \theta)$ is dense using Ted's idea : "I prefer to suppose there's an open interval U in the circle that contains no power of e^iθ, @Gato, knowing that the circle has finite measure."
 
12:07 AM
@r9m I don't know if one would call it slick, but I thought it was worth posting.
 
@Gato OK. And did that get you anywhere?
Also, what is $\theta$? An irrational number?
 
r9m
@robjohn I was trying to prove $e^{x-x^2/2}+e^{-x-x^2/2} \le 2$ in that form too .. ! :-) It's a nice proof :)
 
Only if for an infinite group of $S^1$ there is an accumulation point
 
@r9m I am still trying to think of a way that doesn't require taking a derivative.
@r9m Yours is the only one so far
 
@Gato If you know that for each $n\neq m$, $e^{i\theta n}\neq e^{i\theta m}$ you can deduce this sequence has an accumulation point in $S^1$; since $S^1$ is compact.
But this accumulation point doesn't necessarily fall into the interval you assume is disjoint from this sequence.
 
r9m
12:13 AM
@robjohn I tried Jensen for a long time ... couldn't manage to use it without having to use double derivative ..
 
@PedroTamaroff There is a pigeonhole argument that can be used.
 
@robjohn Yes, yes. We're letting Gato think.
 
@PedroTamaroff since multiplying is continuous every point is as well an accumulation point right?
 
@Gato But not of the original sequence.
You're changing the sequence when you multiply (I assume you mean rotate by an appropriate point.)
 
Right (yes it's why I meant..)! I know we can prove the density using additive subgroup of $\Bbb{R}$ but Ted's idea doesn't need this and I don't know how can I do so..
@PedroTamaroff
 
12:18 AM
@Gato Perhaps @robjohn can help you only slightly with the pigeonhole idea.
Do you know about the pigeonhole principle, @Gato? It is a very important principle!
 
@PedroTamaroff I have already heard that name somewhere..
 
@Gato The principle is very simple: if you have $m$ pigeons and you must distribute them in $n$ boxes, and $m>n$, there must exist a box with at least two pigeons. You can make this more complicated and precise, but this is the simplest form of it.
 
As stated is just logical, I need this for the density?
 
@Gato Yes.
What you want to think is about $nt-\lfloor tn\rfloor$ when $t$ is irrational.
These are numbers in $[0,1]$.
 
@Pedro Do you need a hint, pal?
 
12:23 AM
@MikeMiller Oh, you asked me something about $S^1$ too.
I am doing one thing and I'll think about it.
 
@PedroTamaroff Okay thanks I will think about it.
I need to sleep now, good night all!
 
@Gato Take a look at the pigeonhole argument in this answer, and see if you can adjust it to your needs.
$$\left|e^{ip}-1\right|=\left|e^{i(p-2\pi q)}-1\right|\le|p-2\pi q|$$
 
12:44 AM
PEDRO
 
@robjohn heya
 
@N3buchadnezzar how are things?
 
@robjohn Just back from vacation, you?
 
@N3buchadnezzar work is busy, but I can escape here :-)
 
12:57 AM
when I'm trying to use the universal property of tensor products to prove that some module $Q$ is isomorphic to the tensor product of two other modules $M$ and $N$, what I try to do is as follows: I take a bilinear map from $M \times N$ to some module $P$, and show that it must uniquely factor through $Q$
however I always get stuck on uniqueness, it seems
e.g., I have to prove $F^{mn} \cong F^m \otimes_F F^n$ where $F$ is a field
so I take a bilinear map which sends (u, v) to P somehow
and I notice that I have a nice map from $F^m \times F^n$ to $F^{mn}$ (which I view as $m \times n$ matrices over $F$ by $\Phi(u, v) = uv^T$
forgot a right bracket after "over F"
now it should be true that for any bilinear $L : M \times N \to P$ there's a unique $\hat{L}: F^{mn} \to P$ such that $\hat{L} \circ \Phi = L$, but I don't know how to prove it
my intuition screams that it's true but I just can't figure out how to say why
 
@SamuelYusim Well, here you can simply use that tensor products distribute over direct sums.
$F^m\otimes F^n \simeq F\otimes F^n\oplus\cdots \oplus F\otimes F^n\simeq F^n\oplus \cdots \oplus F^n \simeq F^{nm}$.
Using universal properties is not really optimal here.
 
