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00:56
How do you do on latex the differential equations notation when they use dots above the x's to indicate the order of the DE. Like $x^{..}$ but where the dots are aligned on top of the $x$?
anyone here
@user276387 that's usually known as 'fluxion notation' (going back to Newton) and it's done as $\dot{x}$ for one derivative and $\ddot{x}$ for two
not sure if $\dddot{x}$ works---oh, yep---but past that fluxion notation isn't as convenient as $x^{(n)}$
anyone know if it's legal to split the inequality for
$x < \frac{x+y}{2} < y$ ?
cuz if it's not, there goes my proof :(
what does 'split the inequality' mean?
like in two parts
01:08
@Semiclassical Thank you! I looked everywhere for that!
$ x < \frac{x+y}{2}$
$\frac{x+y}{2} <y$
well, yes. $1<2<3$ means both $1<2$ and $2<3$.
my original question is
If $ x < y $, prove that $x < \frac{x+y}{2} < y$
And by Axiom 8
$ x < y \rightarrow x+z < y+z$
@usukidoll: The only way to prove (or interpret) such a thing is as two separate statements
01:09
OH wait dang no scratch that. I don't think I can split them up T_T
Yes you can
hi @TedShifrin
hi Karim
I am enjoying algebraic topology btw
ohhhh yeah I could
01:10
very nice
but hard though haha
It's been forever since I've done that though, so I was wondering if it was still legal
no comment, Karim
btw @TedShifrin
I found this channel very nice
I wish there was other channels like this all over youtube that would be so nice
01:13
ah! I think I know where this is at

starting at $ x < y$ and by Axiom 8 which is $x < y \rightarrow x+z < y+z$
so there must be at least 2 unique z's to create these end results
$x < \frac{x+y}{2}$ and $\frac{x+y}{2} <y$

so if I start at x<y for the first case I need z = x
if I start at x<y for the second case I need z = y

therefore

first case
x+z <y+z (since z = x)
x+x < y+x
2x<y+x
and then dividing 2 by both sides I should get
$x < \frac{x+y}{2}$
second case
x+z < y+z (since z = y)
x+y <y+y
x+y < 2y
dividing 2 by both sides I should get $\frac{x+y}{2} <y$
I don't know who he is, Karim.
he has some nice set of videos on advanced math
which is nice
therefore
$x < \frac{x+y}{2} < y$
@Semiclassical I only saw your comment after I typed I couldn't split them up, when actually it is possible with that example you gave me.
So I see ... Presumably he knows what he's talking about. He appears to be a graduate student at George Washington U in Washington DC.
01:17
is it good uni ?
Not known for its mathematics ...
more for law, I think?
yeah, maybe social science
i think the following is just a reordering of the proof given above, but you can also do the following
$x< y\implies x-y<0<y-x$ by Axiom 8 with $z=-x,-y$
Karim: For what it's worth (probably very little), I've heard of zero of the faculty members in that department.
01:20
then adding $x+y$ to each term preserves the inequalities, and dividing by two gives the desired results
Clever, @Semiclassic.
eh, only difference is that i subtracted rather than adding $x,y$
so it's not a reordering, but it's really no different
well, you made it into one simultaneous proof.
eh, i guess. but you could also say the first part of the other proof as $x<y\implies x+x<x+y<y+y$
so mine is actually longer :p
01:24
@Ted: Were you here for the involution question? I forget.
Yes, @MikeM ... I made the irrelevant submanifold comment.
Ah, right.
going back to what i was talking about during that conversation
@ted do you know much about Clifford algebras re: K-theory?
I know nothing.
figured as much, but thought i'd ask.
01:25
Generally it's cool to figure I know nothing :)
hah. i think the problem for me is that the physics usage of K-theory is not the same as the obvious math one (i.e. topological K-theory)
there is many seminar going about those stuff here @Semiclassical
but it way over my head
I want to get into it over the summer though
what i'd like to be able to appreciate is the following
there's a table which you see bandied about in discussions of topological condensed matter systems, a so-called 'periodic table' for topological insulators and superconductors
01:29
@Semiclassical: I think it is, because tgere's a physics student who's been asking me about topological K-theory for a talk he's giving about the classification of topological insulators.
oh, i don't mean they're not related.
But I mean I'm pretty sure his talk is to actually give the classification and he's been asking me about bundles and not about Clifford algebras.
the table lays out, in various dimensions, the kinds of topological invariants (usually Z- or Z/2Z-valued) which go with the 10 'classes' of such systems
Good night, all.
night, @ted
and that seems to be about the fact that one can classify 3 real Clifford algebras and 7 complex ones
is probably the most obvious reference
though i really just work with $d=1$ :P
01:42
very nice graphics
to imagine the CW complex of how we build the sphere
by identifying the boundary of $D^2$ to single point $x_0$ which in the picture the top point of the sphere
@L33ter Now do one for the 3-sphere
any n sphere can be done by same intuition same way
But give me a picture @L33ter
lol
I can't give a picture in 4d
this is from wikipedia btw
Yeah I know
01:53
I guess the n-sphere can be realized as a CW complex structure by attaching a n-cell to $e^0$
where $e^n = \{ x \in R^n : ||x|| \leq 1 \}$
by attaching what I mean is we identify the boundary of $D^n$ to a single point
so $S^n = S^{n - 1} \cup D^n / x \sim \phi(x)$
where here $x \in S^{n - 1}$ and $\phi(x) : S^{n - 1} \rightarrow e_0$
02:16
A nice quote:
"Pigeonhole principle: When you have $n$ pigeons and $n+1$ holes, there is at least one pigeon with two holes in it."
6
Also: "$\log(\log(x))$ is bounded above by $6$."
02:44
@Semiclassical Are you aware of an Abel-Plana-like formula involving cosh? I derived a formula that seems to be not quite correct.
I forget what the Abel-Plana formula is
ah. can't say i know any
02:56
@Semiclassical I derived it using the Boole summation formula, which is like the alternating version of the Euler-Maclaurin summation formula. But I made a bunch of assumptions. That's probably why it's not quite correct.
03:24
hey @Semiclassical
here
 
