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07:02
I thought it looked like a trig identity and id verify it by taking the series expansions of the lhs and rhs demonstrating it to be true that way I wouldn't know how to work with recurrence relations in the traditional fashion for something like that ie finding the characteristic polynomial, etc I can see it is linear in f but I thought the presence of q as you defined it makes it non homogenous or some word like that
07:19
$q$ is just a constant
ah ok that's all right then
so how are these kind of radical expressions calculated
$\cos \left( {\frac {3}{20}}\,\pi \right) +\cos \left( {\frac {7}{40}}
\,\pi \right) =-1/8\,\sqrt {2}+1/4\,\sqrt {5+\sqrt {5}}+1/8\,\sqrt {2
}\sqrt {5}+1/2\,\sqrt {2-\sqrt {2}} \left( 1/4\,\sqrt {5}+1/4 \right)
+1/8\,\sqrt {2+\sqrt {2}}\sqrt {2}\sqrt {5-\sqrt {5}}
$
like how would I find the radical expression for $\cos \left( {\frac {7}{50}}\,\pi \right) +\cos \left( {\frac {9}{80}}
\,\pi \right) -\cos \left( {\frac {13}{36}}\,\pi \right) +\cos
\left( {\frac {29}{100}}\,\pi \right) +\cos \left( 1/18\,\pi
\right)
$
@Adam with a lot of pain?
ok determine the polynomial for which $\cos \left( {\frac {31}{150}}\,\pi \right) $ is a root\
instead of caring what the horrifying radical looks like let's just start there how do I determine the poly
i mean you have an algebraic number, so it can always be shown to be computed using summations of rational numbers some how, infinite in this case ill assum e
what is the best way to do this in terms of generality of the functions for which we can determine if an evaluation is algebraic or not like i have asked for cosine
07:36
Hello one and all! I have a question about non-dimensionalisation of an ODE; What happens when it comes to numerical methods? So for example I have some quantities that involve the speed of light so when you have the actual value in a Runge Kutta (for example) it can often blow up or not work out correctly. When I non-dimensionalise everything it looks great and more palpable but can you actually do any numerical methods when it's in this form?
that's fine pain isn't always a bad thing
Or when you want to do numerical methods do you then have to re-instate all the terms that have been non-dimensionalised?
nice i feel like a capitula pop show be used here
pop up should be used here anyway it seems that it is grammar loss o clock\
or bed time as they say in normal
@MatheinBoulomenos is there anything interesting about Spec(Z-hat)?
That's just some sort of product of local rings, right?
07:44
it's pretty big
for any infinite product of rings, there is some ultrafilter stuff that comes into play
@MatheinBoulomenos You had my curiosity, now you have my attention
why do i get the feeling that they have the same size
Argh the set theorists are out in force
3
@AlessandroCodenotti the point is, if you have a product $R=\prod_{i \in I} R_i$, then for any ideal $I$ in $R$, the set $\{J\subset I \mid \exists (a_i)_{i \in I}: a_i = 0 \Leftrightarrow i \in J\}$ is a filter on $I$ and if the ideal is prime, then it's even an ultrafilter
that's how you can show that the spectrum of an infinite product of fields is the Stone-Cech-compactification of the index set (as a discrete topological space)
07:50
I guess I do like Stone Cech
But so now we have SC compactification of $2^{\aleph_0}$?
For $R=\prod_p \Bbb Z_p$, we have the map $R \to \prod_p \Bbb{F}_p$, that realizes the Stone-Cech compactification o $\aleph_0$ as a subspace of the spectrum of $\widehat{\Bbb{Z}}$
@MikeMiller just of $\aleph_0$, our index set is countable
Hey everyone!