1:18 AM
Hey everyone. I had a question regarding topics in mathematics: What are some mathematics topics that have a small, warm community?
 
@JulianRachman Perhaps your internet failed? I've deleted the duplicate message just some minutes ago.
 
@Pedro Let him keep the question
he seems to want to know the answer
 
Oh. Ok. Thank you. It probably did @Pedro
 
Of course, though, i'm not the mod
@Julian I think number theory would be a good community
 
@user1111 but it is a HUGE community of people.
 
1:21 AM
why do you say that?
do you have any stats?
 
@JulianRachman Yes.
 
@user1111 No, But I have heard.
 
But the message stays in the history for other mods, IIRC.
 
@Pedro Can you delete all my comments from January 19 and 20 jsut ot be safe? I made a mistake
 
@JulianRachman What's the problem?
 
1:23 AM
@Julian sorry for interrupting, but who did you hear it from?
 
@PedroTamaroff I appreciate that, but it's just one of a set of similar problems, so a direct answer would still be helpful
 
r9m
@robjohn I'm waiting for probability theorists to have a look at that inequality :-) .. I think I learnt about the Hoeffding Inequality from one of Jack D'Aurizio's answers .. someone or the other is going to spring that I suppose .. :)
 
@Pedro It is just that I said some things that really was not good and I would like to take them back.
 
@SamuelYusim How is my answer not direct?
 
I want to know how to show uniqueness of the $\hat{L}$ maps
 
1:24 AM
@JulianRachman Can you link to the discussion and delete the comment? I don't want to be looking for stuff.
 
user105491
@Pedro Please do not do that. He had made some rude comments about me
 
Ok. No pron
 
user105491
and therefore he wants to delete them
 
@SanathDevalapurkar If the comments are offensive, they should be deleted.
 
user105491
@Pedro Yes, I agree with you on that
 
user105491
1:25 AM
But did not contain bad words
 
Thank you. I appreciate it.
 
It's good that Julian is apologizing, too.
You two should try to avoid getting into discussions here or in comments in main. It seems you go to the same school, so you can probably discuss such things personally.
 
user105491
@Pedro I understand that.
 
user105491
And that is precisely the reason he wants them deleted
 
I am truly sorry... @Pedro Thank you for understanding. It was my fault in the first place
I got my lesson so I appreciate it @Pedro
 
1:27 AM
@SanathDevalapurkar I think I've already told it is not nice to smother Julian. Sometimes you seem to get obsessed with something and press on him too much.
Just give each other air, Julian and Sanath.
 
user105491
@Julian It's no problem. But it seems like you're apologizing more to Pedro than to me. :-P @Pedro I did not realize I was doing that. I apologize, @Julian
 
user105491
and thanks for the advice, @Pedro!
 
I did appoligize to you. I will knee if I have to because i am sincere
 
@Julian I know I'm being very very inappropriate, but I am pretty interested in knowing from where you got the stats that number theory is a huge community
 
user105491
@Julian And as I said, i apologize to you too. So we are even.
 
1:29 AM
Oh. I heard for certain opinionatic places.
 
@Julian what do you want to study then?
 
@Sanath I jsut want us to get along.
 
because that general field will help narrow down this broad question
 
@user1111 Idk. There is a lot! and I am young
just curious
 
I know that
that is why i'm asking you what you are generally interested in
I recall that you said that you wanted to do homotopy theory, in the chat thing that you linked to
 
1:30 AM
I currently am working with topology. But know very little. But find it sort of interesting.
 
@JUlian do you know any algebra at all?
or analysis?
 
I do
'Analysis
I am taking a course in Real analysis at a college
 
user105491
how much analysis do you know?
 
