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7:00 PM
old school style
 
I haven't touched my FB since 2012, I think.
 
I guess because you haven't cooked for me, @Eric.
 
fair play
 
I'm running into documentation with the following statement: "the standard 2-simplex, i.e. a right-angled isoceles triangle"
 
Hmm, you might not even be able to add me as a friend.
 
7:01 PM
@ÉricoMeloSilva where did they get the admit list from
 
Hi @semiclassic.
 
which to my eyes is very much -not- the standard 2-simplex
hi
 
Sure it is.
 
@RyanUnger presumably theyre friends w someone who could see it
 
I mean, it's a projection of it from RR^3 to RR^2
 
7:01 PM
standard n-simplex is defined to be convex hull of (0, 0, ..., 0) and the points you wrote down, in R^n
 
Oh, I guess I don't usually think of it one dimension up.
 
your definition is doing the equilateral face of this guy one dimension up yeah
i dont think most people do that
 
Tammo tom Dieck goes as far as composing everything with an homeomorphisms from [0,1] to the standard 1-simplex because he's that formal
 
LOL
 
7:04 PM
I'm using the definition that Wikipedia lists (which is dangerous of course) on en.wikipedia.org/wiki/Simplex
 
good old tammo tom bombadil
 
rolls $23+\pi/7$ eyes
 
though they cite it from Boyd's "Convex Optimization"
 
@Semiclassical yeah I wouldn't trust Boyd's Convex Optimization to be universally standard
 
${\displaystyle \{x\in \mathbb {R} ^{k+1}:x_{0}+\dots +x_{k}=1,x_{i}\geq 0,i=0,\dots ,k\}.}$
 
7:05 PM
Check Hatcher chapter 2, where he defines n-simplex, maybe
 
@TedShifrin it’s true u can’t, i found u but there’s no button to add
much private
 
yeah, I only let friends of friends add, I think.
 
are you not friends with Mike
oh wait he deleted
 
I used to be.
For sure.
 
Boyd and Hatcher both have the advantage of being available for download on their author's websites
 
7:07 PM
Oh he defines n-simplex like that as well
RIP
 
The reason I've been preferring that equilateral definition is because you can think of the n+1 coordinates as probabilities
i.e. nonnegative and summing to one
 
hatcher has the advantage of having sick exercises
 
Right I agree.
It's easier to understand eg the Dirichlet distribution on the equilateral thing
 
in which case the standard 2-simplex would be a 2D subset of 3D
 
You can add a variable and funk it up which iirc is easier for calculations?
 
7:08 PM
now I'm curious what books use that definition
 
(Dirichlet is like beta but on an n-simplex, where the whole mass is concentrated somewhere on the interior of the face, in a way that the induced mass on the faces aka the various conditionals are also Dirichlet)
The pdf is $\text{Beta}(\alpha_1, \cdots, \alpha_{k+1}) x_1^{\alpha_1} \cdots x_k^{\alpha_k} (1 - \sum_{i = 1}^k x_i)^{\alpha_{k+1}}$ if you want to think about it supported on the right angled $k$-simplex in $\Bbb R^k$
where that big beta is the multibeta function Gamma(blah + ... + blah)/Gamma(blah)...Gamma(blah)
 
@ÉricoMeloSilva this Zhenhua Liu guy, somehow I know of him
he had a paper as a junior or something
 
But you can say $x_{k+1} = 1 - \sum_{i = 1}^k x_i$ and then you're supported on your equilateral $k$-simplex. Becomes simpler to visualize
 
@RyanUnger i met him he was nice
ik less geom analysis than both of u lol
 
@BalarkaSen yes, well, if I'm working with a 13-simplex then I generally don't care about visualization :P
 
7:13 PM
@ÉricoMeloSilva what does discrete math actually mean here
logic?
 
combo maybe
 
or like, cs
 
@Semiclassical RIP
 
oh I have literally no idea what combinatorics is
 
me neither it’s a spook to me
 
7:14 PM
the hilarious bit of this
is that polymake (the program this is from) also has the command "hypersimplex(k,d)"
which "Produce the hypersimplex Δ(k,d), that is the the convex hull of all 0/1-vector in Rd with exactly k 1s"
So Δ(1,d) is the convex hull of (1,0,...0), ... (0,0,...,1) in Rd
i.e. the standard (d-1)-simplex as I know it
So their hypersimplex doesn't simplify to their simplex :P
 
@BalarkaSen Did I tell you that Song used Cech cohomology in a minimal surfaces paper and it shocked everyone
 
Nope
@RyanUnger What are these Riemannian h-principles by Lohkamp
 
7:31 PM
its okay to an user for ask personal twitter?
 