Oh, 2 x aleph_0
it's really the set of all primes here on which the ultrafilters live
Yes, I just mean that there are two points in Spec Zp
07:53
@MatheinBoulomenos So it actually is just the space of ultrafilters on the index set
@AlessandroCodenotti it is and even the topology checks out
I guess I am content with Spec L^infty(X) hehe
@LeakyNun you should google "profinite number theory", there's some stuff by Lenstra about this, I read about it some time ago, but I forgot the details
But in the discrete case of course I have just used your description
I just prefer if other people think about the ultrafilters
but I’m thinking that (p,1,0,0,0...)(1,q,0,0.0...) = (p,q,0,0,0...) so you can’t have more than two components being a prime ideal? by component I mean every ideal is the product of ideals of the p-adic integers right
so I’m thinking all prime ideals look like pZp times whole rings
07:56
@LeakyNun there's a prime ideal that contains the ideal of all elements which have only finitely many nonzero entries
(that's basically the same argument as for nonprinicipal ultrafilters)
but what is wrong with my thinking?
it only works for finite products
so not every ideal is a product of ideals?
yes
consider the ideal of all elements which have finitely many nonzero entries
08:01
in general the spectrum of every infinite product of nonzero rings will contain $\beta \Bbb N$ as a subspace, which means it's pretty big
@MikeMiller ah, that's not really related to this ultrafilter thing, the filters only look for which entries are zero and nonzero, that's why they don't give a complete description here, unless for products of fields
What isn't related? The 2 points thing?
(On phone so I do not see arrows)
I see. Is there a not much worse complete description?
I guess I should follow link
I don't know of a complete description (which doesn't mean that it doesn't exist)
Jan
Jan
Hi just wanna ask about majorization by tangent. $\hat{H}$ denotes the value of $H$ at the current iteration.

For $Ω(H) ≤ Ω(\hat{H}) + \langle ∇Ω(\hat{H}), H − \hat{H} \rangle$, should $∇Ω(\hat{H})$ w.r.t $\hat{H}$ or $H$?
08:08
Hi @Balarka
Hey hey hey
@MikeMiller Back before the millenium byatch
@Balarka I got some good answers for the thing with flat bundles and locally constant sheaves
Mmk, shoot
The sheaf of parallel sections of a flat bundle is locally constant
It is true.
It's foliated by covering maps around the zero section
08:11
and for the other direction, you can tensor a locally constant sheaf with smooth functions for a locally free sheaf. Apparently there's some DeRham stuff which gives you a flat connection on that tensor product
Gotcha.
The algebra wasn't clear to me. Thanks!
(Yeah, the de Rham differential $d : \Gamma(TM) \to \Gamma(TM \otimes T^*M)$ is the flat connection, because $d^2 = 0$)
@BalarkaSen Only for line bundles
There is something quite interesting here. There are the abelian local systems and the nonabelian local systems. The abelian local systems are not so hard to compute; you need some finite cover usually. But the nonabelian ones encode interesting information about the representation theory of the fundamental group
Ah yeah dumb me
08:15
For instance, that the Zariski tangent space to an irreducible representation in O(n) is $H^1(X;\rho)$
Where $\rho$ is thought of as a locally constant sheaf of $\Bbb R^n$
it says "there's a more recent edition of this book"
and then there's a link to "Desk Reference for Neuroscience, Second Edition "
wtf
@MatheinBoulomenos do I lose anything if I just consider the function evauated at the closed points?
08:33
No in the classical case , yes in general (ie you can have two different ‘functions’ with the same value at every closed point)
08:43
do you have an example?
@loch also thanks for pointing it out that the A_p version is sending each point to a function defined near it; that really cleared it up for me
0 and x evaluate to the same value on the unique point on spec C[x]/(x^2)
You can think about whats happening here (hint this ring has nilpotents)
@loch might you by any chance be around?
Nah im going back to the US today
You can take a DVR if you just wanted an example where functions agreeing on the closed point is not the same
@loch so it isnt reduced?
whatever that means
the key players here are the nilradical (intersection of all prime ideals ) and the jacobson radical (or wtc this is called - the intersection of all max ideals)
Yes
08:47
yes, jacobson
If two elements of your ring evaluate to the same value at every prime (max) ideal, then that means that their difference is contained in every prime (max) ideal
@loch Where are you now!
?
London
could you tell me things about the map Spec Z[i] -> Spec Z?