'but I self studied before-hand for about 3 months
 
user105491
ok but how much analysis do you know?
 
user105491
1:32 AM
sorry if I sound inappropriate
 
Up until the continuity and the Topology on $\mathbb{R}$
 
user105491
i'm just trying to help @user1111 help you more
 
@SanathDevalapurkar @JulianRachman I have gone through the messages. I don't find anything offensive. The only thing I find is me and Julian agreeing that Sanath might have made Julian think one should study mathematics in such and such way, which I think it is no the way to go.
 
@Pedro I agree with you. I can see that there is nothing offensive there, but such things can hurt @Julian and @Sanath's friendship
 
Ok I am sorry for saying this. But i sound like a bitch and I really an so sorry about it.
 
1:33 AM
so i mean, morally, it's the right thing to do
 
@JulianRachman No, you don't.
 
user105491
@JUlian Hey, part of it was my fault too
 
user105491
So we can just be friends
 
user105491
:-)
 
user105491
Sorry i gtg
 
user105491
1:34 AM
bye
 
But -- in my opinion -- you need to stop worrying about algebraic topology and homotopy theory (for now!) and start learning from the bottom up.
 
Please I will be glad to welcome you again and be great mathematicians. :)
 
@Julian what topology on R?
 
@Pedro I have already changed my mindset
 
@user1111 I'm not going to back this up with statistics, but you could do so by looking at the database of AMS members. Number theory and number theorists, as a proportion of mathematics and mathematicians, is indeed quite big. Certainly I wouldnt call it small; if you want small, you start specializing. The number of people working on the applications of tensor triangulated geometry for the sake of p-adic hodge tbeory is probably quite small.
 
1:35 AM
And ahve gottne a lot of advice
 
@Mike Sorry, I don't have access to MathSciNet or anything like that
@Julian ok, how about algebra?
 
Not to butt in, but I'd also agree with @Pedro. :)
 
abstract @user111
@user1111
 
@AlexWertheim I love pomeranians.
 
do you know abstract algebra?
Sanath's website seems to show that he's teaching something like that
 
1:37 AM
@PedroTamaroff: I love dogs. But I confess, this was actually a friend's choice of avatar. She had great amusement in picking it. =P
 
and i presume you go to the same school?
 
@AlexWertheim Though I just like the ones with round heads. Not the chihuahua looking ones.
 
@Mike ah, thanks
i never saw that before
@Julian
 
@user1111
yes
 
1:38 AM
WHat about that thing that Sanath is doing?
 
that seems valuable if you don't know abstract algebra
@Pedro That's cute
 
Awwww. What do you think of samoyeds, @Pedro? Now that's my kind of dog.
 
@user1111 Actually, this is more direct. Note that it counts people more than once. Compare that against a count of total memvers if you can find one.
 
1:39 AM
@AlexWertheim Those look pretty majestic.
 
@user1111 What does that mean?
 
I like German shepherds for majestic. Or collies.
 
@Julian He seems to introduce abstract algebra from the basics
quoting his website:
 
They are. :) I had one for many years.
 
"introduction to group theory, ring theory, vector spaces, and modules"
@Julian
@Mike as you go down the line the numbers kinda start to decrease :-P
@Julian hello?
 
r9m
1:43 AM
@PedroTamaroff Anyone who'd want to have those for dinner are simply monsters ...
 
Anyone available to clarify some language regarding sigma-algebras for me?
 
@Arkamis Perhaps.
What is troubling you?
 
@Arkamis Can't guarantee anything, but i can try
 
the following wording from Folland:
 
@Julian are you still on are have you left?
 
1:45 AM
"If $\mathcal{M}$ is the $\sigma$-algebra generated by $\mathcal{E}$, then $\mathcal{M}$ is the union of the $\sigma$-algebras generated by $\mathcal{F}$ as $\mathcal{F}$ ranges over all countable subsets of $\mathcal{E}$."
 
