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00:00
so if $f$ is the homeomorphism use the metric $d(x,y) = |f(x)-f(y)|$
00:37
Get a homeomorphism from $\overline{\mathbb R}$ to $[0,1]$ (such as $\dfrac1{1-e^x}$) and use the metric on $[0,1]$.
($y=\frac1{1+e^x}$ is rotationally symmetric around $(0,\frac12)$, I believe. Not that it matters much.)
*I meant $\dfrac1{1+e^x}$ in the first thing.
01:32
@Disc Can you help me understand the fundemental theorem of finitely generated abelian groups?
what is there to understand? @Incurrence is there something lacking in the things you read. It just classifies them
I understand for finite groups
But for infinite groups I have trouble
What in particular?
I suppose the cyclic nature of them
That means nothing to me
01:35
So if a subgroup is generated by some elements, we say that we take every word in the alphabet of those elements
Which reminds me of the free group
And the definition of a finitely generated abelian group is that there exists finitely many elements in the group, such that every element in the group can be written as some linear combination of them
If this is the case, we say that these elements are the generating set
So if we want to generate $(\Bbb Z, +)$ we can just take linear combinations of $n_1 \times 1$ where $n\in \Bbb Z$
So $1$ is the generating set of $\Bbb Z$
If I wanted to setup a generating set for $\Bbb Q$, I am not sure how I would go about this. I know it will fail, but pretty much I want to setup $x_i\in \Bbb Q$ to fail it. I am just choosing the denominator as coprime values right?
And hence I will be able to inductively add elements to this generating set forever
Hence there is no generating set
Every group has a generating set
Not necessarily finite though
Okay, so what is the generating set of $(\Bbb Q,+)$?
Generating sets are not unique
What is one form of the generating set of $(\Bbb Q,+)$?
$\{1/n \mid n \in \Bbb Z_{>0} \}$, $\Bbb Q$, the set of all rationals less than 1, etc
01:45
Okay
Thanks, I'll go work on that. I think I'll stop asking questions here until I have spent a few days on this
I think I was underprepared for this part of the course
Were you able to figure out the direct product thing
user143442
@DiscipleofBarney I don't understand what you mean with 'pull the metric'
Nope
I couldn't get it to split across the second monomorphism or the first epimorphism
(for the direct product)
@user You since the space is homeomorphic just treat it as if it was $[0,1]$, and use the metric there, I gave you an explicit example
But I'll go do more textbook
Actually I'll take a break from Algebra and do some complex analysis and functional analysis, talk later guys
01:50
Okay, maybe mess around with more explicit examples., see yah
user143442
02:08
@DiscipleofBarney so if $f:X \to Y$ is an homeomorphism and $X$ is metric then $Y$ is metric too?
user143442
I've been looking for that result and I think it's not true
morning
Evening, @MikeM :)
how's things
Better than yesterday - can manage to eat dinner tonight. You?
02:20
Are you guys gonna eat together
seems unlikel
he had the stomach flu, hence the comment
that's good to hear, @Alex
Not unless Mike has a teleportation device, lol.
finishing up this talk
Cool, what on? I'm excited to actually get to come to some of these next year.
I thought you were chatting on here while simultaneously giving a math talk right now lol
02:24
lol @ᴇʏᴇs
@AlexWertheim The title is "The dimension barrier in topology"
the diff between low dimensions and high dimensions
I'm thinking strongly about making this into a blog post
Ah, I believe I was here when you were describing it before. Good stuff.
MSE blog, you mean? Or do you have your own?
@MikeMiller I would subscribe, like, follow and read your blog every day
I don't have my own, I would make it
OK, I'm done
The punchline: why is high dimensional topology easier than low? because 2 x 2 < 5
That'd be cool. I bet a lot of people here (myself included) would enjoy that.
LOL. A deep theorem.
the only downside is I have to draw to write this post
there's no way to get around it
02:28
You're not an artist like Ted, @Mike?
If I'm a topologist, I have to be... but I'm not so much
You can't use TikZ?
Nope
I don't know how
The other issue is that doing this properly takes explaining handlebodies, which take a couple months to actually grasp
So I will probably lose some folks when doing that in 5 minutes
02:30
Nice, a couple months worth of blog posts
All the more reason to start a blog!
Hahaha
Lol @ᴇʏᴇs, same thought :)
At least, it took me a couple months to grasp
And by grappling lots of examples
You should post an example every week or something
02:31
It's not that exciting on its own sadly
Explaining hard things well is always exciting, especially if they build to something cool.