08:56
I thought @MatheinBoulomenos did a very good job explaining it!
or maybe Spec Z[cos(2pi/7)] -> Spec Z
that has galois group C3 right
London is nice. Conference?
Nah my girlfriend is studying in london so i visit her when i can
Aha
That's some dosh but so it is
@loch if you vanish at the generic point, then you vanish everywhere?
09:07
Well this is nice
So what's the best route for me to take to translate a whole pdf that is in German, into English?
London is a generic point, and loch's vanishing from London, which means he will vanish everywhere
like just copy paste into a translator in a browser window?
@loch u do not exist
@Adam learn german dude
@LeakyNun yeah absolutely not
09:13
Heh, it seems like Daniel Tubbenhauer is making a habit of slightly odd acknowledgements. Previously, it was thanking the football world cup for providing a peaceful working environment. And in this paper, he thanks "Darkness, his old friend".
4
Hahaha
@LeakyNun the vanishing locus of a function is always closed, so if it contains the generic point it contains its closure! hence everywhere
@BalarkaSen i can exist as a nilpotent element!
although im not sure if that's a good thing
Clearly, once you have cos(1°) in radicals you can get any trigonometric function of any integer number of degrees in radicals by applying secondary-school-level identities.
What math are you interested in @Adam
@loch and well-definedness?
09:23
?
what can I say about functions defined at the generic point?
Ok well can someone tell me how this guy knows that a cubic polynomial with a root equal to $cos(\frac{\pi}{180})$ please
@Adam What does that even mean?
cubic polynomial $*has*$ a root equal to $\cos(\frac{\pi}{180})$. how would I determine that polynomial is cubic
How to determine that an a priori cubic polynomial is a cubic polynomial, given that one of its roots is blah
09:29
Hmm I think maybe it's worth being more precise on what we mean by functions now. I guess probably a good way of saying this is that functions are elements of $\mathcal{O}(U)$ for some open set $U$, where $\mathcal{O}$ is your structure sheaf.

So in this sense it doesn't quite make sense to ask for functions defined at the generic point - but maybe you want to know what are the functions that are defined on any open set containing the generic point - here's the catch - every open set on an irreducible space contains the generic point.
Maybe saying this more concretely - if you take an integral domain $A$ - what is its stalk at the generic point? The generic point is the ideal $(0)$, so you are localising at the prime $(0)$, which gives you the fraction field.

So if you remember at some point I said a rational function is a function defined on some open (dense) set of your space (and irreducible implies all non-empty opens are dense anyway) , you can also just view them as elements of the fraction field
@loch shock, Spec(integral domain) = fraction field???
@LeakyNun nonono
im saying rational functions on spec (integral domain) = fraction field of integral domain
maybe this is not very meaningful if you haven't seen rational functions defined as functions defined on some dense open of your space
lol
09:46
@LeakyNun He's talking about $\mathcal{O}_{\text{Spec}(A),(0)} = A_{(0)}=\kappa(A)$ for $A$ an ID
Structure sheaf of in this case an affine scheme
10:25
7 messages moved to Trashcan
1 message moved to Trashcan
Mooooving on
Hi @Parth
Hello, @BalarkaSen
How're things
Pretty good, I suppose
10:38
Nice nice
What've you been up to lately?
Math mostly
And has there ever been a time in your life when that wasn't the answer to the question?
Yes :)
22 messages moved to Trash
I took the liberty to move that ultra screen-real-estatey monologue to the trash along with the reactions.
I hope I didn't catch anything unrelated in the crossfire
10:44
Thanks. It was distracting.
@ParthKohli Right now I'm doing math while listening to music, eg.
Can I always take a union/ an intersection of infinite amout of sets?
More precisely Agalloch's "The Mantle" (thanks to @Alex for recommending, it's great)
How do you concentrate while listening to music lol
I can't, honestly. My mind wanders around, but ideal when I'm LaTeXing stuff
I see you're not the grumpy 14 year-old grand-daddy anymore.
