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2:00 PM
Last night's nightmare included a sequence that started with me playing catch with a little boy and ended with me begging him for mercy
It was a weird nightmare
 
If I consider an algebraic group as a functor $\mathbf{G}$ from the category of $\mathbb{Q}$-algebras to the category of groups, can I consider $\mathbf{G}(\mathbb{Q})$ as a subgroup of $\mathbf{G}(\mathbb{R})$ as a subgroup of $\mathbf{G}(\mathbb{C})$?
 
@AkivaWeinberger what did the boy did?
 
@abenthy I don't know but I suspect the answer is yes
 
But functors need not send monomorphisms to monomorphism, so even through $\mathbb{Q} \hookrightarrow \mathbb{R}$ is injective, I don't see why the induced map $\mathbf{G}(\mathbb{Q}) \to \mathbf{G}(\mathbb{R})$ should be injective.
 
2:15 PM
It is not Balarka, the source of the problem is not in topology, but something related, and lurks in h bar...
Soon we will sniff out this source of all that drop in weirdness
One of the Lockdown Trio is responsible for that sharp drop in weirdness in h bar and making it very hostile to our kind
Our response to this hostility is simple: Erase the source
2
 
Is it possible to spice up this limit to something exotic?

$$\lim_{n\to \infty } \, \frac{(2 i \pi ) \zeta \left(1+\frac{2 i \pi }{\log (n)}\right)}{\log (n)}=1$$
 
what do you mean "spicing up"?
 
@Secret So that the limit would converge to something else than one.
 
Cannot think of anything exotic. Easy ways to change the value of the limit is to multiply it with some other limit expression that equals to some desired real number r
 
2:33 PM
@AkivaWeinberger That's me irl when I play catch.
 
Sup nerds
 
Hey hows it going
 
Hey Alessandro
 
@BalarkaSen I just heard someone near me say "imperialism isn't that bad though"
 
2:44 PM
squash them
 
So I guess thats how it's going
-infinity/10
 
@Eric you're a functional analysis person, right?
 
I understand that people wants to criticize capitalism but putting imperialism in a higher pedestal to build an argument against capitalism is naive and wrong-headed
A lot of people are fond of doing so
 
You mean you don't immediately burst out in laughter when someone tries that, @BalarkaSen
 
2:59 PM
I’m not sure to what extent imperialism can be disentangled from capitalism
 
They’re not the same, by any means
 
@anakhronizein I would if the population which tried that argument was in the minority
in the same way I would laugh at Donald Trump if he wasn't the president of US
 
I still have this doubt that Donald Trump can do something that is irreversible.
 
3:01 PM
Gorsuch.
Which is to say, his appointment of Neil Gorsuch to the Supreme Court is about as permanent as anything is in politics
 
Well that's the thing, my belief is politics is very temporary.
 
@BalarkaSen Were Donald Trump’s presidency a TV show, it would be a brilliant farce
 
Not permanent at all.
Then again I don't live in the USA so I guess I am allowed to escape much of the urgency.
I instead have the delightful Justin Trudope, my rasta-brother.
 
Well, Supreme Court justices serve until they voluntarily retire or die
 
he'll butcher Medicare soon
that'd be a permanent hit on the healthcare system
 
3:05 PM
And Neil Gorsuch is forty now
 
@AlessandroCodenotti I'm a person who likes functional analysis I suppose
 
Healthcare reforms do occur.
 
So if he lasts until he’s 80 (which isn’t at all absurd) that’s another forty years
 
@Semiclassical oof
 
Add to that the effect of Supreme Court decisions in setting precedent
For reference, he replaced Scalia after his death, and he served 30 years
and Scalia was appointed at age 50
 
3:09 PM
So what is wrong with Gorsuch? I am not familiar with him.
 
@Semiclassical I also think this is a difficult thing
 
Very conservative. But I don’t want to argue whether him being a Justice is good/bad, though I do have my opinions. My point in bringing him up is just to emphasize that Trump’s actions will outlive his presidency
 
I'm looking for a counterexample, specifically a normed vector space $X$ and two convex subsets $A$ and $B$ one of which is open that are separated but not strictly by an hyperplane, do you happen to know one or where to find one?
 