Fair enough!
Okie doke, done
02:52
You're done writing the blog up already? That was quick! (I kid, I kid)
03:37
Anyone have a closed form for the generalized harmonic numbers in terms of special functions (like digamma)?
22
Q: Do harmonic numbers have a “closed-form” expression?

SrivatsanOne of the joys of high-school mathematics is summing a complicated series to get a “closed-form” expression. And of course many of us have tried summing the harmonic series $H_n =\sum \limits_{k \leq n} \frac{1}{k}$, and failed. But should we necessarily fail? More precisely, is it known th...

In mathematics, the n-th harmonic number is the sum of the reciprocals of the first n natural numbers: This also equals n times the inverse of the harmonic mean of these natural numbers. Number n such that the numerator of is prime are 2, 3, 5, 8, 9, 21, 26, 41, 56, 62, 69, 79, 89, 91, 122, 127, 143, 167, 201, 230, 247, 252, 290, 349, 376, 459, 489, 492, 516, 662, 687, 714, 771, 932, 944, 1061, 1281, 1352, 1489, 1730, 1969, ... (sequence A056903 in OEIS) Harmonic numbers were studied in antiquity and are important in various branches of number theory. They are sometimes loosely termed harmonic...
 
2 hours later…
05:21
@Disciple are you online?
playing team fortress 2 at the moment, whats going on?
i have a doubt....@DiscipleofBarney
You know you could just say what your doubt is... @Rememberme
i have to prove that AX=Y will have solutions if and only if the row rank of A and the row rank of the augmented matrix are equal@DiscipleofBarney
@Disc i was thinking that the augmented matrix will be m*n+1
so the row space will be $\beta=(\alpha_1,\alpha_2,.......\alpha_m)$ of the matrix A
Now since the augmented matrix also contains m rows i was thinking will the row space be the same?@DiscipleofBarney
06:04
can someone please check that i am right or not
I would, but I don't know much about matrices :v
you know about ranks etc@Regret
nope! can you explain it to me?
What do you mean the same? they have a different amount of coordinates in the space @Rememberme
but it also contains m rows right @DiscipleofBarney
06:08
@Rememberme When you augment a matrix, you add a coordinate so they can not be exactly the same
so how should i prove it....what i thought was the augmented matrix will also contain m rows
@Rememberme could you give the verbatim question, I don't think yours makes sense, or is wrong
AX=Y Prove that this system has solutions iff the row rank of A is equal to the row rank of of the augmented matrix of the system@DiscipleofBarney
@MikeMiller: Please do it! I've always been astounded at the claim that 5+ dimensions is easier than 4- but I'm a course or two short of getting into technical details. I'd love to see your exposition :)
Hi@EricStucky long time no see
@disc according to this is what i am saying that the rows of the augmented matrix will be the same as that of the matrix A
06:16
Where is it in Hoffman and Kunze
Hoffman Kunze
Where in Hoffman and Kunze @Rememberme
Vector spaces:- computations concerning subspaces
last question@DiscipleofBarney
@DiscipleofBarney I think what i am doing is right the number of independent rows in the augmented matrix will be the same as the number of rows in A and due to this the row space will be the same and hence the row rank
That is what you are suppose to prove
Except they are not the same exact space
i am supposed to prove that the row rank of A is the same as the row rank of the augmented matrix and i do that by saying that the number of rows in both of them are same .....AM i right @Disc
06:31
No, because they have the same number of rows when there is no solution
SO how do i prove it@Disc
I am in the middle of playing a game, I can't think of exactly how I would go about it. I would probably reduce to row echelon and when something is not a solution you will have extra stuff on the zero rows, that shouldn't be there making the rank of augmented larger. I would have to put more time than I care to to make sure that actually works. @Rememberme
@Rememberme If the augmented matrix has a higher rank, then it is linearly independent, and hence there is no solution
Hi @Will
@Incurrence Hi Alex.
@Incurrence i am saying that the rows are equal in both of the matrix is it right?
06:44
@Rememberme All you know is that the number of rows are the same, yes.
@Rememberme I don't know what this means, and I have to leave in a few min
yes @WillHunting
The number of rows are the same yes
You have an additional column
@Rememberme Your sentences do not make sense at all. I think you are still not clear about writing proofs in general and should return to Hammock.
