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22:02
@AkivaWeinberger WTF, you even copied the spelling error correctly.
@AkivaWeinberger yes
without AC is consistent that there is no injection $\omega_1\to\Bbb R$
you should join the set theory study group, we discussed that exact question (which is an exercise in the book we're following) just today :P @Akiva
Wait hold on, you need AoC to prove that if $|X| \le |Y|$, then there
's an injection from $X$ to $Y$?
you gotta be careful how you define stuff
$|X|$ is not as easy to define without choice
@Daminark anyway that should hold regardless of AC, the problem without AC is that you can't always compare the cardinalities of $2$ sets
@Ted: in problem 4, section 1, is there an "intelligent way"? I don't want to make each of the for cases...
22:18
Well I mean when they're defined
Like $|\omega_1| = \aleph_1 \le 2^{\aleph_0} = |\mathbb{R}|$, I think
So here there isn't a problem
(Also wow I didn't think losing choice would be that nasty of a hit)
the problem here is that without AC $\Bbb R$ might not be well-orderable so its cardinality doesn't need to be a cardinal (an aleph number)
see this question for example
Yeah okay this is overpowering just about any reservation I might have had about the axiom of choice
@Daminark "There exists a surjection from $A$ to $B$" is not necessarily equivalent to "There exists an injection from $B$ to $A$"
(The second implies the first)
(and the first implies the second under AC)
I don't know exactly how much choice you need
22:26
You know I'm just gonna take AC wholeheartedly and not look back at this point
Hopefully not all of it
actually "AC" and "a function has a right-inverse iff it's surjective" are equivalent over ZF
Oh, OK, so all of it.
Here they say something about this. I have a full proof of both implications in my notes somewhere, but I have no access to them at the moment
My set-theory question got flagged as a duplicate
but I got 17 rep points on it so I still win
22:30
For once I entered the room the same time as Mike.
Hmm, one direction is clear, since if you have choice you take your surjection and map back by choosing a point in the pre-image
The question that your was marked as a duplicate of was linked earlier in the set theory study group room :P
Now, assume every surjection has a right inverse
Which resulted in a Three Stooges-esque moment as you both tried to fit through the door at the same time @JasperLoy
@Daminark I guess the idea is this
Ah, first you prove that "AC" and "AC for disjoint families" are equivalent over ZF
22:33
There's a clear surjection from socks to drawers
but mapping the drawers to their socks is the same as choosing a sock in each drawer
@Alessandro okay I can see how AC for disjoint families immediately becomes this
So like, let's say you have some product of sets
That are disjoint
@AlessandroCodenotti Oh. Do you work with ordered pairs (sock, drawer) if, uh… some sock is in multiple drawers…?
The analogy breaks down a bit
Really really big socks
then you take a family of disjoint sets $F$ and consider the function $g:\bigcup F\to F$ that sends $z\in\bigcup F$ in the only $A\in F$ such that $z\in A$ (this requires no choice), this is a surjective function and then you need to show that its right-inverses are choice functions for $F$
Okay wait let's not look at it as a product, $A$ is a collection of disjoint sets, so map their union to it such that a point is mapped into the set containing it, the right inverse is a choice
Gah fucking sniping
Aw, now my socks are tangled
22:37
anyway I should go to sleep, bye everyone
Now, assume AC over disjoint sets is true, maybe you can take a general collection and try to force it to be disjoint by taking products with ordinals?
See you @Alessandro
@Daminark take $A\times\{A\}$ for all the $A$ in the collection
@AlessandroCodenotti So (sock, drawer), essentially?
Yep
Drawer containing nothing but that sock
I'm really going now
22:44
tries to push Ale in general direction of the door
He's gone, let's party
parties in LaTeX
@Lucas: Well, there's some symmetry going on and some patterns in the numbers. Rather than thinking about 4 triangles, what can you think about?
Hey @Ted!