Hmmm
 
I mean, with those conditions they must be separated by an hyperplane, it's the "not strictly" part the interesting one
 
3:13 PM
Mostly what comes to mind is an old line of Keynes
“But this long run is a misleading guide to current affairs. In the long run we are all dead.“
 
@Alessandro the set of points above 1/x and the lower half space
 
easier is (-infty, 0] and (0, infty)
 
That one is lame
 
lol
 
I like mine better
 
3:17 PM
duh I was thinking about infinite dimensional stuff and missed the obvious examples
thanks
 
Can you make an example by drawing a tangent of a circle?
 
mhm
one side of the tangent and the interior of the circle
 
right
 
@Balarka my anti-imperialist feelings are definitely guided by emotional concerns is the thing, I'm a native person from a place where imperialism has devastated native peoples, so IDT I can give an argument without getting really really heated
 
And one can of course do that more generally
 
3:20 PM
@Semiclassical I still like my asymptotey boi
 
@EricSilva I understand. I have strong opinions against colonialisms too, but very obviously it would be nowhere close to as strong as the arguments you would have.
 
It also seems like ‘imperialism’ is a bit too broad a word, as though it functioned identically a hundred years ago as it does now
 
There are various versions of imperialisms, yeah
 
There is continuity, of course
 
3:25 PM
Medieval imperialism is of course of a different flavor than New imperialism
 
And then there's the imperial fleet under command of our supreme leader snoke!
 
It's an umbrella term for a lot of different structures and institutions that manifest differently over time and space but there's probably a few characteristics that are present in all its forms or the term would be useless
 
The role of multinational corporations isn’t a new one (East India Company, Chiquita Banana, etc)
 
(I didn't make this)
@BalarkaSen
 
god meme
 
3:27 PM
B, are you free this Sunday?
 
I don’t actually know how multinational corporations fit into ‘imperialism’ though
 
"economic" imperialism
 
Kk
Different institutions
 
@SohamChowdhury After ~12 in the morning, yeah
You want to do that ISI thing?
 
I imagine ‘neoliberalism’ fits in here as well
 
3:29 PM
At least in Latin America there's some degree of continuity between explicit political domination in the form of colonialism and economic imperialism in the form of multinationals turning Nations into enclave economies @Semi
 
I don’t know how all of these fit together, though
@EricSilva I buy that.
 
I would say economic imperialism is often wielded as a blunt tool of neoliberalism @Semi
 
@Semiclassical The multinational corporations that emerged in New Imperialism is of a distinctly different flavor than the ones appearing now, of course (one is an attempt at colonial exploitation, the other is a characteristic of capitalism). But it's a good point of similarity
 
And it’s not as though Chiquita Banana, for instance, was entirely a force unto itself. It had US support/backing
 
@BalarkaSen yes
 
3:32 PM
@SohamChowdhury I'd be down for it
 
Great line from the talk I’m at just now, after the speaker picked up two pads of paper to illustrate: “Even as an Italian I can’t handwave with more than two hands”
 
@Eric too many rich ideas in symplectic geom
 
Have you seen the symplectic proof of the Lefschetz hyperplane theorem, @BalarkaSen
 
Ah not yet, I have been learning the basics so far
 
3:44 PM
What are you learning from?
 
primarily looking at Eliashberg-Mishachev's book on h-principles which has a chapter on symplectic/contact top
But I remembered someone once sent me these: ocw.mit.edu/courses/mathematics/…
it's p awesome so far
 
hello
does any one know about regular languages?
 
Ana Cannas da Silva also has some lecture notes online that are not that bad.
 
I have heard of those
 
the regular languages that are related to automata theory
 
3:49 PM
Since you are doing H-principles though, you are probably much more advanced than Ana's notes are set up for.
Her notes sort of read like a 2nd course in modern differential geometry.
Like a continuation after you learn about de Rham cohomology.
 
