#serious-discussion
1 messages · Page 464 of 1
I think the honors pure math courses are just designed for a challenge for those who are up to it
It's geared towards making you into a pure mathematician
I don't think anyone is implying this
I'm implying it
imma take em
i've never seen a single proof in school
and i wanna see rigor and abstractness
Cool
@atomic cypress I mean let's put it this way
I'm happy to have theory LA and practical LA
And both split as regular + honors
Honors will cover more in each case but the emphasis is different
Maybe practical honors emphasizes more advanced matrix factorizations, coding, numerical stuff, algorithm analysis
Theory honors will do stuff like modules, determinant as a multilinear form, etc
So that's my point when I say practical vs theory in my mind should be orthogonal to the question of regular vs honors
Because there are classes which will be, perhaps on the upper level of "reasonable intensity" for normal students
And which will be boring for others
But having an available honors class allows you to push the more advanced students to something closer to their potential
Right, I don't care about honors classes, I mean creating a select group of students called honors students is irksome. Elitism is a pest, unless it's me, in which case it's cool.
Lol. Idk what I think of elitism
I guess like
It has too much potential that "the elite" at X extrapolate from being skilled at X to simply "being better™️"
I didn't see in math ppl saying they're elite for taking honors courses
Instead I saw "Yeah I dunno why I chose this"
what are honors courses?
It varies from school to school
But in general honors math course means in someway it will challenge you
e.g. in honors real analysis at UCLA we used Rudin's principles and had harder exams and problem sets
Honors Linear Algebra covered significantly more material
I think they're a waste of time frankly, just skip calculus and do real analysis.
Or alternatively do calculus like the rest of us plebs.
There's no point doing shitty real analysis instead of calculus.
That way you get neither the computational training of calculus nor the rigor training of real analysis.
well what if I took an honors calculus sequence that gave me both
Through spivak's calculus, you get good at computation
and you get rigor
Isn’t Spivak like the calcbook that teaches u proof
Through spivak's calculus, you get good at computation
and you get rigor
Unless you felt that Spivak let you skip Rudin Principles for Real and Complex, I'm not sure one gets rigor from Spivak.
Or sufficient practice in rigor, at least.
Hello All. Where can I find information on the financial viability of doing a Masters Degree on Pure Maths? Thanks in advance.
ty
also #❓how-to-get-help is for hw problems mostly no?
yeah, this question feels reasonable for discussions
@neat lintel This channel should be fine. It would help if you explain your background with respect to education, why you'd like to pursue a math degree, and your particular interests (if any) within maths.
@atomic cypress you could jump to Royden from Spivak
Maybe not big Rudin
Because big Rudin wants you to know metric spaces
Spivak is basically like
How to put it
Let's say you haven't seen computational calculus or any sort of proof-based math before
Unless you're really clever, you do not want Rudin to simultaneously be your intro to calculus and your intro to proofs
Spivak can function as such
And then you do smth like Royden to get metric spaces, measure theory, and baby functional
Thanks @devout nacelle I'm an Industrial Engineer. I have completed online courses in Abstract Algebra, Number Theory, Topology, Probability Theory, Real & Complex Analysis, and Differential Equations. My particular interest is Analysis and Topology. However, I seem to have learned all these subjects for practically free, buying the reccommended books on the areas (Rudin, Jacobson, Willard, Hoffman and Kunze, Rosen, Dummit and Foote, etc.) and searching online whenever I get stuck. My question is how to make Maths my daily job without having to pay for $50k+ Degrees.
Yeah this is kind of what I mean. You can read Spivak -> Abbot/Royden/whatever -> Big Rudin or Any Calculus Textbook -> Baby Rudin -> Daddy Rudin
You end up learning the proofs anyways but at least on the latter path you have more calc practice
Not quite
Royden is basically half of big Rudin lol
Plus some metric spaces
Abbott is basically Spivak + epsilon
So I'm saying you either do Spivak -> Royden + complex analysis
Or Stewart -> Rudin -> Big Rudin
Have you looked into masters programs that are willing to take in students who did not major in math? I don't think a masters in math should cost as much, although I don't know much about the specifics of the place you're at.
Will do, @devout nacelle Thanks for the help.