Night @Alessandro
Hi Demonark
What's Lucas trying to prove?
Sum of four triangles is all numbers or something?
(Word order is weird my)
22:47
How's it going?
No, a problem to show, given four points, that they make a tetrahedron whose sides are all equilateral triangles.
@Lucas should figure out that some of the exercises are more worthwhile for him than others.
I don't understand. Are these four points supposed to lie on a face or something?
DogAteMy: I'm not sure everyone's familiar with the Three Stooges. I always had to teach my students who the Marx Brothers were.
Also @Ted for reference, I think a number of bootcamp people didn't pick up forms all too well
I know, Demonark. That's what I told Eric.
22:49
@TedShifrin The wackiest Marx was Karl
They're the vertices of the tetrahedron, DogAteMy. ... Well, I think I'd take Karl over our current situation.
4 of us did manifolds, and had a more general treatment of forms from there (rushed, but we did have a pset dedicated to it), and from just talking around to friends a lot of people were just playing it by ear wrt even Schlag's plane treatment
Heya @anon @arctictern.
so sad
Well, I don't think you guys should sacrifice actual differential geometry for days of learning about forms, much as I love forms. I think it's better to learn more curves and surfaces and develop geometric intuition. But Eric thinks you're all baby Erics :P
22:51
Well, I mean we still did officially do some forms stuff
I like Henri Cartan's book on forms
This is the problem when people don't teach what they should in the important formative courses ... and go off the deep end too much into fancy analysis.
Yes, it's a nice book.
heya
@TedShifrin Oh I thought the points were arbitrary
uchicago is a place that prides itself on doing fancy analysis
22:51
So I think it'd probably be fair to just go and say hey guys, review forms from 208
No, no, DogAteMy: $(1,1,1)$, and then possible permutations with two negatives, one positive.
@TedShifrin formative?
@TedShifrin Oh. That's just the one in the cube
I'm allowed to pun, Demonark. You're still banned for 28 days.
That's a nice tetrahedron
22:52
Symmetries of that tetrahedron have cool matrix representations, too, DogAteMy.
I'd even say it's the nicest tetrahedron
(Just measure the side lengths I guess)
@anon: Did you get my ping about the post wanting help with spin and spin-c structures?
couldn't figure out what the question was
didn't see it in the transcript
@TedShifrin The symmetries of the cube being $S_4$, I think the symmetries of the tetrahedron are precisely the $A_4$ in it?
It was a long post and I didn't read it carefully. Just said to myself, "Self, this is for tern."
22:54
Even permutations of the diagonals
not sure I'd be useful anyway, I don't usually put structures on manifolds
Yes, DogAteMy, that's true :) Do you see what they all are geometrically?
@AkivaWeinberger yes
Well, I thought it was more of an algebra/representation theory question, in fact.
Also @Eric are we gonna do just surfaces?
22:55
ah
@Daminark as opposed to what else
Curves + surfaces
You have to do a certain amount with curves or else nothing else makes sense.
oh i mean schlag had us only do surfaces last year but souganidis did curves with us
idk what you guys did
@TedShifrin Everything but 90 degree rotations about the faces and 180 degree rotations about the edges
22:56
You need the basics of the first two sections. You can skip the third, although things like Fary-Milnor are amazing mathematics.
But what are they in terms of the tetrahedron, DogAteMy?
That is, it'd be 180 degree rotations about the faces and all rotations about vertices
@TedShifrin Rotate about a vertex, edge, or face, no?
although given my recent conversations with some of the other TAs they don't really know much about curves either...
Plus identity
Soug did curves for about 2 days
Or I mean maybe 3 idk
22:57
@TedShifrin Line joining opposite edges
Do you know Frenet equations, Demonark? That's essential.
midpoints thereof, yes, DogAteMy.
@TedShifrin Oh, wait, so rotating around a face and a vertex are the same
Yup.
OK, I need to go prep a chicken to roast it. Back in 10.