10:46
I wasn't, I was just a pretentious ass
But yes I grew up I guess :P
omg, hi @ParthKohli :O
@student Hi! (But, like, who are you?!)
skullpatrol
Ohhh, I guess he deleted his MSE profile then
10:56
nah, i go by just "skull" now
I've exclusively been doing competitive programming all summer.
nice, what language?
Doesn't matter tbh, but C++ (and occasionally Python3).
hi @DenisNardin
& bye
perhaps, you could ask for some tips about competitive programming on CSEductors.SE @ParthKohli
Hm, competitive programming isn't truly CS.
11:04
i'm sure they could help though
just ask for "Buffy" in their chat room
he has a PhD in math; and has been teaching CS at uni for 40+ years
 
3 hours later…
14:11
Where will I get the brief motivation for differential forms and cohomology? Planning to study. Can anybody give a suggestion?
@LeakyNun my example of failure of unique ideal factorization wasn't correct, here's a correct one: the unique prime ideal in $\Bbb{Z}[2i]$ above $2$ is $(2,2i)$ and we have $(2,2i)^2=(4,4i)=(2)(2,2i)$ if there was unique prime ideal factorization, this would imply $(2)=(2,2i)$ which is wrong
you can also see the singularity of $\Bbb{Z}[2i]$ at $(2,2i)$ by looking at the tangent space
$(2,2i)/(2,2i)^2=(2,2i)/2(2,2i) \cong \Bbb{F}_2 \otimes_{\Bbb{Z}} (2,2i)$ now $(2,2i)$ is a free $\Bbb Z$-module of rank $2$, so you get that this a two-dimensional $\Bbb{F}_2$ vector space
that's the cotangent space, the tangent space which is the dual will also be two-dimensional
but the ring itself is one-dimensional
so you get a tangent space of higher dimension than the curve you have, which is exactly what happens at a cusp
 
2 hours later…
16:07
https://oeis.org/search?q=cohomology&language=english&go=Search
There are 900 sequences in the OEIS that mention the word cohomology. Not sure it helps though.
@FailedtobeaMathematician
Hi all!
@mercio & maybe @MatheinBoulomenos (or anyone who can answer that): Why do I have $[(V_1\oplus V_2)\otimes(V_1\oplus V_2)]^- = [V_1\otimes V_1]^- \oplus [V_2\otimes V_2]^- \oplus (V_1\oplus V_2)$, in particular how did $(V_1\oplus V_2)$ get "outside" the anti-sammetric part (which is $[]^-$)?
16:23
how is it called when you use the same concept in the definition of that concept? like Set of Natural Numbers (Nat) is {x | x = 0 or if x in Nat then x+1 in Nat}
Impredicative
thanks!
recursive definition?
recursive definition is different, the set construction does not refer to the set itself as part of it
see how the von neumann universe is defined recursively for example, no stage is the set to be constructed at the current stage referred to in the recursion
Hey everyone
16:30
Hi @Perturbative
Heya @Daminark @AlessandroCodenotti
Been a while since I last spoke to you @AlessandroCodenotti
What are you up to these days?
I've started studying more algebraic topology recently
what about you?
@MatsGranvik Thank you very much
But my question was related to differential form.
@Alessandro Pretty much the same boat is you, gonna really get stuck in with some alg-top over the next 6 months
What kind of AT?
16:34
Hey @Perturbative! Also hey everyone!
hamster $\ne$ chipmonk :(
@AlessandroCodenotti Probably just the basics + homology and cohomology
though maybe it is a gopher or prairie dog?
it ain't no chipmonk, that's all I'm sayin'
It's a prairie dog
16:46
I think you already did most of that stuff though
Not really, I only know about fundamental groups and Seifert-van Kampen, I learnt how simplicial homology works yesterday and now I'm reading about singular homology
Ohh okay nice
Howdy @Perturbative, demonic @Alessandro, Xander, Secret, et al
Hi @TedShifrin :)
16:59
hi @Ted
oh, it's a lurking Leaky
Anything interesting going on?
my poster got displayed for open day
ah, good
Hi @Ted!
Hi, a @Balarka
17:01
I'm still somewhat convinced that all the sections of Spec(integral domain) combined, modulo some equivalence relation, gives you the fraction field
@TedShifrin I wrote a thingy on stratified spaces.