For a standard [xyz|a] augmentation
If this new column is dependent on the others, it is solvable and the rank remains the same after augmentation
if the new column is independent the rank will be increased, but it will not be solvable
Okay I have to go now, cya later :)
06:46
cya
@Rememberme Reading these three lines, I do not think you are ready to study linear algebra yet.
@WillHunting you said you wont make any comments about what i do but still you are .... and infact i did hammock and please dont interfere
@Rememberme Sorry, I won't interfere then, goodbye.
@Rememberme Figure it out?
@DiscipleofBarney I think that if you want to help him, you should comment on what he originally wrote. The first line already does not make sense.
06:59
@disc i really cant
@Rememberme Okay just wondering. You should figure it out.
@WillHunting Are you talking about the rows on the matrices being equal?
@Rememberme I am sorry if my words have hurt you. I won't talk to you again. Thanks for your encouragement.
If the row rank is more what happens does it mean there is one row extra
@DiscipleofBarney "i was thinking that the augmented matrix will be m*n+1" this already does not make sense.
@WillHunting Yah
07:01
@DiscipleofBarney "so the row space will be $\beta=(\alpha_1,\alpha_2,.......\alpha_m)$ of the matrix A" this also does not make sense.
i mean the augmented matrix will m by n+1 that means m rows and n+1 coloumns
@DiscipleofBarney So there is no point in going further.
"since the augmented matrix also contains m rows i was thinking will the row space be the same" again does not make sense.
@DiscipleofBarney it does make sense that what hoffman kunze says
@Rememberme You should acually write out what row rank is, do some examples, and then do some examples with augmented matrixes, it seems like you have some misconception that points to you either not spending enough time/ wrongly convincing yourself you do understand.
@Rememberme the problem in Hoffman and Kunze does make sense, I was just reading it wrong.
07:04
See i told you
i am not making sense to you
@WillHunting What brings you back? Once you go math chatroom you cant go back? :D
@DiscipleofBarney Well, no idea. =) But I think if you really want to help him, you should encourage him to study math properly, and I don't think he is and if he continues like that he will just be wasting his time.
I am studying math properly others can understand what i say why cant you @Will hunting
@Rememberme I already gave you an example of what you are writing wrongly.
@Rememberme Others do not set high standards for their students, maybe. I do.
i am just saying that an augmented matrix is an m by n+1 matrix is that what you mean i am saying wrong@WillHunting
07:09
@Rememberme Before we can make sure you understand, we need to be sure you are clear, and being clear means writing down every sentence clearly.
It is very important to be precise in math. That day when you talked about n points in a plane, we don't even know what you were asking.
@Rememberme Yes, just one example. The following two sentences also do not make sense like I said.
and what i think row rank is the dimension of the row space of a matrix A now this is wrong @WillHunting
What you were saying wasn't making sense. Maybe I am just feeling extra grumpy, but I also think you put an inconsiderate small amount of thought in some of the questions you ask, solutions you give, and how you ask them. I basically have to pry every detail out of you. @Rememberme
@Rememberme What I am saying is that first you need to express your ideas clearly in writing.
But what i say is right it is an m by n+1 matrix
@Rememberme Yes, that is right, but different from what you originally said.
07:12
@Rememberme The row rank is the dimension of the row space, I am not sure though you understand what the row space is, at least from some of the things you were saying.
@Rememberme So first you must write your sentences clearly before asking for help.
@Rememberme Yes, that is also right.
Just because two matrices have the same number of rows does not mean their row space are equal, or even their row rank @Rememberme
yes i understand the row space it is the space spanned by the vectors
$(\alpha_1,\alpha_2,.......\alpha_m)$
where
$\alpha_1=(A_{i1},........,A_{in})$
now i think i am right @DiscipleofBarney
How about you right up a full proof @Rememberme
@Rememberme Yes, you should write up a full proof and then ask. Then it is clear whether you know or do not know.
07:18
fine let me write.....but tell me something first if the augmented matrix has a row rank more than A then what does it mean does it mean that there is a row with all 0's and one 1 or something else @Disc
@Rememberme Again, it is not clear what you are asking here.
okay let me write a proof straight away and then see what happens
@Rememberme It means it has row rank more than $A$... At least how you phrased the question, it does not mean what you said. As Will says, I am not entirely clear as to what you are saying. You should be able to figure whatever it is out though
@DiscipleofBarney I don't play any computer games.