Some applied maths masters may be recruiting non maths majors, if your major had sufficient maths. As an industrial engineer it shouldn't be a problem
Might be trickier with pure maths but you can still try
Thanks @eager crescent . Boy do I love pure Maths 🤓
hi
hi
how are you
i also like pure maths
but cant understand
any part of it
like really

That's how people start
@neat lintel . Do you know how to do proofs?
well there is the difference of not being able to learn well and overthinking too much and getting burned out too much and die and respawn
i remember when every math wikipedia page was absolute gibberish
well i know types of proofs but not every
then slowly over the course of undergrad, i was like, hey this is actually readable
well when did you start your undergrad
4 yrs ago?
and do you want to become the greatest mathematician of all time
no
i am sorry but your age
how old are you 
isn't undergrad first year of college?
yes
i am 15 just 3 yrs behind joining in an undergrad
Most people don't want to be the best
And if they do it gets beaten out of them fast
then they started with motivation and not curiosity
what
then why arent yo still in your doctorate program
how many years will it take to complete undergrad
i finished this year
5 years from now?
what
you will be 30 by them
then
where do you study
yes 22 +5 = 30
Big Rudin -> Baby Rudin
???
surely you just switched them
Put a parenthesis between Big Rudin and (or any calculus...)
I dunno, I feel like papa rudin is hard to learn from without guidance
Like there's a part where he just states that every open set can be written as a countable union of rectangles
Probably, he meant more the level than the book
surely you just switched them
Actually one should start with Stein's monograph on singular integrals before even attempting Principles of Analysis
There is a definition of "continnuos function" with predicate logic?
Good,pal...good...
what is a simple region?
usually means something like connected and simply connected?
so
every loop contracts to a point inside the region, there are no holes
if this is in the context of vector calculus it usually means contractable as well
since you don't want higher dimensional holes either
???
lmao
Has anyone studied Optimisation here?
rudin is garbie
terrible
pugh is the five guys of analysis books, messy but delicious
tao is homophobic, even.
rudin is actually more like in n out tbh
like
fine, and good value, but not actually very good if you look at it in a human way instead of as an accountant
Pugh chapter 2 is not good is the problem
I haven't read 3 and 4 but heard they're solid
But 2 is just
Worst treatment on metric topology

Problem is it doesn't really have a clear delineation about what's going on with sequences vs open sets vs continuous functions
It just kinda does everything at the same time
No real organization

do you need to study more than any one analysis text?
im using abbot, is that enough?
or should i like, read rudin after?
in general it depends on what you wanna do
If you want to learn more analysis then yes
If you get a taste from abott
And you don't like it
Just learn abbott and put it away
(I personally feel revisiting the same subject again with different texts after an interval of time helps me understand stuff better)
The key thing being after an interval of time
A non-trivial one
At least a few months
I mostly just kinda wanna get to the good stuff. like complex analysis. and differential forms
For complex analysis you need a complex analysis textbook, and differential forms it seems tend to be treated in a rush at the end of intro analysis textbooks
I say it seems because I don’t have much experience with differential forms
So I decided to sit down and finally listen to an album by the Clash, having only previously heard a bunch of their songs. I guess I was expected a bit less reggae, honestly.
light mode 🤢
@light needle does
But also honestly comparing something to chick-fil-A is actually a genuinely good insult and one that I will add to my repertoire.
what is the point of chic fil a now that so many other quality fast food chicken sandwiches exist
do they need to exist anymore
Why do I constantly want to drink coffee?
Coffee just tastes so good
As long as I don't overdo it
Ye it just doesn’t feel right to drink like more than 4 cups a day
Imma just make some tea instead
But then I fall asleep
For a while i had a medical reason that i couldn't drink coffee
It was horrible
Never again pls God
Ouuf that sounds harsh
coffee and calm nerves should only go together with a negation in between
Nah it's kind of like how cigarettes calm ppls nerves. I dont smoke but i see it in my friends faces after they do
i'm just talking physiologically
Also the taste and the "roundness" of the coffee are so calming, i can't describe it
coffee is a stimulant and is linked to increased anxiety
I honestly might get some caffeine free coffee
once you drink it often enough that it doesn't tickle you anymore, then sure
And just drink that all day
just enjoy the taste 😛
Without caffeine it's not as addictive :-( you stop making the mental association
But I’m like drinking coffee for the taste anyway it feels like
When I’m reading I just want to sip on something
And that something is always coffee
I don't drink coffee I drink tea my dear
Tea is way too hot tho
But it staves off the headaches I'd get if i drank <5 cups a day
You have to wait like 4 years before you can finally drink it in peace
It's a song
@deep mango do you drink coffee
not really
And are you snobbish about it at all?
i like tea better
Aww damn
I used to drink coffee and now I drink water
And sometimes tea, with milk
It's expensive tho

Ahhh I was about to send that
owo
OwO
Hey i need friends. pls add me
Hmm. No
Hey i need friends. pls add me
moniker plsss add him
Just dropped by to say I'm writing about what I've learnt this week for my master's thesis and I love it! I haven't felt so motivated since... well, forever!