Aha, I see
 
This H-principles book looks good.
 
I think my "expertise" and comfort zones in mathematics are rather nonlinear and sparse so I don't mind reading books that are more readable to me :)
 
I was looking for a reference since Etnyer references H-principles and the wikipedia page is mysteriously quiet about what Etnyer says.
 
@anakhronizein I like it a lot
h-principles are an illusive thing. I spent the last month understanding the basic statements
 
3:51 PM
Well in that case you might like Ana's book. It's not bad. It's got some exercises which are nice.
 
Very cool, I shall have a look.
 
I need to do more symplectic geometry. Right now I have been caught in contact geometry.
So much to do, so little time. :(
 
Truly
Maybe we should exchange what we have read so far ;) I'll tell you about h principles so you can teach me symplectic geometry.
 
Heh, I am actually quite sure that you probably know as much about symplectic geometry as I do.
I know very little.
 
I am reading the Moser trick right now -- so I don't know the proof of Darboux yet
 
3:57 PM
I don't know the proof of Darboux's theorem either!
 
Oh lol.
 
I was given that as an exercise to do it (of course without Moser's trick).
But I never did figure out how to do it.
 
Oh wait, I misread. You are caught in contact geometry, not symplectic
 
But I could very well show you the proof of the Lefschetz hyperplane theorem using symplectic geometry at some point if you are interested.
 
That's all the better! I need to know contact geometry
That's what my primary interest is but I am spending all the time figuring out symplectic instead because it's so attractive
@anakhronizein I'd be very interested to hear that!
Not right now, though. Need to scurry off to somewhere else in a few minutes
 
3:59 PM
Heh, don't worry I lack the time right now too.
 
I'll ping you when we both have some spare time :)
 
4:13 PM
Hello
 
I attended a talk on (basic) tropical geometry
the guy was clearly on drug, as he called a Y-shaped thing "a line" and called some collection of squares and octagons "a quartic curve"
 
I have the following limit $\lim_{x \to x_0} \frac{f(x)-f(x_0)}{x-x_0}$. I know it's the definition of the derivative but I want to treat it has a formal limit to apply L'Hospital to it. But to be able to use L'Hospital I need to justify that the limit is one of the two cases "$\frac{0}{0}$" or "$\frac{\infty}{\infty}$". I believe it's the "$\frac{0}{0}$" case but how do I justify it?
I know that $f$ is continuous at $x_0$
 
@philmcole apply the definition of that
 
of what?
 
24 secs ago, by philmcole
I know that $f$ is continuous at $x_0$
 
4:17 PM
@LeakyNun I vaguely remember that
 
ok so then $\lim_{x \to x_0} f(x) = f(x_0)$
 
and then subtract $f(x_0)$ from both sides
 
but that's essentially all I remember from tropical geometry
 
@AkivaWeinberger lol
 
@LeakyNun Got it, thanks!
 
4:18 PM
meanwhile I'm getting stuck on 100% of the problems in Atiyah
 
That's normal, I would say.
Atiyah is not easy.
@LeakyNun what chapter are you at in Atiyah?
 
Which Atiyah?
 
Did you want any hints? If I have done them, I can help.
 
commutative algebra
@anakhronizein I don't
 
4:25 PM
Ah, sure, A-M is a classic
 
what torture am I submitting myself to
 
You can find another book or topic if you don't enjoy this one
 
no, I'm fine
 
I like Miles Reid's commutative algebra. Balarka does too
 
Proposition 1.9. $x \in J(A) \iff (\forall y \in A: 1-xy \in A^*)$
proof: prove both arrows by contrapositive
me be like o.O what
 
4:28 PM
I will check that one out, I have not heard of it before, @AlessandroCodenotti
 
maximal inconstructiveness confirmed
 
I've only met two mathematicians who ever cared about constructivism.
 
which two?
 