2 hours later…
05:30
Hey @MikeMiller available ?
drawing picture in lower dimension as a way to understand something is effective
05:59
@L33ter I am here.
good :D I am learning a lot
just want quick question
Ok?
I have to leave, so be quick, please.
Hatcher defn of real projective space is just the all vectors that go through the orgin topologized as the final topology obtained from R with the relation x ~ cx where x is a vector passing through the orgin.
Yes.
$c$ is a nonzero real.
The reason we can restrict to only vectors of norm 1 is that we only go in two opposite direct and so if we have two vectors in same directions we don't really get any new information
so that is why we can restrict to vectors of norm 1
right ?
yeah exactly c is non-zero real
06:03
@L33ter Eh. Well, OK. But more rigorously, $v \sim c \cdot v$ means the equivalence class of $[v]$ consists of two distinct representatives: one of norm 1 and one of norm -1.
But the intuition is right.
oh oke good
I see ok
the reason we can now restrict to only the hemisphere with anti-podal points on the equator identified is because of also any vector that pass through the orgin will also to have to pass through the equator
That's not true...
There are lots of lines through $(0, 0, 0)$ in $\Bbb R^3$ which doesn't pass through the equator of the unit sphere.
hm
why are we allowed to restrict then to the upper hemisphere or lower hemisphere then ?
We aren't. Any line through the origin must either pass through both the (open) hemispheres or the equator.
Anyway, I have to go now.
 
1 hour later…
07:22
Hello ! everyone, would you like to check this integral question. Thank in advance.

http://math.stackexchange.com/questions/1579690/the-value-of-double-integral-int-01-int-0-frac1x-fracx1y2-dx
Huy
Huy
n
I think I have an interesting result now
it is fairly intuitive, but it hasn't been done before
i will show my advisor next week
It seems that American politicians enjoy learning mathematics. For example, we have Sarah Palin and Ben Carson among users.
Huy
Huy
thanks Obama
the Palin profile is funny
"a penchant for going rogue" lol
is there an Obama?
Huy
Huy
07:47
@ForeverMozart: you know some basic topology, right??
Huy
Huy
I have a question
@ForeverMozart I want to ask a topology question
specifically in algebraic topology
Huy
Huy
say that two transverse simple closed curves $\alpha, \beta$ in a surface $S$ form a bigon if there is an embedded disk in $S$ (the "bigon") whose boundary is the union of an arc of $\alpha$ and one of $\beta$ intersecting in exactly two points
07:48
Huy
Huy
I have the following Lemma: if transverse simple closed curves $\alpha, \beta$ on $S$ do not form any bigons, then in the universal cover, any pair of lifts intersect in at most one point
here I don't understand the statement thus as a set $X^n = X^{n - 1} \cup e_{\alpha}^n$, where here the union is taken to be disjoint.
@ForeverMozart It seems that there used to be, but deleted his account: math.stackexchange.com/questions/377211/…
Probably this is a better link: normalhuman.github.io/oldusernames/…
Huy
Huy
the proof for $\chi(S) \leq 0$ starts as follows: note that $S \cong \mathbb{R}^2$ in this case. let $p: \tilde{S} \to S$ be the covering map. suppose $\tilde{\alpha}, \tilde{\beta}$ intersect in at least two points. then, there is an embedded disk $D_0$ in $\tilde{S}$ bounded by one subarc of each of the two lifts.

by compactness and transversality, the intersection $(p^{-1}(\alpha) \cup p^{-1}(\beta)) \cap D_0$ is a finite graph, if we think of the intersection points as vertices.