also I love summing diagrams lol
I just realized, despite the ongoing global pandemic and the lockdown, I know more people who died in (totally unrelated) car crashes since the pandemic began than people who have died of covid.
seatbelts don't work confirmed

so many questions
like what does a square root do to a number and why
and why do we need it
why do exponents multiply a number by itslf
yeah man
yeah man
nah man
Whats it about? Roughly
Thanks for asking! Here goes 😛
So firstly I don't know yet where we're going to end up with this but the starting point is a paper where the aim is to construct a topological quantum field theory (which to my understanding is a topological invariant like homology but the properties are a bit different) for a class of surfaces, and said TQFT is constructed with the help of a coxeter group and its associated hecke algebra (so more or less like you fix a base abelian group for homology)
oh and by triangulating the surfaces and that's how those squares come about, the invariant is preserved by swapping diagonals on the triangulation and that's what that expression proves 😄
can you please walk me through W=x+xyz for z?
extremely! oh and in the paper there's also some interpretation of those algebras that allow the authors to compute stuff with drawings and it's so nice lmao
I don't know much yet tho, haven't really reached that yet
but it's like performing some operations on the drawings is equivalent to performing operations on the algebra and it's apparently easier
like how can I not like this right away? it's just like undoing knots or something (like the 2nd one)
it's so neat 😂
yeah string diagrams are weird as shit
string diagrams 
I want to learn them but like
it's hard to do examples when it's more drawing
Oh god I forgot what it's like to struggle to learn a new computer algebra system. I'm trying to pick up Macaulay2 for my commalg class right now and I'm half inclined to just compute these Groebner bases by hand.
Maybe it'll be easier tomorrow when I'm less tired.
when you posted this in #discussion , you were met with the following message:
Stick to one channel and don't post the same question in multiple channels. Please don't ask for help in other channels if no one is responding in the one you have posted your question in.
seems you didnt listen.

i suggest you do in the future.
(Daman)x+c
b
1+1
=2
2x2
=4
actually c
cuz it says Hint
is this a part of multi variable
I only know multi variable
dang
to some extent
is anthropology a controversial field ? i see a lot of theories on it but no strong answer
This is not math I think.
this channel is for general discussion
about as controversial as any other social science really, it's got its own methods which are validated by the community
Any topic humans are involved is controversial.
that's to say, not controversial at all except for like hardline scientists who like to give social science a hard time lol
i read into anthropology a bit more and i think i can draw a conclusion that humans are vastly different in everything
bruh and it says we are closely related to apes and stuff but how come all most apes look a like but we don’t
do apes not look alike? or are you more sensitive to the differences because you are a human?
that said, we do have a fair amount of differences because humans diverged from apes longer ago than apes themselves did
human ancestors branch off from, say, chimps and other primates with the Australopithecine subtribe
the rest of this subtribe is entirely extinct
plenty of members of it in the fossil record are fairly human-like
but it seems humans outcompeted with them all (or in some cases, like neanderthals, interbred)
its unclear why humans outcompeted other lines so well.
the old hypothesis was simply that we were smarter and more energy-efficient (required less food, could hunt longer)
and cooking food let us optimize even more energy
but AFAIK most anthropologists think this theory is a bit limited (our pre-human ancestors left behind evidence of tool and fire use, after all)
its not particularly uncommon for an entire evolutionary branch except one member to go extinct, though
humans are special but not THAT special
will we ever find strong evidence on why?
or is it too hard
probably eventually.
Do you want to do mathematics research? Or are you just looking for quantitative finance? Or are you looking for statistics?
Umm... wdym fractals
You can study self-similar sets
And it's not as easy as it seems
Why would I want to motivate you to pursue math research?
@neat lintel
assuming you already done calculus and linear algebra, but if you haven't that's definitely part of the math curriculum
well, there's group theory which leads to representation theory, which is something you might want to look into
and there's metric spaces and topological spaces
and there's differential equations, ordinary and partial
beyond that there's set theory where they cover ZFC rigourously and ordinals
and more...
combinatorics
number theory but not just modular arithmetic
number fields
numerical analysis
and there's two main routes afterwards, algebraists and analysts, algebraists typically focuses on structure to prove stuff and analysts are more of bounding and inequalities to prove stuff
group theory leads to a lot of things, representation theory is only one of them and by no means is it the best of the bunch
What do you find appealing in pursuing a math degree
homelessness impending
yeah there's other stuff in group theory too but I just know representation theory
maybe stuff about homotopy and homology groups tho
math degree is impressive to almost everyone
it shows that most people dont care what you learn for the most part
thats a good sign
shit is boring af
honestly, you should ask yourself that question first, does math drive you
you are essentially learning arithmetic right now
don't worry if the answer is no
you might be interested in computing or physics or other related stuff that uses math
if you want to learn why things work the way to do and being able to show that then math is for you
see the numbers? well some math has almost no numbers
Oh, sad, calculus is what interested me into math
then, maybe it's philosophy you might be into
you should try and learn about proofs too
then maybe topology might pique your interest
not your standard coffee cup stuff, no, we have topological spaces and open sets here
if you like counter intuitive results topology is probably best bet
Topology and analysis?