Only one actually claimed to be a constructivist.
Two professors of mine.
 
well I am a constructivist in my free time
ah, ok
 
4:30 PM
Oddly enough, both category theorists and logicians.
(not odd at all)
 
I also like category theory and logic, lol
 
Every ideal being contained in a maximal ideal is kinda important here and there
 
What sort of stuff do you like in category theory and logic, @LeakyNun?
 
@anakhronizein oh I haven't studied them much
 
4:31 PM
like incompleteness theorem, maybe
 
I am not too fond of pure category theory.
But I like mathematical logic for sure.
And recursion theory.
Took a course on lambda calculus and proof theory, that was neat.
 
there's a constructive proof and a non-constructive proof of the following:
if x is nilpotent, then 1+x is a unit
non-constructive: if 1+x is not a unit, then it is contained in some maximal ideal, which is also a prime ideal, which contains x, contradiction
(is that how you did that exercise? :P)
that's exercise 1 of ch.1
 
@BalarkaSen I wish I had more time to learn it
 
No, I factored 1 + (-x)^n
 
nice, me too
 
4:35 PM
1
Q: Showing that the Jacobi method doesn't converge with $A=\begin{bmatrix}2 & \pm2\sqrt2 & 0 \\ \pm2\sqrt2&8&\pm2\sqrt2 \\ 0&\pm2\sqrt2&2 \end{bmatrix}$

RichardAn exercise asks to find the values of $a\in\Bbb R$ such that, given a linear system with $A_a=\begin{bmatrix}2 & a & 0 \\ a&8&a \\ 0&a&2 \end{bmatrix}$, the Jacobi method converges. I've found $\rho(A_a)=5-\sqrt{9+2a^2}$ and setting it to be smaller than $1$ I get $\lvert a\rvert <2\sqrt2$. How...

Could someome help me with that?
 
@LeakyNun That's how I did it, which of course works for the following part/question where you show $x+u$ is a unit for $x$ nilpotent and $u$ a unit.
A&M exercises are quite pleasant. I recommend the noncommutative algebra text by Dennis and Farb as a companion. They are similar in style.
 
good ol' Farb
 
4:58 PM
Is it OK to call $s = \sum \left\{x \in \mathbb{Z}\,\middle|\, 0 \leq x < i \land (3 \!\mid\! x \,\lor\, 5 \!\mid\! x) \right\}$ Mathematical logic?
I'm trying to find a name for a first-order logic in which mathematics is used (mathematical symbols like +, -, <, >, summation, set theory symbols, etc.)
 
Peano arithmetic uses +
Well I guess that is just +1 technically by axiom, then the rest by definition.
 
Most first order languages have symbols
 
5:13 PM
Does @DavidRobertJones want symbols in general or arithmetic symbols specifically?
 
@anakhronizein I'm mostly interested in mathematical symbols. I'm translating English sentences into first-order logic, and the "domain" that the sentences use is mathematical. For example, the sum of all the multiples of 3 or 5 below 1000.
 
I don't quite follow what you are asking for though. Symbols aren't inherently mathematical or not mathematical. + has no universal meaning. I can define + to mean very well whatever I want it to be, as long as I make it clear.
 
For other sentences I've used quantifiers, that could make the use of first-order logic more evident
@anakhronizein So, say that you are given the task of translating an English sentence that involves mathematical concepts into a formal language, like first-order logic. Would you say that the task is "Translate the following English sentences into first-order logic" or something else? Like "Translate the following English sentences into mathematical logic"?
 
The former, I guess.
 
5:29 PM
@AlessandroCodenotti A-M is basically Atiyah's notes when learning commutative algebra
It's good, but too terse and is hard to motivate
 
@Leaky: No, the "right" proof is geometric series.
Hi @Balarka @anakhronizein
Oh, and hi demonic @Alessandro
 
Hi Ted.
 
Hey @Ted
 
hi chat
 
Hi @Ted and chat
 
5:34 PM
Hi @Semiclassic and @Mathein
 
Hi @Ted
 
Heya Eric
 
The clouds are gathering
 
Ominously?
 