@BalarkaSen "can you decrease Ricci curvature an arbitrary amount by an arbitrarily small $L^\infty$ perturbation"
 
open manifold surely
 
I think it's just in a ball
 
Gotcha
I believe this
 
@BalarkaSen it's not so easy
what I said turns out to be exactly true for the scalar curvature
for Ricci curvature you need strongly negative Ricci curvature outside of the ball and you can "propagate it in"
it's a very hard paper and I don't know any more than that for the Ricci curvature story
 
7:37 PM
Huh
 
scalar curvature is done completely by hand in another crazy paper
 
Hm, tell me what the whole theory is called where they think about which functions on manifolds appear as scalar curvature of some metric?
I think it's true that Gauss-Bonnet is the only restriction on compact surfaces?
 
prescribed scalar curvature problem
 
Aha.
Maybe you can ask which given functions on a manifold can be arbitrarily well approximated by scalar curvatures? :P
That'd be a nice h-principle
 
Gauss-Bonnet is the obstruction in 2D yea, but it's not a simple proof
 
7:42 PM
You should teach the proof to me one day
 
the theorem is: $f$ is Gauss curvature iff $f$ somewhere has the same sign as the Euler characteristic
 
Makes sense, yes.
What is the thing in higher dimension?
 
you define three classes of manifolds
(A) M has a psc metric
(B) M has a Ricci flat metric but no psc metric
(C) M does not have a metric with nonnegative scalar curvature
and the theorem is that any manifold has to be in one of these classes
 
scary. ok
 
furthermore, a manifold in class (A) has surjective scalar curvature map
in classes (B) and (C), as long as the function is negative somewhere, it's a scalar curvature
 
7:45 PM
Wow ok
 
in particular, any function is a scalar curvature on any manifold as long as it's negative somewhere
 
Pretty amazing.
 
a much harder problem is answering this in a conformal class
prescribing scalar curvature on the sphere is a very hard problem too
kind of crazy
2-dimensional sphere
 
Right so what I was wondering is if $f_t$ is a small homotopy through functions which are negative somewhere can you also choose metrics $g_t$ with $f_t = \text{scal} g_t$ such that $g_t$ is a small homotopy through metrics
Maybe Lohkamp says yes?
 
oh...that should follow from standard PDE, I think.
 
7:50 PM
Huh OK
 
uh let me check my thesis
I also have to run soon
 
You can tell me later. I should go to sleep or something
 
Ok so here's the strategy
The first step is that since $f(p)<0$, we can in fact assume that $f$ is close to $-1$ on a "good portion" of $M$, that is, $f$ is $\varepsilon$-close to $-1$ in $L^p$.
I think you can just fix a background metric for that to make sense
I didn't specify in my thesis
But you do this by taking $p$ to be the inf of $f$ and using a diffeomorphism to blow this up a nbhd of this point to be a good portion of $M$.
since the problem is diff invariant, this is fine
now there's a cool trick for getting a background metric with scalar curvature identically $-1$ on $M$
now the idea is that you can take this background metric and prescribe its scalar curvature in an $L^p$ nbhd
this follows from the inverse function theorem and elliptic theory
(you need to work in a Sobolev space)
so you just need to check that the inverse function theorem gives you a curve of solutions
 
@RyanUnger That's a very cool idea.
 
which I imagine it does since the map you're linearizing is Frechet-$C^\infty$
 
7:57 PM
@RyanUnger I see, so you can "invert the differential operator" on an nbhd of $-1$ since it's $\varepsilon$-close to $-1$
These are all Gromov ideas man
I need to read your thesis now lmao
 
this is before Gromov tho
this is from like 1975
before Gromov was super famous I mean
the diffeomorphism trick is super cool
but inverting the differential operator is standard elliptic PDE
now why it's invertible at $-1$ and not at $+1$ is...deep
 
@RyanUnger ya, it's how you spread holonomic approximations outward if you have a Diff-invariant differential relation
to make global holonomic approximations
 
so you first need to know that any manifold has a metric with scalar curvature $-1$
 
surprising
 
by a theorem of Yamabe, this is true if the manifold has a metric for which $$\int scal<0$$
and there's a clever trick for constructing such a metric (by hand) on any manifold
it's completely local
 
8:02 PM
@RyanUnger So constant function $-1$ is a regular value of scal, is what we're trying to prove here?
 