oh yea true
cantor sets are pretty counter intuitive
abstract algebra is another branch of math
where you talk about algebraic structure of sets of objects including numbers
its pretty much everything youve done until now and more
but you formalize which rules are allowed
it gets more interesting the more math you learn
I loved abstract algebra when I first learned the basics of it
If you want to have more fun with calculus you can try to learn symbolic integration
Algorithms for calculating indefinite integrals and stuff
It has a lot of abstract algebra in it
@neat lintel calculus is fun when you find different paths to the same answer
optimization and approximation are also fun, newtons method & gradient descent are nifty
like try approximating n! by approxomating ln(n!) with an integral (since it is a series)
[strlings formula basically]
sitrling's formula 
sush tterra I'm still amazed by baby math
what are prereqs for AG mr ttera
dont wanna invest time into reading over my head if that makes sense
i thought u were geometry guy
differential geometry, yes
oh thats still good for mee
even if it's been a while since i've done any classes in it
what ur classes now innit
measure theory, analytic nt, and control theory
last one might do some subriemannian geometry (DG) at the end
i kind of spaced out during the lecture explaining what it is
seems like optimization on crack
analytic number theory so far just feels like proving a bunch of random identities and estimates
without really any focus
that should change soon
hmm
online says i should be ok to learn AG
but I would benefit from learning comm algebra
ask like
shamrock or chmonkey or someone
they can probably tell you what you need to start ag
look you're the AG guys in my mind
@ someone
My recommendation is that being fluent in commutative algebra before starting is incredibly helpful if you can stomach just learning commutative algebra without knowing why you’re learning it
i probably cant
I wasn’t able to really like commutative algebra until I saw how you use it in AG
But it was incredibly painful to do AG without it
so rn im taking a grad algebra sequence
when will i learn com alg
i think second sem maybe?
yea
Does it state what you learn in each semester somewhere
im learning it in second sen
Ah
Do you have a copy of the description
and learn something else in meantime that relates to what im learning rn
The thing is there’s ways to learn AG that aren’t comm alg heavy
It’s just like… you can’t really do everything
Hmmm
What school is this or if u don’t wanna dox is ur school good
Lol
Because if you’re at a decent school the comm alg you learn will probably be sufficient for a while
But if not it could be a crapshoot
nothing spectacular
hi i need friends. pls add me
Well it's a topics course so it's not gonna be super in depth
This is just a bit too advanced to put in questions, but far too stupid to put in any advanced channel.
How do you find upper and lower bounds for $T(0) = O(1), T(n) = T(n / 3) + T(n / 2) + O(n)$ ?
jcob_the_student
my classmate swears that its bound above and below by a linear function O(n)
not swears
but thinks
I don't know...
thoughts on making a discord server for a math class? i'm debating whether or not it'll be worth it to run one this quarter
For a single class it's usually not that useful
If it's a discord for a cohort of students at the school where there are specific channels for different classes
That's more useful
Oh anyone know a topic that would be good for analysis noobs to explore PDEs?
I'm writing some proposal for a mini-REU, but the math background of students is lack-luster
this is so much easier said than done loool
idk i think i just have really low tolerance for this stuff.
That’s why I said
“If you can stomach it”
And immediately qualified it by saying I was not
Yeah, yeah, i get it. i'm just groaning
Yeah
as soon as I see shit i don't understand i basically close the book
i mean the purpose
woke approach is to read szamuely which has no details and then read comm alg to understand all the details
this is something of an exaggeration, i just have a really low tolerance for wading through derivations i don't understand the point of.