The very first sentence isn't a sentence :P Is it supposed to be a thingy that's notes for you or a thingy that's presentable? :P
Notes for me. I didn't care about grammatical correctness, but tried to be mathematical correct about the various definitions flying around
OK, just surprising for the very first thing not to be a sentence. :P
Heheheh
17:04
@famesyasd it's an inductive definition
@BalarkaSen What have margins done to you?
I like small margin
That way no one can write notes in the margins.
Balarka 1 - Fermat 0
15
P.S. I'm with Alessandro
Is this mathematical World Cup?
17:05
Ted talks.
I lol'd irl
gg @Alessandro
@TedShifrin We didn't qualify for the football one, maybe we have a chance in the math cup?
hi @Mats
@TedShifrin Hi.
That's why I asked, Alessandro.
17:12
Lol I'm not fond of margins either, I'd rather just use more of the page. More time spent on each line when reading, I guess? Also hey Ted!
@Daminark If you use very wide margins you can study more pages per day
top 10 lifehacks to increase your productivity
margins are good so you can write in them after the fact
Just write a page with small margins and then increase margin so that it becomes 3 pages
observations, thoughts, questions
Give yourself a pat in the back. What do you want, a medal?
17:14
No, I want everyone to print books with good margins
If a page is too cluttered/full, I just give up and quit reading.
0.5 inches is good margin my dude
hi Demonark
I think LaTeX's default margin is maybe a little too big, but I write a lot of stuff in the margins so I quite like it
I use 1 inches (inch?) when writing most often
17:15
I usually use the fullpage package
I've never used that.
And then don't change the margins from there
I have my own defaults, depending on whether it's a class assignment, exam, letter, book, etc.
Balarka, are you done with your visit or are you still there?
17:16
I think it has something to do with how I don't like zooming in too much if necessary, but I also don't like to have whitespace on my screen
@TedShifrin I return on 29th, a night + a day after
Ah, cool.
Seems decent, but something about this document throws me off. Maybe the font?
I am rather exhausted so it's a relief kind of
@Daminark Good, you hate on Hatcher's font
Exhausted from talking math with pros all the time?
I typeset my algebra book with Lucida, which is the same font Hatcher used. After that, I switched to Times Roman.
And, full disclosure, my algebra book appeared before Hatcher.
17:18
Comic Sans?
So he must have copied me.
@TedShifrin Exhausted from just doing/thinking/anticipating to do math 24 hours I guess
Still overall a positive experience, I'm sure.
Yeah it was good
17:21
Are we discussing Hatcher’s font and why it’s awful
We are discussing why you are a bum
I like Lucida, but its vs and nus are indistinguishable. So that was sorta OK for an algebra book, but not for geometry :P
I am indeed a bum but that doesn’t make Hatcher’s font good
I paid my own money to buy Lucida fonts (and also New Times Roman fonts) back in the day.
I published Lucida stuff before Hatcher, so I get priority.
looks up when Hatcher first appeared
I want to like rewrite it in a different font Bc it’s too good a book for me to hate on it so much
17:24
I found it slightly strange but not enough to really care. Best font for a math book is definitely comic sans though
I think it's a pretty font, actually.
If you ever write a book in comic sans I’ll actually hire a hit man
So it was published in 2002. I wonder when he produced the text.
I think Hatcher writes about how he came to the typeset somewhere
When did you write your algebra book?
17:26
I was just looking that up.
@EricSilva There is a section on his website about it
It appeared published in 95, so I must have started late 80s. Probably didn't put it in Lucida font until the version I typeset for publication. Still, early 90s.
I was teaching out of typeset versions of it starting in 90 or so.
Have you all read Hatcher's essay on why he used the font? Oh yeah, I should have reminded everyone the AMS Notices use the same font.
I’ll have it known that my vendetta against it is super unserious
Back to reading moving frames, @EricSilva.