@WillHunting I don't play to many games, I have only recently started getting into playing computer games again, and I don't even play that much, couple hours a week... maybe
07:30
@DiscipleofBarney i have got a brilliant idea for the question(at least in my perspective)
2
Hey guys, I'm trying to prove that $\{B_r(x) : r \in \mathbb{Q}, x \in \mathbb{Q}^n\}$ is a basis for the euclidean topology on $\mathbb{R}^n$ but I feel like this is one of those problems where I just don't know the trick I'm supposed to know to easily solve the problem.
Well, fully prove it using that brilliant idea if you can @Rememberme
Any advice?
$B_r(x)$ is of course the open ball of radius $r$ around the point $x$.
the idea is to do this thing in two different cases that is first if i say that AX=Y and the this matrix Y is a matrix has all zeroes in its column then the augmented matrix will be just the matrix A.
Second case:-
If the matrix Y contains some non zero values lets say then i can show that these two matrix are row equivalent to each other and if they are row equivalent to each other then voila!!!!!!they have the same row space right @DiscipleofBarney
I can do similar problems, but I just can't come up with a nice way to deal with the fact that the important values need to be rational
07:37
Am i right @DiscipleofBarney
like if I have an open set $U$ then I want to pick an arbitrary point $x$ in $U$ and show that I can fit an element of my supposed basis around it which is entirely in $U$, so I'm trying in particular to squeeze this basis element inside of the particular $B_r(x) \subset U$ which I already know must exist, but doesn't necessarily have a rational $r$ or $x$.
@SamuelYusim The idea is you can put rational points arbitrarily close to a real point, so any ball around a real point you should be able to put a rational point so close that some ball will be contained in the ball around the real point and contain the real point. (although it sort of sounds like you get this)
honestly I might have figured it out from just typing my question out
Thats good, I have done that quite a few times @SamuelYusim
@disc am i right????????????
07:45
@Rememberme Didn't you read what I said: fully prove it! You will see what is right or wrong when you do that, maybe you are half right and half wrong, or onto a pretty good idea but doesn't work, maybe it is spot on... It is good to understand your full proofs, if you made and actually good proof you wouldn't need my confirmation. I don't want to read all your half-assed attempts
Plus I have already explained why that doesn't exactly work
to you
@DiscipleofBarney Why aren't you sleeping yet? It's late there.
Where am I? @WillHunting
@DiscipleofBarney I thought you were in the US.
Don't worry, I don't check people's IP addresses, LOL.
Haha, that is what @Incurrence did
Why did he do that?
That's a little scary.
07:54
I had been screwing with him, making him think I was in one of his classes and he tricked me. He was like "Can you check my blog post" and then his stats showed him where I was. I did say it was a VPN though...
Oh OK. I don't use a VPN these days, even though many sites are blocked here.
Anyway I really should not come here, bye.
@DiscipleofBarney there goes my proof
It didn't work ?
And why?
it did work and it s right i am sending you a pic
there we go
@DiscipleofBarney
@DiscipleofBarney how is it............. fine? i feel it is
i have showed that it would be inconsistent is it okay#@DiscipleofBarney
@Balarka please check my proof
08:15
@MikeMiller I think it really does prove it for in general $M$.
@Rememberme I don't even know what you are trying to prove.
@Rememberme It looks pretty good, I don't think claim one is necessary. More detail in case three would be nice, and I don't think case one is necessary
thanks so i am right.....
thats what i was trying to tell you from the first @DiscipleofBarney
By more detail, I mean expand on how certain things imply certain things (even if they are quite simple). Yes @Rememberme It doesn't sound much like what you were trying to tell me, in fact it basically looks like what I told you.
You have it though, if you think you understand it all and how each step is done then it is good @Rememberme
@Rememberme Can you please define right now clearly, what does rank refer to?
@Incurrence Hey. Did your analysis? And when is that project for algebra?
due
08:25
@DiscipleofBarney Analysis study was alright. I was just recovering heaps of the basics to make sure I don't have any hole in my understanding(did first 50 pages of my complex text)
Project for algebra is 5 questions(with many subparts) + the presentation
And that is due on Wed 6th
Okay, so about a week
what is going to be the topic of the presentation?
@BalarkaSen Central extensions
08:27
What is the other one you are doing (free groups, or braid groups I think)
With an example of $H_3(\Bbb Z)$
you know what group (co)homology is? cool!