Yeah I don’t really mind them
i've been trying to read this chapter on derived categories for like 2 weeks and made zero progress
I just didn’t get comm alg until I saw it in use a bit
because i keep just going "but why"
It helps that I like algebra a lot so I now enjoy learning CA for its own sake
Idk I just like it
what have you studied that most gave you a taste for it
Chmonkey has no motivation but unending masochism
True
I basically did AG because I liked algebra
And this was the first algebraic ___
I did
And I was like “ok I do this now”
To an extent 
if anyone wants to discuss Spanier's Algebraic Topology with me, i'd be down lol
hahaha. gotcha
yeah grad school kinda blew my doors off in terms of exposure to new ideas, i thought i was just going to study computability and model theory and shit
together with like, more philosophical questions in logic
What i've liked most in commutative algebra is like, the chain of results that culminates in Hilbert's Nullstellensatz. and here i'll include as part of that chain the krull-cohen-seidenberg 'goingup' stuff
those chapters in altman-kleiman were most interesting to me.
i feel like i've asked this before, but idk if i've asked you
if i have no intuition for DVRs and know zero number theory
what is the best way to gain intuition for them
also is there any motivation for regular local rings other than smoothness?
or do we exclusively study them because they code smoothness
are they of any a priori interest from a purely CA POV
this is probably a lot of questions lol
its based i would never support piracy in accordance with the discord TOS and server guidelines®
the quality varies
there is some science on the effect of technology on learning and it seems to have a negative effect (if its not a purely work computer)
okay unironically a neat one is that if you fix a field k and an extension K of k with transcendence degree 1 then the set of DVRs of containing k with field of fractions K forms a 1 dim scheme, say X
and if you pick some f in K transcendental over k then normalizing k[f] and k[f^{-1}] in K (meaning take the curve coming from the integral closure of each in K) gives you two affine dim 1 schemes say X^+ and X^- covering X, with X - X^+ containing exactly the points of X coming from prime ideals above (f^{-1})
this happens because every ideal will contain either f or f^{-1}
This is my intuition for them anyway and i think historically how they came about, its a kind of abstract projective analogue
oh over C this will look pretty much like gluing together noncompact riemann surfaces to get a compact one
like let k be a field, K = k(t), then normalizing k[t] and k[t^{-1}] will get you two copies of the affine line C and they are glued together in exactly the same way we glue together riemann surfaces C to get the riemann sphere
are you saying that if you take all these DVR's there's some canonical way to glue them together into a scheme, based on like, the prime ideals in their common intersection or something
i'm not sure I get how this construction would work. a superficial sketch is fine
wait, is the rest of your message sketching the construction
Uh not really
ok.
kind of but i can do it explicitly
the topology on X has closed sets be finite ones not containing K itself
the local ring at a DVR R is R itself
so the sheaf is given by taking intersections
like the sheaf on U is the intersection of all DVRs R in U
this is truly weird. i assume this identification of rings with points works somewhat because the dvr only has one nonzero prime so it's fine to think of it as a point?
Hahaha.
this is called a zariski riemann space and its like
in dim 2 theyre all trivial
in dim 3 theyre not
shit like that
and for dim higher than 1 theyre not even schemes
But for dim 1 they are!
if its any help like if X is an integral affine normal curve over k with function field K(X) and coord ring O(X)
then the local rings of X are exactly the DVRs with field of fractions K(X) containing O(X)
so this is sort of expanding the net a bit and making the local rings all the ones containing the base field k
in dim 1 theyre all going to contain either k[x] or k[x^{-1}] for some x transcendental which is what i talked about here
wait, if K is transcendental over k, is it necessarily infinite
Sorry, nvm, i misread "subsets of (DVRS in) K" as "subsets of K" for a minute. the question is irrelevant.
Ah
Over C these are equivalent to riemann surfaces so this is a nice lower tech way to start working with etale things 
Yeah. I'm still digesting this but certainly Riemann surfaces were the example I was working out.
i'm stuck on something basic, maybe somebody can help me out. Say I have a point x in a Riemann surface, X. The stalk of the sheaf of holomorphic functions at x should be a DVR. And I think that the big field K we are supposed to be thinking of is the field of meromorphic functions on X. But I am thinking that a locally defined holomorphic function does not extend, necessarily, to a global holomorphic function. For example if X is the punctured complex plane, and x is, say, 1, I can take the germ of a branch of the natural logarithm. This should not extend to any global meromorphic function. So the DVR associated to x is not a subring of the field of functions.
I suppose in the Zariski topology, since every open subset is cofinite, this issue does not occur. So it is more a hangup of being overly literal about the classical interpretation.
Fwiw it should correspond to compact riemann surfaces
idk enough complex geometry to answer if this problem still occurs in that case
Hmm, I see. That is helpful. The issue I am describing seems to arise more from a nontrivial fundamental group than compactness. I could be wrong about this. I don't know of any good examples wrt an elliptic curve structure on the torus off the top of my head.
I suppose this question is something like: is the sheaf of meromorphic functions on a compact Riemann surface flasque?
No?