@user1111 sorry had to do something
 
@Julian no probs
 
@Arkamis Yes.
 
i havent done that yep as to what you ahve described
 
What is meant by "as $\mathcal{F}$ ranges over all countable subsets of $\mathcal{E}$?"
 
1:45 AM
so what about sanath's thing?
 
isnt that done in topology?
@user1111
 
abstract algebra?
 
he does homotopy theory
 
no, it's not done in topology
@Julian I can see that from his website
 
Does that mean $\mathcal{F}$ is a family of sets, containing some number of countable subsets of $\mathcal{E}$?
 
1:46 AM
but abstract algebra is not done in topology
 
'And what about abstract algebra? this as a research fireld?
 
@Arkamis No.
 
rather, they combine (kinda) in homotopy theory
 
I mean, I know what the theorem is trying to prove, but I'm stuck on the language.
 
no, it is not a research field
 
1:46 AM
yes I know @user1111
 
at least not as much as you might expect
i mean, there's (non)commutative algebra and stuff
 
It means $\mathcal M$ is the union of all the $\sigma$-algebras of the form $\langle S\rangle$ where $S\subseteq E$ is countable.
 
So waht are we trying to get at?
 
@Arkamis They mean that F is some collection of countable subsets of E
 
I sort of dont want to copy sanath
 
1:47 AM
but basic abstract algebra, as he's teaching it, you can't do much research in
@Julian I understand what you're saying
 
I know that, @PedroTamaroff, I'm curious as to what the term "ranges over" refers to
@KevinDriscoll Ok, that's what I thought.
 
@Arkamis It means "sweeps through" or any equivalent visual aid.
 
Sanath seems like a nice and hardworking guy but the way he reacted above seemed kinda harsh and protective
@Julian That is not to say he does not seem like a nice guy
 
@Arkamis For example, $\Bbb N$ is the union of $\{i\}$ as $i$ ranges through the natural numbers.
 
But he works in homotopy theory, and homotopy theory contains a lot of every field
 
1:48 AM
Well, a $\sigma$-algebra generated by a set doesn't make sense unless you're implicitly either talking about a set of sets or the powerset of that set
 
@Julian
So the question is
 
@user1111 yes I understand. He can be a bit discouraging, but he still motivates me. :)
 
@Pedro Have you solved the $S^1$ problem?
 
@MikeMiller In part. I was doing something else.
 
Yes? the quesin?
 
1:49 AM
do you want to do math with intuition
or do you want math with symbols?
homotopy theory is kinda a combination of the two
 
thanks, @PedroTamaroff and @KevinDriscoll
 
i should know, i study htpy theory
 
I do like both
 
@Arkamis If you need strict notation.
 
@Arkamis Well, F is the subset and then that generates the sigma algebra
 
1:50 AM
i mean i do what bothe @user11111
 
I think
 
$\langle E\rangle =\bigcup\{\langle S\rangle :S\subseteq E,S\text{ countable }\}$, @Arkamis.
 
I do like symboles
 
@Julian then you can do algebraic geometry, but homotopy theory contains algebraic geometry
 
@PedroTamaroff I'm not stuck on that -- I know what the theorem is saying. I know what the statement means.
 
1:50 AM
like derived algebraic geometry
which is what it seems like sanath is doing.
 
@Arkamis Then what is bothering you? I'm confused.
 
@PedroTamaroff The term "ranges over"
 
I like it! But i have ALWAYS had an interest in category theory ever since
@user1111
 
@Arkamis What's your mother tounge?
 
why category theory?
@Julian
 
1:51 AM
I wanted to be sure it meant what I thought it meant.
 
Please be Spanish please be Spanish
 
@PedroTamaroff Terrible english.
 
Oh, OK.
 
you don't know abstract algebra but are interested in category theory?
i'm confused
as heck
@jUlian
@Julian ever since when?
 