---

compactness of what is used here? I don't see why this intersection should contain only finitely many
@BalarkaSen maybe you can explain this to me
when you come back
07:52
@L33ter you dont understand disjoint union?
no no
what I don't understand is the following
@Huy what is meant by universal cover?
here the final set is just $X^{n - 1} \cup D^n / \sim$
Huy
Huy
@ForeverMozart: the simply connected cover
i.e $X^n = X^{n - 1} \cup D^n / \sim$ why is that equal to
$X^{n - 1} \cup e^n$?
07:57
@L33ter I think you are taking closed discs and bending them so that the boundary goes to a point, then attaching that point to $X^{n-1}$
so as a set its the same as $X^{n-1}$ with some open discs
(the boundaries of the closed discs are identified with points of $X^{n-1}$, leaving open discs)
@Huy I'm thinking about it. Algebraic topology is my weakest subset of topology.
Huy
Huy
no rush, I'm thinking about it too. I just want to finish this chapter of a geometric topology book and then I'll go and do some real algebraic topology so I can proceed
@L33ter does that make sense?
1 moment reading
ohh
I see
ok good I understand
just one more question
do you know about the projective plane @ForeverMozart ?
so RP^n = S^n / (v ~ -v), where v and -v are antipodal points identified
why are we allowed to restrict ourself to hemisphere with antipodal points identified
08:12
I dont understand the question
Huy
Huy
because of the identification?
when they mention this is equivalent to saying that RP^n is the quotient space of hemisphere with antipodal points identified
why is are they equivalent ?
Huy
Huy
because the other hemisphere is identified with the upper?
so you only need to worry about the equator
ohh I see
oh ok I understand
thank you
Huy
Huy
note that (for n=2) if you draw the equator as a square, you get the fundamental square of the real projective plane and see that it contains the mobius strip and hence isn't orientable (in case you didn't know yet)
08:18
cool
hey huy can you give me some geometric intuition of the following
Then I will very satisfied for today
Huy
Huy
sorry, but I can't satisfy you for today
:S
I have been trying to develop intuition of why we get it like this
I understand the torus issue
like I understand the geometry of identification of torus,projective plane,klein bottle
but trying to understand the geometrical intuition or have it for my mind for genus g orientable surface
@Huy CompCtness of the disc $D_0$
Huy
Huy
hey Mike, you're up early (or still
I assumed, but how is that used?
Poker was long.
Huy
Huy
08:27
hope you won
@Huy: That thing is a closed subspace so compact.
Huy
Huy
so you got more than 1 dollar and 33 cents in your bank account now
If you believe the vertices are discrete we then see that they're a finite set.
Huy
Huy
hm
The discreteness should come from the transverse intersextion.
Huy
Huy
08:29
yes, that I believe
Well, a compact discrete set
Huy
Huy
ah
um
so the compact set we're looking at is actually the set of intersections
Huy
Huy
that makes more sense
ok thanks
We just know it's a compaxt set because it's a closed subset of $D_0)
Huy
Huy
08:31
yes
are you drunk?
lol
probably his c doesn't work
so, he is using x instead
Huy
Huy
@MikeMiller: wait a second. the set we're looking at is $(p^{-1}(\alpha) \cup p^{-1}(\beta)) \cap D_0$, not $(\dots \cap \dots)$, so it's not the intersection points of the curves but the union intersected with the disk. why should this be discrete?
@Huy: The discrete thing is the intersection points of the curves. What's left is then a finite number of points with arcs between them. Aka, a finite graph.
Huy
Huy
08:40
ah, ok
hello, i have a small question please : here http://math.stackexchange.com/questions/1038406/convergent-squence-in-topology
from where we get the idea to use the set A ?
@L33ter What is le question?
from where we get the idea to use the set A ?
If you haven't figured it out already that is.
yeah I figured it out first my question was why is $X^n = X^{n - 1} \cup D^n / \sim = X^{n - 1} \cup e^n$ as a set
08:44
have you an idea @Huy ?
but I figured it out with help of mozart
Okie.
The thing is attaching maps leaves interior of attached disks alone. That's all.
yeah
By the way, they are not equal as sets. Merely in bijective correspondence.
I see
hey @BalarkaSen maybe you can help me with some geometric intuition of the following picture
08:46
@ForeverMozart Herro.
@L33ter What is unclear?
I can't see it
why does it generate those shapes
I see it from the CW complex if we started with a point and keep attaching n-cell
but I don't see why the picture above produce those shapes
Look at the top octagon. Cut off from the diagonal going from the small dot to the opposite vertex. Identify. You'll get 2 torii with big punctures. Then paste along punctures.
You'll get the genus 2 surface.
You should try it out with a piece of paper.