@DiscipleofBarney We only have to present one, and then prepare another for unknown reasons, and I am learning some of all of the topics
@BalarkaSen I do not
Its the Heisenburg group @BalarkaSen
:p
oh, blah.
08:28
I will in 6months :)
I'd have done free groups and braid groups if I were there, but that'd involve alot of non-algebraic stuff.
Its a five minute presentation
@Incurrence Good for you, 'cause I've never studied group co/homology.
"Topics from groups, rings, fields, algebraic number theory, category theory & homological algebra, with applications to quantum algebras. Offered in odd numbered years only."
Although I probably would have done free group or, the tensor products of groups
08:31
i don't even know what tensor product of groups are.
"Algebraic structures & their representations of importance to current mathematical physics research: Lie algebras & superalgebras; quantum groups & algebras; Hopf & quasi-Hopf algebras; affine & Kac-Moody algebras. Illustrative applications to knot theory."
@BalarkaSen You don't want to
what the heck is it about? we already have a symmetric monoidal structure on $\mathbf{Grp}$.
When I take algebraic topology next semester, it is a 6xxx course, where the first number usually refers to the year you are in :-)
@BalarkaSen The definition is as follows:
"the condition or quality of being sad." - Tensor product of finite groups( aka sadness )
08:33
Well the tensor product of non-abelian groups is very local
They look interesting, don't really know anything about them. Looks like semidirect with two actions. I guess they came up in some higher dimentional van Kampen theorem.
And since I don't know pretty much any module theory, it was inaccessible to me[only because I can't appreciate it in that form, which is better documented by far]
no, no, i don't think that's ad hoc at all.
$\pi_3(X; A, B) \cong \pi_2(A, C) \otimes \pi_2(B, C)$ looks very interersting.
I can't see where this $\pi$ map is defined
Seems to be a group action?
08:36
it's not a map : it's a functor $\mathbf{Top} \to \mathbf{Grp}$
google homotopy groups
Ok
Wait is this going to lead me down a rabbit hole
yep
:p you'll learn about them when you do algebraic topology nevertheless
So it refers to the third homotopy group
Hey @Incurrence.
Hi @lliW
Gotta go
08:39
@Incurrence Are you busy now? Shall we talk in the other room?
i thought the best one could do for higher homotopy groups is homotopy excision.
Girlfriend needs me to weigh some rocks lol
@Incurrence OK, bye.
Doing sample testing for her thesis
@DiscipleofBarney I consider case 1 proof wrong though.
08:43
@WillHunting Formally it isn't great, and maybe there is some misunderstanding on rm's part
But I think I should keep my mouth shut. He doesn't like me, LOL.
he probably didn't do the proof-writing bit of Hammock, thus the horrible logical reasoning.
I think I can't read what he writes anymore, so maybe I should press ignore.
I was laughing when a few days ago you (I am guessing it was you) when I mentioned him supposedly completing Apostol but either expressing himself terribly or did not know about "density" and he was like "who starred this"
I am not sure what that question about n points was.
Maybe he will say two different things and then say they are the same, and after that you don't know what he wants to say.
There is no point trying to guess what he is thinking.
08:49
I am not sure either, I feel like we ended up figuring out what it was, but I am not sure.
It's very dangerous to guess his thoughts and say they are right when they are actually wrong.
i feel like i don't care about his questions anymore :p
At this stage, we must be strict and say that case 1 above is wrong.
I have been having a similar feeling
A matrix with a column of zeros added is no longer the same, and why should the rank be the same immediately?
08:51
i was doing this from the very beginning
If you want to help, be thorough. Otherwise, don't say anything and do more damage.
@BalarkaSen Answering questions?
I don't think someone who has been through Hammock and Apostol thoroughly will express himself like that.
@DiscipleofBarney no, i mean, being strict while checking his proofs.
Especially after doing all the exercises, which I seriously doubt.
08:54
@WillHunting Probably right. Past few days my caffeine levels have been a lot lower than usual, and my patience is thin
It's my bad but I can no longer tolerate his mathematics.
And I already told him multiple times it didn't make sense, so I just didn't care anymore
Who are you guys talking about?
@Regret You
08:55
Oh no. :v
He seems to take it badly though, so I should just stfu.
I think he should ignore me too. I don't wanna upset him either.
I think maybe he thinks my math is not good enough to help him too, LOL.
Maybe I am the one being stupid and not getting what he says.

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