The meromorphic functions on the riemann sphere is only rational functions, but there are a lot more meromorphic functions on the complex plane
im not actually sure that the sheaf of holomorphic functions should be equivalent to the sheaf on the zariski riemann space though it... sounds like it should
or like
to be clear the closed points will form a compact riemann surface
Ok, thank you, good counterexample.
But anyway i hope thats neat enough motivation for DVRs 
I think probably it will be once I finish parsing it. I'll let you know
It definitely seems interesting. Thank you
Over an integral variety, one can define a sheaf of fields of rational functions. But actually all the restriction maps would be isomorphisms, right? As a consequence of the space being irreducible. I am not sure if the same thing is true if the variety is not irreducible, but that's fine, I don't mind that assumption.
So there's a slight obstacle with me trying to think of the field of rational functions over a variety as being too much like meromorphic functions.
Every inclusion of an open subset is a birational equivalence.
And something analogous isn't true for meromorphic functions.
If you stay in category of projective things analogy is almost perfect though.
If variety is not irreducible than it's not true, think about variety=two points, say. I know that you emphasized that you are not interesting in this case, just side remark
Ok, I think I understand the basic idea. That's cool.
I'm not your Guy, pal.
Ok. I remember from the first chapter of hartshorne that one can associate to every finitely generated field K/k a variety which has K as its field of functions, and we can take this variety to be affine, because every variety is birationally equivalent to an affine one.
So this construction is something similar, but it's more fine grained and doesn't involve arbitrary choices.
And it won't be affine in general.
which construction?
The construction that Moth sketched above. It associates to a field K of transcendence degree 1 a scheme whose points are exactly the DVRS in K whose field of fractions is K, and the field of functions on this scheme is K.
ok I see
Where is this from?

Than you ryc
Why do set theorists keep buying up land owned by the Catholic Church?
They’re interested in large cardinal properties

I'm studying analysis
using Marsden elementary classical analysis
does anyone prefer a different textbook/suggestions?
Not Rudin
a lot of people like abbot's understanding analysis
i like pugh's analysis but it's a bit goofy
oh I didn't know about that one thanks
I mostly heard about marsden and tao
100 years of Frobenius Uniqueness Conjecture by Aigner
Not enough people know about Markov's theorem
Pi_1 is a functor from Top* to Grp that preserves products, so it maps group objects in Top* to group objects in Grp. So fundemental groups of topological groups are abelian
so cursed lol
that's fkin cursed
Ikr lol.
Hey i need friends. pls add me
Hey i need cup of twinings tea. pls do not add sugar to it
hey i need a back massage
i need a bear

anyone know where to practice like good derivative questions (that use like chain, product, quotient, power, trig functions type of stuff)
obligatory khan academy recommendation
you could also just come up with your own functions to differentiate
you can check answers on wolframalpha
They don’t seem that difficult
Not a bad idea thanks
pauls online notes have many if need

Hello there
yo what is the difference between the direct sum and the direct product?
they seem identical to me
The difference is when you have an infinite collection of stuff
The direct sum says "all but finitely many of the terms should be 0/e/1, whatever the identity is"
The direct product says the terms can be anything at all
So you should think of direct sums like finite sequences (of any length), and direct products like infinite sequences.
E.g. the direct sum of infinitely many copies of Z is all the sequences like (a_1, a_2, ..., a_n, 0, 0, 0, ...) while the direct product of infinitely many copies of Z is just all sequences of integers
yeee okay I see I see
The point is that direct sums are algebraically a lot easier to work with (cause algebraic operations are finitary)
But once you have a topology and can take limits, suddenly direct products are a lot easier to work with (because I want to be able to say that the limit of the sequence of sequences with n 1's and then 0's is the sequence of all 1's)
yee okay now I get it. Thank you so much! 
Kind of reminds me of like the box and product topology

@dusk relic
mmmm
Where can I study "graph collisions"?
Or "stream interaction"?
I need to study other subjects really bad but I just can't get this collision idéa out of my head
@slim meadow are you submitting an abstract to JMM?
ive determined my lin alg teacher uses pruposefully weird notation
yeah i looked at this recently and it's pretty casual
very informal, i can see it being a bit rough for the student who is new to this stuff. it flies through the initial stuff pretty fast.
No such thing as a
a...?
What does that symbol even mean?

Nyone have time
Explaining taking integrals on manifolds
I poorly understand partition of unity
but that seems like its essential
i can try
ok
what exactly are you confused about
are you reading Lee?