@Vrouvrou suppose a sequence is not eventually constant. let $a$ be any point. pick an increasing subsequence of points in the sequence different from $a$. the complement of this subsequence is open containing $a$. Thus the sequence does not converge to $a$. since $a$ is arbitrary, the sequence does not converge at all.
08:48
Well, very elastic piece of paper, I think.
@Huy What's up.
Huy
Huy
@BalarkaSen: I wanna finish this section before I go to Hatcher
I will try it with a paper
I am gonna try it with clay
I think that is better
@Huy Sure. But note that I can't help you much for the next - uh - 10 days because of my exams.
Then of course there's Mike.
:)
Huy
Huy
sure, maybe Mike will be around
when's your last exam?
08:52
@BalarkaSen every point of the interval $(0,1)$ is a cut point
and each point divides it into two connected components
@ForeverMozart Is that a recent breakthrough?
:P
lol no
but this was open question:
is there a metric space so that every point divides it into 3 connected components
um um um
it was open until i solved it ;)
Huy
Huy
wow
that must be like
08:53
Wow, really?
Huy
Huy
$$\huge{\text{THE MOST AMAZING RESULT YOU GOT THIS YEAR!!!!!}}$$
3
@ForeverMozart What was your result?
@ForeverMozart so we do by contradiction
Did you come up with an example or did you prove there's none?
08:54
there is an example
Hm.
Actually, let me ponder a bit. I know lots of topological space of that form but I am not sure if any of them are Hausdorff.
yeah think about it maybe you will come up with something different
@ForeverMozart Are you going to publish this?
I don't know, I have to show it to my advisor first
Ah, OK.
Who proposed this problem first?
08:57
previously there were some hausdorff examples
i.e., which paper?
but none of them were metric
ah, so not second countable.
i dont think so, cause they were regular
the hardest thing was proving the triangle inequality for my metric
Interesting.
Well, congratulations, @ForeverMozart. It's exciting.
09:00
thanks, hopefully i dont find any errors
i think for each $n$ you can get each point to divide the space into $n$ components
but i havent proved that
how about for $\kappa$ components
hmm
maybe but I'm not sure
Huy
Huy
Ted is so popular on mathed.SE: "My favorite textbook for an undergraduate course in Abstract Algebra, Ted Shifrin's Abstract Algebra: A Geometric Approach, uses a rings-first approach."
3
and yet another "geometric approach"
09:04
I used Hungerford's book
almost have it memorized cover to cover
Ted's books are great.
@Vrouvrou yes or contrapositive
Joyce is a good mathematician. Have heard of him.
iirc, he's Silverman's student
I think it is contrapositive
i feel like getting some donuts
alrighty guys I am going to sleep
cya all tomorrow
09:10
l88ter
@Vrouvrou does it make sense?
we pick a sequence which is not eventually constant, and show it does not converge to any point
@ForeverMozart why an increasing subsequence , A is the set where x_n do not equal to a no ?
why we work with a subsequence ?
because then the complement of the subsequence is an open set containing $a$, but it misses infinitely many points of the sequence... so that the sequence does not converge to $a$
we need to work with a subsequence possibly, so that we avoid $a$. ($a$ might be a term of the sequence)
try to think about it visually
but the limit of a sequence is not necesserly a term of the sequence
like (1\n) and 0
@ForeverMozart
yes. $a$ is just any point of the space.
but it MAY be a term of the sequence
but we can choose a subsequence that avoids it because the sequence is not eventually equal to $a$
@ForeverMozart when we suppose that $x_n$ is not constant at a by definition $\forall n\in \mathbb{N}, \exists n_0\geq n, x_n\neq a$
so we can directly set $A=\{x_n\in E, x_n\neq a\}$
09:26
i lost the link to your question
what is $E$?
$E$ is an uncountable set
ok so $E$ is the entire space?
look...
Let $(x_n)$ be a sequence in $E$ that is not eventually constant. Let $a\in E$. There is a subsequence $(x_{n_i})$ of $(x_n)$ such that $x_{n_i}\neq a$ for each $i$. Let $U=E\setminus \{x_{n_i}:i\in\omega\}$. Then $U$ is open and contains $a$. Also, $U$ misses infinitely many terms of $(x_n)$, so that $(x_n)$ does not converge to $a$.
@Vrouvrou
we must use a subsequence ?
09:33
because what if $a$ is a term of the sequence?
Then $E\setminus \{x_n:n\in\omega\}$ would not contain $a$
we take it from the definition of not constant $\forall n\in \mathbb{N}, \exists n_0\geq n, x_n\neq a$
so there would be no contradiction
$x_{n_0}\neq a$
you mean
i don't understand
we take the set $A=\{x_n\in E, x_n\neq a\}$ then $E\setminus A$ is open containig $a$
then $x_n$ do not converge to a
not eventually constant means for each $a$ and each $n$ there exists $n_0\geq n$ such that $x_{n_0}\neq a$.
@Vrouvrou that works, but you need to show $A$ has infinitely many terms of the sequence... that's why I used the subsequence
ok i understand
09:40
you are french?
mmm Algerian
i just saw that today
i have lots of french movies
Godard
jean luc godard
he is the best director
 