Like one day he just went through a bunch of definitions of stuff, one day where he did arclength and parametrization, and one day he proved that conformal mappings satisfied C-R equations
So that's $1$ identity, $6/2$ edge 180 degree stuffs, and $2\times4$ other 120 degree stuffs
22:58
Hi, I'm here from PPCG. We're currently having a bit lot of a struggle understanding OEIS A000236. Could anyone help explain parts of it?
and that's twelve
@Ted we didn't do Frenet equations
Souganidis did a week with us and a long pset
Lol that was our shortest pset all quarter
This answer seems to help out a bit with what residue classes are, but what is an n-th power residue class?
22:59
I mean well... No, it was only the shortest because we didn't have a linear algebra pset
We had the Frenet-Serret stuff allllll as an exercise
He was originally gonna do Jordan form but decided against it
which is astounding because he didn't teach us ODE stuff
@HyperNeutrino Consider the set of all things that are 2 mod 5 (that is, two more than a multiple of five)
(so, like 7 or 12 or 27)
23:00
And raise them to the 3rd power
only me and a few other people who had seen the stuff in high school could even remotely follow what was going on @Daminark. I think it put a lot of people off
Turns out they're all gonna be 8 mod 5, which is the same as 3 mod 5
oh we also did some basic stuff about surfaces
(Ex: 7^3=343, 12^3=1728, 27^3=19683)
Yeah there was a day when our TA gave the lecture, she did a bit on surfaces
23:01
So 3 mod 5 is the third power of the 2 mod 5 residue class
Note that 2^3=8
Actually @Eric can you send his curves pset?
Okay. Thanks.
no, I don't have it anymore
The reason 8 mod 5 and 3 mod 5 are the same thing is that 8 and 3 are a multiple of 5 apart
Ah okay.
So, the sequence's description says this: Maximum m such that there are no two adjacent elements belonging to the same n-th power residue class modulo some prime p in the sequence 1,2,...,m
23:02
it's been almost two years
> n-th power residue class modulo some prime p
Oh, is the chalk site gone?
he emailed them to us
The way you termed it, it has 3 numbers: the power, the remainder, and the base. But n-th power residue class modulo some prime p only has 2 numbers. What does that mean?
Oh weird
He put them on chalk for us
I mean I guess if he had the Schlag problems for you guys it would make sense that he emailed them
23:03
Anyway, I have to go for a bit; I should be back in about 15 or so minutes hopefully. I'll read responses when I get back. Thanks for your help :)
@HyperNeutrino I haven't read it yet but this reference might be helpful cms.math.ca/openaccess/cjm/v16/cjm1964v16.0310-0314.pdf
(One of the links on the OEIS page)
those problems were pretty cool
something something heisenberg uncertainty somethign something
@AkivaWeinberger thanks
DogAteMy: Yeah, you got the tetrahedral symmetries. Now the matrix representations are cool (you could even say 3-dimensional representations) :P
Demonark: Read through the first two sections of my notes. I can recommend particular exercises if necessary. You should do a few computations (with chain rule, even) to make sure you understand basics.
23:20
@TedShifrin Do you have any notes on forms by any chance? :)
Differential forms? They're in my multivariable book and the 112 YouTube lectures.
Yep Differential forms, I'll take a look at your book and lectures now!
It's a less abstract treatment than you'll find in chapter 4 of G&P ... also, I hate their numerical convention that has the unit cube in $\Bbb R^k$ having volume $1/k!$.
I think that's the first complaint I've heard about G&P from you :p
Oh, I'm vocal about that complaint. And I complain about a few other things (too many/too few hints on problems).
23:26
@TedShifrin Hm. Cycling through the coordinates should work, that's 3
Negating two coordinates at a time, that's 3 more
i remember the hints being annoying when I read that book
sometimes everything is just given away and sometimes it's opaque
But then there's one problem to lift maps $S^1\to S^1$ where he gives essentially no hint. Ridiculous.