I should be
but ive been going off lecture mostly
we are just starting chapter 3
but went through 2 very quickly
three is tangent spaces
on manifolds
and I followed the entire construction in class
partition of unity sort of alludes me
tbh, I've never used the construction of partiton of unity. You really only use its properties once you construct it
why do we want functions from the manifold to 0,1
and what does compact support mean
Well, you can think of the functions as being to R, but the image is contained in [0,1]
locally finite means every neighborhood instersects finitely open sets iirc
Compact support means the support is compact lmao
support of f is all the inputs not sent to 0
and you want that to be a compact set for some reason?
not sure what this corresponds to
You want each function to be bounded is how I think about it
oh wait wtf
and by bounded, I mean like, outside of some closed set, its 0
So we cant havr support being bounded since support contains points on manifold
So the only form of boundedness we can get is having it being compact
Finiteness*
yeah exactly
why do we care about the support though
I mean, in some sense the function will tell us things about the set its supported on
and be 0 outside of its support
Like, if we have a function f on the manifold
and we multiply it by the function with compact support, this new function tells us about f on the support
and you can replace function with vector field or anything on your manifold really
So if we have f:M->M and compactly supported q:M->[0,1] ?
when you mean function on manifold
you mean from manifold to reals?
not like chart
yes
oh wait
im piecing it together
slowly though
Hmm
So like
say we have a function on manifold f
no its good you're asking good questions
We have our partition of unity being a collection of continuous compactly supported functions f_alpha(x) with the sum of f_alpha(x) equal to 1
oh rip lol
Yep
but pretty much
bad choices wtf
multiplying each f_i by f
is a way to talk about the values of f we care about at small sections
Yeah thats right
and if you sum f*f_i you sort of get the values you should care about for f
so you arent trying to sum ff_i but talk about individual f_if
how exactly is partition of unity used from here
i sort of understand why we want compactly supported instead of having f_i have compact preimages
so one of the important things is that you can actually choose your partitions of unities with respect to some covering of your manifold
So if you choose some covering of your manifold by charts
yea
It's usually called choosing a partition of unity subordinate to this covering
ive heard that phrasing
and paraconpact
You first define the integral on a coordinate chart
And that's easy because you just pull back the function so its a function on R^n and just take the usual integral
Then, partition of unities allow you to extend this definition to functions on any open set
pull back like with category theory pullbacks?
no
or do you just mean the coordinate function
pullback is like a functor in this sense
from Rn to M
what I mean is that
if you have some chart from U \subseteq R^n to some open set V on your manifold M
and you have a function from V to R
then you can precompose with your chart to get a function from U to R
precompose just means left compose?
sure, I'm letting the image of that be U
I mean you can let charts go in either direction
since charts are diffeomorphisms
ok lol can i clear something up really quick
So one book i read said charts/coordinate maps/coordinate functions are diffeomorphisms from U to Rn where U is a neighborhood of a point x in M. It gave a name to the inverse chart and called it coordinate somethings
is there a name for the inverse of a chart?
Coordinate functions?
Because you have a function from some open subset of R^n to U
yeah
and you can think of it like laying coordinates onto U
the convention for whatever god forsaken reason is x^i
i still see no reason for it
cause of Einstein summation conventions mostly
i definitely need to review that
it was used when we differential I think
or derivative
can’t remember which one anymore
i have no clue how it works either tbh
tysm
i feel like intuition is super important for this topic atleast right now
also im a little weird on the entire smooth mappings/atlas/smooth structures on manifolds idea
so like
every topological manifold I can think of
yeah its important to understand that stuff well first
has a smooth structure
i was given examples in my class
about people who won fields medals for proofs about which dimensions had topological manifolds with no smooth structures
i dont even think there is intuition to be had
like apparently R4 or R5 have no smooth structure
or wait
R4 and R5 definitely have smooth structures
yeah I believe that
Right that
this sorta sucks thoug
yea i have 0 intuition for this too
because I can’t visualize past 3
i dont think anyone can lol
i also can barely visualize 3
So i sorta have no quick and easy pathological examples
and nothing worth computing ig
Also when you need to show a function on a manifold is smooth
you need to show psi o f o phi-1 smooth for different charts phi and psi?
but isnt f:M->R and psi M toRn?
So what that formula is for
no
is when you have a function between manifolds
ok
when f : M -> N
that makes more sense yea
you need to only check f o psi^-1 is smooth
Technically when f is from M -> R you can still do this, but charts on R are trivial
so ig this is where i get tripped up
i have an easier time telling for good examples when a function isnt smooth
how do I in general show a function is smooth, differentiate and then induction or something lol?