I like the diagrams?
LOL
 
1:52 AM
that is not a reason! :-)
 
sorry...
 
but honestly, category theory is very abstract
it requires a lot of effort
 
@Arkamis I though a sigma algebra generated by a single set does make sense?
 
ya. I really like abstract concepts
I want to THINK
 
i find it amazing that sanath did that when he was so young
@Julian yeah, i know
but every math field requires thinking
but the thing is that category theory requires you to spend a LOT of time understanding the abstract concepts and translating them to mean something intuistically
 
1:54 AM
Ok. I'll bite. What is so wrong with calculus? Calculus is amazing. It's some of the most beautiful, important math ever done. There's plenty of thinking to be done in calculus.
 
@Alex haha, yeah
i mean you can do things in like analysis or something
like PDEs
and in fact that is what i was going to ask you about, @Julian
Do you have experience with differential equations
@Julian
 
I just don't see the point of fluttering about in topology and analysis without having any idea why analysis and topology are so important in the first place. Which, and this is only my opinion, just seems beyond belief without a thorough, solid understanding of calculus.
 
Yes @user1111
 
@Alex exactly why i think @Julian you should spend some time studying calculus
 
I dont like analysis. and I can say that because I am taking it
 
1:56 AM
why not?
 
Yes. i understand
 
it's beautiful!
@Julian Let me give some advice
you seem to be so very concentrated on doing some research that you don't remember that the most beautiful part about math is the learning process
 
How could you like analysis if you're trying to study it without knowing calculus? Analysis encapsulates calculus. That's like saying you don't enjoy french movies without knowing how to speak French. There's only so much you can enjoy if you don't speak the language.
 
@Alex precisely what I am telling @Julian
 
I have learned calculus
but on my own
 
1:58 AM
@julian introduction
sorry, integration?
 
With the little perspective I have, I have to agree with something someone told me: one should study category theory once one knows a load of examples of concepts of category theory. It makes no sense to learn what a projective object is in a category, or what a terminal object is, or what a universal property is, or a kernel or an equalizer if one hasn't bumped into the obvious examples of such things in the practice.
5
 
some @user1111
 
@Pedro I love you. That is great. @Julian @Pedro's comment is exactly why you shouldn't want to do category theory simply for diagrams (im joking here)
@Julian Can I give you a question?
Additionally category theory simplifies many constructions as special exmples of things
for example (co)products as (co)limits, etc
@Julian Hello?
 
IDK anything about category theory. sorry. only an overview'
@Pedro thanks for the advice
 
@Julian Can you summarize what it is, then?
I mean, you shouildn't even be worrying about categoyr theory at this point in your education
it's a waste of time right now
Can I ask, slightly unrelated, on why you have a math genealogy profile? You are learning calculus!
 
2:03 AM
it has something to do with a collection of objects and arrows having morphisms, homeomorphisms, and etc. acting on it?
 
@Julian
@Julian what's a homeomorphism? Do you know?
 
@user1111 I am trying to get it removed. I did not know how professional it was until later
@user1111
 
ok
but why was it made?
 
Guys you needn't ping each other all the time.
It is basically you two in the conversation at the moment.
 
Haha, yeah
i know. it's just fun though
jk
i'll stop
Ok @Julian (last ping) so what is a homeomorphism? Just want to know if you know what it is
because though you used it in the right context, knowing calculus does not mean you know what a homeomorhpism is
@Pedro and did i say i was a guy? :-)
 
2:08 AM
Pedro solve the $S^1$ problem or you're in trouble
 
@MikeMiller There's nothing to prove.
Done.
 
"Is trivial!"
Not acceptable
 
@user1111 Sadly, i dont
 
you do not have to be sad
it is not necessary in the first place for you to know what a homeomorphism is at this point
but can i give you an integration question?
 
that is true
 
2:11 AM
@MikeMiller Tomorrow noon I'll give you a solution.
 
Noon what time?
 
OK, closer to night. Like 6 or 7 pm. Or something.
 
3:02 AM
I have a question. Perhaps a silly one.
I was looking at this question math.stackexchange.com/questions/1087999/a-riddle-for-2015 , and I found myself asking: can every number up to 9! be written in that way?
 
3:16 AM
up to 1+9!, I mean.
 