1 hour later…
11:11
@Huy nice starred message. But there is an interesting thing though: when I say the most amazing result this year I omit to say that I get amazing results every day which counts a lot. :-)
@Huy and one more thing: at the present moment I have far more results, numerically speaking, than Ramanujan had.
What is the meaning of that? Who knows?
BBL
11:37
It means you are about to die.
11:52
@BalarkaSen It means you have actually counted the two things.
12:21
@BalarkaSen you're right, very hard work can be dangerous. Hope to survive and produce very good work for a long while.
You misinterpret me.
But true nonetheless.
@BalarkaSen I'm sure you have only noble thoughts to me. ;)
It was a mere joking reference to Ramanujan's early death, nothing more.
@BalarkaSen I know, and I was pretending I didn't get your point. ;)
No more work. Trying to watch a movie.
BBL
Huy
Huy
12:52
@I'manartist I'm sure you understand the concept of quality vs. quantity - not that I'm saying anything about the quality of your work. I leave that to experts in the field.
@BalarkaSen: are you studying for your exams atm ?
13:12
@BalarkaSen Oh hi
@I'manartist I find this extremely unlikely. Where can I see expositions of your work?
@PartlyPutridPileofPus Do you think I should have an exposition with my work?
13:28
If your results are comparable in quality, to those of Ramanujan, of course.
@Huy By experts I understand those that can finish these integrals in no time, say
17
A: Two integrals involving logarithm and polylogarithm function

Cleo$$\begin{align*}{\large\int}_0^1\frac{\ln(1-x)\,\operatorname{Li}_3\left(\frac{1+x}2\right)}xdx&=\frac{29\,\zeta(5)}{16}-\frac{19\pi^2}{96}\zeta(3)+\frac{5\,\zeta(3)}{16}\ln^22+\frac{\ln^52}{40}\\&-\frac{5\pi^2}{72}\ln^32+\frac{11\pi^4}{1440}\ln2-3\operatorname{Li}_5\left(\tfrac12\right).\\ \\ {\...

@Huy and far harder integrals. But wait I wouldn't call these ones hard.
They are more of a joke.
@Huy and at least any other questions answered by Cleo with much ease. Most of the people that show amazing faces on main to the Cleo's answers is because they are not trained enough.
It's OK, but I just wanted you to know this little point.
So your field of research is calculus?
I've tried to specialize myself in the calculation of integrals, series and limits.
I see, I am surprised to hear someone could find an advisor for such a specialization.
@PartlyPutridPileofPus I'm not affiliated to any institution, I didn't follow a math uni, I'm self-educated, and I'm not advisor for anyone. I teach (tutor) some kids, but some easy math.
13:38
I think I am misunderstanding. You are doing research in the field of calculation of integrals, series and limits, and you are not working with anyone else on this? While your results are as copious as those of Ramanujan?
@PartlyPutridPileofPus Yes, I do everything alone. I was only saying that numerically I have more results than him.
@Huy Somewhat. But if you have questions, feel free to ask.
Hi @iwriteonbananas.
14:03
helpability decreased quite remarkable level inhere
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