5:36 PM
Potentially
 
do clouds ever gather non-ominously
 
reminds me of an old george carlin reversal: "Behind every silver lining...there's a dark cloud!"
 
over the desert of Sahara, I suspect
@Semiclassical The gatherings are clouded?
 
sounds right
 
@Ted still needs to learn me a thing about that complex frobenius thing
 
Yeah, I haven't worked out the end yet. But it's using the theorem that you dug up from Nirenberg. I'm amazed that's not in books.
hi @Antonios
 
i knew i heard nirenberg's name associated to a frobenius type thing before
 
I saw the Nijenhuis tensor in E-M
Very cool
 
I think that's maybe different, @Balarka?
What's E-M? Electricity & Magnetism?
 
It's cool how the symplectic and complex geometry is so intimately connected... $d\omega = 0$ is the condition for integrability of almost symplectic structures to symplectic ones, and Nijenhuis tensor = 0 is the analogue for that.
@TedShifrin Ah I meant Eliashberg-Mishachev
 
5:43 PM
Ohhh. :P
 
Or as Eric likes to say, Eliahjbjhbhjbjhberg Mishaljkjkkykev
 
That's too hard for me to spell.
 
lol
 
5:58 PM
Afternoon MSE
 
What is the difference between MSE and math overflow?
 
User base
 
@Kiro one's MO is for research related things
 
by and large, the people on MO are people doing research and the people on MSE aren’t
 
@Semiclassical yeah
 
6:01 PM
Okay, I see. Thanks!
 
There’s not a hard boundary there, but I only post questions to MO when I can cite literature to explain what I know and don’t know.
If it’s a textbook question, definitely not
 
@Semiclassical isn't when one begins to do research it just becomes reading a whole bunch of "scattered" papers
 
yo
 
@TedShifrin bonsoir
 
I guess I'm doing research, but not on mathematics. I'm just asking because I want to make a new question but not quite sure where to post it. Basically I'm looking for algorithms to solve a system of linear Diophantine equations. The problem itself is of course mathematical but since I would like to know if there are computer algorithms for it then it's sort of computer science too..
 
6:16 PM
@balarka speaker just invoked the Hopf fibration and higher homotopy groups on the sphere
Weee
 
noice
 
Hey @Akiva
 
@Kiro Have a look at scicomp.stackexchange.com too.
If you're looking for algorithmic solutions to mathematical problems, I don't think math.stackexchange.com is the most appropriate site to ask your question.
 
@nbro thanks for the suggestion, I'll take a look!
 
6:24 PM
Furthermore, I doubt people are going to be happy if your question is: "is there an algorithm to solve a system of equations". At least, provide context, what you have searched so far, etc.
 
Of course.
 
Does anyone know one applications of Contour Integration in Physics ?
 
@BalarkaSen a line on his conclusions slide: “[These solitons are] a test-bed for exploring $\pi_3(S^2)$ and $\pi_3(S^3/Z_2)$ topologies”
Neat
 
@MikeMiller is there one particular thing people are meaning when they refer to a linearisation of a vector field?
 
@Zophikel There seems to be something relevant in this arxiv prepring arxiv.org/abs/1712.06893
 
6:33 PM
First I always try to think of the push-forward but that doesn't always work.
 
@Kiro O.O that's way above me i'm looking for a simple application to physics
 
There’s a bunch of applications of contour integrals in physics
 
Some of that is just the fact that certain integrals along the real line can be dealt with elegantly by doing contour integration
 
Is $\Bbb{R}^\omega$ in the product topology locally compact?
 
6:39 PM
For a place where it’s endemic, I’d point to Matsubara frequency sums en.m.wikipedia.org/wiki/Matsubara_frequency
One sees similar ideas in other field theory calculations
 
6:53 PM
@Narcissusjewel does it matter how you did the 1+x part, to do the u+x part? I mean, x/u is nilpotent, so 1+x/u is unit, so u+x is unit
 
@anakhronizein Can I have an example?
 

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