Probably not 9!-2.
 
 
3 hours later…
6:43 AM
@robjohn Thanks!!+1
 
@Gato You were able to show density?
 
7:26 AM
I'm having trouble showing that a vector space cannot be the union of three of its proper subspaces. Can anyone nudge me gently in the right direction?
 
@Miguelgondu Hey! I'm the guy who did that computer search in the question you linked.
The answer is a resounding NO, there are tons of numbers between 1 and 9! that are not expressible in that form.
I mean, it is clear that no irrational number can be written that way. But even if we limit ourselves to just the integers, many of them are unattainable.
The first unattainable positive integer is 2284, after which there are 348862 more until 9!.
So 348863/362880 ~ 96.1% of integers between 1 and 9! inclusive cannot be written that way.
It might interest you to know that among the numbers which are attainable, there is enormous variation in the number of ways to express each one.
As shown in my post, there are 536 ways to obtain 2015. That might seem like a lot, but there are a staggering 648,265 ways to obtain 0, and 184,681 ways to obtain 1.
 
Hello @JayeshBadwaik
 
@Fargle You need some more conditions. As is, that's false.
 
@ABeautifulMind Hello, howz it going?
 
@JayeshBadwaik My mental problems have worsened. I am giving myself a few more years to get well and go to grad school.
 
7:39 AM
@Miguelgondu If you try to plot the distribution of the number of way to express each number, you get some rather interesting figures.
 
@ABeautifulMind Sorry to hear that. I hope you get better.
 
That plot includes the full range of numbers which can be produced. You can see that points tend to cluster around the origin, with 0 and 1 huge outliers.
If I zoom in a bit closer and show only numbers between -1 and 1, you can see the behavior a bit more clearly. Most numbers can only be produced in a few hundred ways.
 
How'd you make that figure?
 
@MikeMiller Er, rather, a finitely generated vector space over an infinite field.
 
@Fargle Infinitely generated won't work either, but infinite field is key. (Any $\Bbb F_2$-vector space of dimension greater than 1 can be written as a union of precisely 3 proper subspaces.)
 
7:43 AM
@MikeMiller These were all done in Wolfram Mathematica, after I had finished the original search.
 
@DavidZhang Sure, but I was more curious about the code that produced it. Did you bound the denominator of the numbers in your plot below?
 
No, I didn't do anything like that. There are only a finite number of rational numbers which can be produced in this fashion, so I literally just produced them all and counted them up.
If you'd like to see it, I still have the frequency table stored as a text file.
 
Sure, that'd be cool. I enjoyed your post.
 
It's about 5MB compressed. Can I upload files through Math.SE chat?
 
Only images. My email is in my profile, if that's most convenient.
 
7:56 AM
@MikeMiller Sent.
 
8:33 AM
The idiot who downvoted my posts on main every time I opened a meta post apparently deleted his account, I feel like I should celebrate
Is there a way to see what accounts were deleted recently?
 
@NajibIdrissi Ah, it was my anti-Najib alt. They finally caught me.
 
Ah, apparently "undownvote" in this manner actually means that the serial downvoting was caught, but there weren't enough questions affected to merge it as "serial voting reversed"
Seriously though, what do these people feel they're achieving? It's childish.
 
Well, they're trying to condition you. They suspect that if they downvote you when you make meta posts you'll stop making meta posts. It's silly, of course.
 
Hello @MikeMiller.
 
morning
 
8:45 AM
It just makes me wonder which of the people who usually disagree with me are mentally 5yo
 
Most people stop growing mentally at 20.
 
@ABeautifulMind who said that ?
 
@Ramanewbie I.
 
^^ are you sure then ?
 
No, we are never sure of these things.
 
8:52 AM
@ABeautifulMind can't scientist determine that ? And how do you know it's 20 ?
 
@Ramanewbie Just my gut feel.
 
where to draw a curve online please ?
I mean, you give the formula and it draws it.
 
@Ramanewbie there's a pretty nice online graphing calculator at desmos.com/calculator
 

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