Uh I'm not sure what you're really asking
Almost all the time, you just check the definition
Choose charts for your manifold, compute the composition of f with the charts, check that this is smooth
yes
the last part
how do you check every partial is infinitely differentiable
do you just write them out?
but if its smooth you shouldnt run out
ig this is a problem carried over from analysis
ig, but none of these really come up in practice
ic ic
in learning they shouldnt come up often
so this implies that like, e^x is smooth and stuff idk
what topics are usually after defining a derivative on manifolds
integral*
in a first sem diftop
the whole like, differential form/de rham cohomology story probably
thats what Lee does ig
you could also go onto riemannian geometry from there (not dt though)
riemannian geo
I find diff top really interesting and hate riemannian geo im not gonna lie
i want to learn geometry eventually
you are def right
I want to atleast touch algebrsic geometry
god i love riemannian geometry so much

I just don't get riemannian geo
lol
idk, everything in diff top is relatively nice and neat
actually you know that scuffed wojak meme
and then riemannian geo gets so messy
he is embodiment
what's wrong with messy things
i mean maybe if ur a messy person
i am
makes sense
are you saying messy because notation looks crank
not really, Einstein notation is actually really helpful
christoffel symbols are terrible though
its just more than idk
woah
So riemannian geometry allows you to add a metric onto your manifold
and with that metric, things get a lot more analysisy imo
you get a lot more arguments with approximations and stuff like analysis
so imo it gets a lot messier
I'm not sure what you mean by that
the spaces in algebraic geometry aren't usually hausdorff so you definitely can't put a metric on them lmao
i guess its just that im not sure what key objects in geometry are
my guess is topologies
i dont think anyone knows lmao
is there a study of geometry on things that arent topologies?
i just learned about k vectors 5 minutes ago
the key objects in geometry are geometries just as the key objects in topology are topologies
or that they are a thing
graph theory
No everything in geometry is a topology at least I think
but a topology is way way way too general to do anything close to geometry I think
what is a k vector
dnt sully me
i wish a geometry was an object
a line of ket nami
id probably get excited
vectors have a magnitude and direction, a bivector has area and orientation, then there is trivectors etc. There is different products with them too, the video I am watching is related to Clifford Algebra
what can you do with k vectors
https://www.youtube.com/watch?v=60z_hpEAtD8 no kinda a quirky vid i thought it was a meme or fake at first XD
This video is an introduction to geometric algebra, a severely underrated mathematical language that can be used to describe almost all of physics. This video was made as a presentation for my lab that I work in. While I had the people there foremost in my mind when making this, I realized that this might be useful to the general public, so I ...
is there a mathematical explanation of k vectors tbh
i still dont know what physicists mean when they say these things
100%.
oh so they just mean k as a variable
physics is based on falsificationism i believe
here
like math is probably 10% unfounded bullshit somewhere
damn i got excited
K is my favourite letter
but here its just being a dumb variable
i havent watched it but so many people have asked about it that i get the gist
geometric algebra is a weird thing used by physicists thats arguably a bit more convenient than vector calculus
mathematicians dont really use it much at all
i am interested in physics too so i was watching it 😛
hmm
i dont know the exact details but apparently physicists find it handy for multivar
so i cant judge
multivar is kind of dumb anyway so anything that makes it easier im cool with
i heard from some excited undergrad last yesr that tensors are important when describing quantum entanglement
yeah im in multivar calc rn
how?
tensors are important in all of physics
to the point where "important when describing quantum entanglement" is really vague
idk im not a physicist lmao
physiciast student
i think theyre usually first introduced informally in a QM course
o
or maybe relativity
reference frames lol
in one of those 2 yeah
sounds like pseudoscience
or if u take a grad level course on CM
Celestial mechanics?
how tf does reference frame sound like pseudoscience
you learn about them in high school physics
lol
i get the concept
"if youre in a bus moving forward, and you throw a baseball backwards, what happens?"
as soon as you study GR tensors come up in a completely unavoidable way
they do come up in QM and QFT as well
maybe less so in early QM
if the earth is rotating why we no fly off
i get the concept
but ive always felt like its one hypothesis of many if that makes sense
if the sun is traveling quickly with respect to the galactic center why don't the planets fly off?
thing is, the planets formed around the sun
but its not completely irrational
it was already going fast
like there is always the question in science, what if more reasons?
i dont follow

are you implying the universe ought to have a canonical reference frame
if so, how fast am i moving in it
i think you mean like every object in the universe has its own properties independent of which situation its in if that makes sense
Ooh do we have some relativity crankery
i mean sure, one of those properties is mass
is it not normal to have doubts
yea so I mean the solution to this is to consider reference frames
pfft
but yes plenty of things change depending on situation
thats why we have ways of modelling situations
i agree they are useful but why I called it pseudoscience was partially meme
maybe at one point in the universe there was only 1 reference frame, like big bang or something