@BalarkaSen Uhhhh, that's much harder but also true. What I'm saying is true for the scalar curvature operator in a conformal class.
For this easy case, you only need to make a conformal deformation once you have the $-1$ metric.
 
Ah OK
Right, easier than arbitrary perturbations of the metric I'd expect
 
so the equation is $-a\Delta u-u=fu^{(n+2)/(n-2)}$
and then $scal(u^{4/(n-2)}g)=f$
where $scal(g)=-1$
 
I am not checking it but I 100% believe you
 
@BalarkaSen so you look at $\Gamma(u,f)=-a\Delta u-u-fu^{2^\star-1}$
and you know that $\Gamma(1,-1)$ is a solution
 
8:08 PM
Ya OK
 
so you linearize in the first variable to get...well something invertible by elliptic theory
I also didn't tell you where $u$ and $f$ live
or what $a$ and $2^\star$ are
it's not trivial but not hard if you know some PDE
and you need to know why all the funky powers too
 
It's OK. I might need to read some PDEs. So the theorem in elliptic theory is that the principal symbol is invertible => the operator is invertible?
 
no that's definitely false
 
Invertible symbol just means that the operator is elliptic I believe?
 
it's invertible modulo the kernel and cokernel
 
8:12 PM
Ahh OK
 
elliptic means Fredholm
 
Got it.
That makes a lot of sense
 
so you just check that it is legitmately invertible
for example, $-\Delta+1$ is
that's the kind of operator you get from this
 
This is pretty dope
I wish I knew some actual PDEs instead of topological dumb shit like differential relations lmao.
 
I dunno how you can prove anything without passing to weak solutions
which I guess you avoid with that stuff
 
8:15 PM
Gotta learn this stuff. Lots of things to learn this semester. Inevitably I will end up learning nothing though
Thanks a ton, @Ryan. That was a good explanation
 
np
do you have my thesis?
this is less than one page of it :P
 
@Balarka alas, I have to refresh my rusty brain in 2 months after being out for a year :(
 
Yup I have it
 
so there's one thing I failed to prove
my definition of class (B) in the thesis is not what I said a moment ago
the proof of that in Besse is seriously stupid and I've been working on a different one
 
@ÍgjøgnumMeg u just gotta get in the loop, then it should flow naturally from my exp
 
8:17 PM
no luck yet
I prove everything that I say in the thesis but something significantly stronger is true
 
I see
 
It's really unfortunate because the full proof is stuck in some French papers from long ag
And the result is used in GR even today
 
Rip
 
ok I'm going to the store, cheers
 
Talk later! Thanks again
 
8:19 PM
@Balarka aye, I spend a lot of time reading but not much time doing lol
 
let's take a crack at Galois Groups and Fundamental Groups togather if you haven't made significant headway on it yet
i want to read that book
 
That could be cool
 
Give me 7 days lmao
 
I only read a portion of the first chapter, up until the sidenote on category theory lol
 
I'm making too many promises to read too many books with too many people lol but it should be good. I'm sick of coursework bull
@ÍgjøgnumMeg I need to read first chapter
maybe you can help me with that since it's algebra
I can help with the covering space bits in turn
 
8:22 PM
yeah that'd be cool
I had planned to talk about it at the university before I leave for Germany but I've done nothing on it hahaha
 
i feel u, master procrastinator here
 
well I'm at work 8:30 - 17:00 and then i just go to sleep when I get home
hahaha
 
ah that sucks
when are you checking out of this dayjob lol
 
16th of September lol
then I have a month-ish until the Master's starts
 
That's good
 
8:27 PM
Yeah I think I'll start visiting the library after work next week, so I don't end up knowing nothing when I start
 
Just remember to not stress yourself out.
 