Those are the order 2 ones, DogAteMy?
that book really does do a great service though
Oh and then cycling and negating at the same time @TedShifrin
I think I'm missing some
Wait no I'm not
What does the offset mean in OEIS?
23:30
So this is things with exactly one $\pm1$ per row and column, where an even number of them are $-1$ and the rest are $1$ @TedShifrin
Do you get the right number of elements of the right orders?
I really do not understand this ._.
@TedShifrin I think I do
@TedShifrin Chapter 4 of G&P looks like it's independent of the other chapters (barring chapter 1 on Tangent Spaces)
@HyperNeutrino The offset means the number input to the sequence that generates the first result. For example, in A00478, it doesn't make sense to put fewer then 6 balls in 3 containers with at least 2 balls per container, so the sequence isn't defined for inputs less than 5, and thus the offset is 6.
Not sure what the ,1 means
23:33
well, much of it is, not all the applications and interpretations, @Perturbative.
oh okay thanks
It's nice that $A_4$ can be represented in 3D
rather than the trivial 4D solution
$A_5$ even 'cause of the icosadodecahedron
I think I'll start going through Chapter 4 concurrently from tomorrow
Thanks again for all your help earlier! @TedShifrin
Sure, Perturbative.
I'm off to bed now, cheers everyone!
23:36
o/
Night.
Yeah chapter 4 was nifty
I liked the degree formula big time
Did he fix my volume $1/k!$ complaint, Demonark?
Yes, the degree theorem is very important.
So today I learned that $\bar z$ lacks path invariance in the best way possible
I hope you see all sorts of parallels to complex analysis, too, Demonark.
23:38
'cause apparently $\oint\bar z\ {\rm d}z$ is proportial to the area of the contour
DogAteMy: Not if you do $\int \bar z\,d\bar z$ !!
($2i$ times)
Ronno mentioned that he was disobeying the book's convention :P
Correct, DogAteMy. I guess you never got to Stokes's Theorem.
Like he did the $k!$ stuff early
In order to not deal with $\frac{1}{k!}$ later
23:40
No, Demonark, I mean to correct so that you get the right thing for the volume of the cube. G&P try to save some factorials in formulas and that's the price they pay. I'm not willing to pay it.
Well yeah, I mean he introduced a $k!$ type of term into the formulas early so that down the line when he got to $\mathbb{R}^n$ he wouldn't have to scale the volume of the cube as such
It's in the Alt formula you need a factor.
Or When you define wedge.
That's when it came up (he didn't do tensors in general so we didn't have any Alt)
Also if you integrate along the unit circle, I think $\oint z^m\operatorname d\!z$ traces something homeomorphic to the Hawaiian earring as $m$ varies
23:42
If you do stuff without tensors, then you shouldn't have any trouble. You just use determinants.
See my book.
Will do
I reserve the right to say "I've had it with these motherfucking aches on this motherfucking plane" when it next becomes relevant
Are you flying back to NY now, DogAteMy?
@TedShifrin No
23:51
Then what are you babbling about, DogAteMy?
Ah right yeah, it is clean that way
Though I dunno, I do like how the wedge product is really the alternator applied to the tensor product, it's a helpful way of thinking about it
As in the notes you sent
Yeah, that's standard, although at the graduate level people define it by modding out and taking a quotient algebra instead of a subalgebra.
Huh, how does that go?
You see it occasionally in physics as well, e.g. whether $T_{[ij]}=\frac{1}{2}(T_{ij}-T_{ji})$ or not (though that's the simplest possible case of course)
To make (equivalence classes) be skew-symmetric, you mod out by the ideal generated by all things of the form $v\otimes v$
23:54
@TedShifrin I babble about lots of things
(I'd probably prefer the 1/2 in general so that the antisymmetrization of an antisymmetric tensor is the same tensor)
True.
Yes, Semiclassic, it's best to be a projection.

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