Gromov is a madman
@Eric you might like this
17:45
Ah apparently THE SPACE man is back
@BalarkaSen I told you about him, right?
scratches head
He's the crank I met yesterday
Oh. What does he say
I am not back at the math building just yet
But yesterday was just this mess, among other things, 1=infinity, he got all 7 millennium problems in one go, Cantor was just wrong and there are no uncountable sets, twin paradox in relativity is false, etc
That's a person of some real caliber
How did you meet him
17:50
He had this thing he called "THE SPACE" and was trying to use it as a way to revolutionize math. Basically N with a """""metric""""" and the p-adic "order", somehow this is "topologically equivalent" to Euclidean space and in it all the millennium problems (singled out Riemann hypothesis but later said all) were solvable by it
Jesus that's out there
This is beautiful
So I walked into the math building, saw him, and he was like, is the math department here? I said yeah, he asked if any professors here do geometry/topology. I said yeah, and then went up to the third floor, then he tried to open Benson's office. Benson came out and space man requested a meeting
17:53
Benson said wait 45 minutes, then I went into the Barn,a couple minutes late this guy followed and asked if I had 15 minutes. Then he started his rant, and at one point where it was getting really hard not to laugh I left under the pretense of returning a library book
I emailed Benson like "Fam, might've thrown you under a bus, this guy is mad, sorry :/"
Lmfao
I wonder what the conversation with Benson might have been if it did happen
Always makes me think of that article about trisectors
I come back and see Benson talking to him, by which I mean space man is ranting and Benson is just nodding like "Uh-huh", tried to contest at one point that p-adics aren't equivalent to R^n
LOOLOLOLOL
And then space man wasn't having any of that, tried to say that he did prove it, and then kept going
Afterwards he was like yeah, I emailed so many professors at MIT and got no response, Benson's like yeah just email me and if I have time I might do something. Responded to me saying "Thanks for the email, this is indeed unfortunate"
17:56
Jesus
lol I thought that kind of cranks were just a legend
And then space man just kept going for hours, I tried to be like, yo this is great and shit but I've got work and I'm probably too underqualified to judge anyway, but then after a few minutes he'd come back and keep going
He's already teaching his, I think < 7 year old daughter this shit
jesus god im dying
So how come he's back
Also he was just a really strange character. At one point while he was talking someone else (who was watching the game) said "Oh my God!", then this guy called his daughter and said "What do we say when someone says "Oh my God"?" She says "No More!" and then he looks back to my friend and repeats that with such a defiant look on his face
And I was wondering, like, should I just say "GOTTEM!" or something?
He's probably lunatic
It is unfortunate
18:03
Yeah I kinda don't want to interact too much with him anymore. He's too nice to just tell off, but he's like glue
That story was something else
Lol @Perturbative
I feel sad for his daughter though :(
And for his daughter's future math teachers
Plot twist: he's actually a math teacher
Yikes
18:27
@Daminark david told me about that guy
didnt u lead him to Farb-y boi
18:40
Yup
Check fb
I only realized he was a crank after he found Benson
what a story
18:59
@BalarkaSen i do in fact like that
I keep on accidentally writing "order-isomorphism" with a hyphen
1st world math problems
Isa
Isa
19:52
Does someone here knows about pde's?
I have a question
20:43
If we have $x < y \le z$ does this mean $x \le z$ or $x < z$?
I'd read it as $x < z$
it's standard to interpret each pair - so you know $x < y$ and $y \le z$, ergo $x < z$
yes makes sense thank you
hail grothendieck
Hail Leaky nun
21:08
enter buttons are annoying
So I have a sequence that smells funky, and I'm not sure what sort of thing is producing it. I feel like it's weird enough that it's probably something someone's seen.

For each prime number p >= 5 (currently looked up to about 100), there is a number n = p-1 or p+1 such that each entry whose index is divisible by n is also divisible by p.

For example, N_6, N_12, N_18, etc... all have exactly one 5 as a prime factor.
But N_10, N_20, N_30, etc... all have exactly one 11 as a prime factor

But as it turns out, N_3, N_9, etc, also have a factor of 5 too. Sometimes it's not just N_{n*i} for i
it grows exponentially, too.

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