yeaaah I have some faith in my ability to learn fast when under pressure so I'm not too worried
 
Cool
 
Hi
I am following the Lucas Probable Primality test described on page 83 of nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.186-4.pdf
Can U and V be negative? I know D can be negative and since V is computed with D (and U is computed with V) it would make sense that U and V can be negative.
$V_{temp} = \frac{V_{i+1}^2+DU_{i+1}^2}{2} mod C$ I know normally the square only applies to $U_{i+1}$ (order of operation) but in this situation is it somehow different?
 
taking mod C gives an output 0,1,2,...,C-1
not really any opportunity to get a negative output there
 
Sorry I don't understand. As a counter example -1 mod 2 = -1
@Semiclassical
 
How?
I can't see U and V on page 83.
 
@northerner $-1 \equiv 0 \bmod 1$
 
8:53 PM
@ÍgjøgnumMeg I corrected the last message
 
alright so $-1 \equiv 1 \bmod 2$
 
how do you get latex to display?
 
tinyurl.com/cfqcvpc
make this a bookmark
 
Then what? I tried clicking on the links, reloading this page too, but nothing changed?
@ÍgjøgnumMeg
 
you make that page a bookmark on your bookmarks bar
and then open chat and press the bookmark
 
9:06 PM
I still don't get "taking mod C gives an output 0,1,2,...,C-1" shouldn't it be -C+1,...0,1,2,...C-1 ?
 
the usual set of representatives are those positive remainders 0,...,C-1
$-1$ mod $C$ is just $-1 \equiv C-1 \bmod C$
 
Ok
so just take the absolute value?
 
No, it's not an absolute value, the numbers are just taken to be positive normally, of course you can write $-4$ mod $C$ but it's the same as $C - 4 \bmod C$
 
9:23 PM
You can keep adding multiples of C till you get to the "other side" of zero: $-5 \pmod{7} \equiv (-5+7)\pmod{7} = 2\pmod{7}$ and $-14\pmod{3} = (-14+3(5))\pmod{3} = 1\pmod{3}$ etc.
@ÍgjøgnumMeg Are you doing your PhD now?
 
nnooo
I'm "preparing" for a masters
 
I'm in the same boat, Meg.
I technically still have one undergrad semester left, but I consider that as part of the prep.
 
@Rithaniel luck to you :)
 
9:39 PM
I just glanced at chat for a moment and my sleep-addled mind read an insulting message.
 
But indeed, I hope things go well for you, too.
 
Cheeeeerz
When's your start date?
 
January 2020. I'm still gonna be at the same college, but the intention is for that to be a temporary setup.
 
I see, you want to move away from Madison?
 
9:42 PM
I'm not in Madison. I go to a "football college" called Clemson.
 
Ah fair
A football college?
 
But yeah, I've been told that, in the long term, it's good to receive lots of points of view in your math education.
A college that makes most of it's income from it's football team.
 
Oh I see
weird
 
Clemson is commonly a contender for "best football team in the US"
Not the best place to get a math education, but they don't have a bad math department, either.
 
I'm in a similar boat
Maths department at my uni has 3 pure mathematicians hahaha
 
9:47 PM
What are the criteria for "pure mathematician?"
 
I guess someone who keeps themselves busy with pure mathematics
 
Cause Clemson might have several pure mathematicians, in that case.
 
There is an algebraist, an algebraic geometer, and an analyst
 
that good
 
I know Ben Jaye and Shitou Liu are analysts, Jim Coykendall is an algebraist, and the Michael Burr is a geometer, so that gives at least four.
There are many professors I don't know, too, so maybe this place is better for math than I would have suspected.
 
9:56 PM
@ÍgjøgnumMeg is that the beginning of a joke?
 
Anybody agree with his conclusion that n \geq 23? defproc.co.uk/blog/lattice-hinge-design-minimum-bend-radius
 
 
1 hour later…
11:11 PM
@ÍgjøgnumMeg I see. I wondered that after looking at your project (thesis?) gave me nightmares the other day lol.
 
11:53 PM
Let $(X,d)$ be a metric space whose metric topology is discrete. Is the reason why that $(X,d)$ is not Geodesic essentially due to the fact that there are no continuous injections from $[0,1]$ to $X$?
 
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