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16:08
@TedShifrin yeah not been doing that great thie last 10 days plus a couple midterms and a few assinments. Making it difficult aced both the midterms i think though. Might have to ask for an exstension on one of my assinments due friday. Next dosage increase in 5 days hopefully be better then!
16:30
How do the 2-Sylows in A5 intersect? There are 5 of them, each coming as a normal V4 in A4, embedded in five different (but conjugate) ways in A5.
I think they pairwise intersect at one non-identity element.
(The full intersection, as in of all of them, is trivial of course)
@BalarkaSen Well, you can quite easily explicitly write them up as sets of double transpositions.
I know, I am asking for a sanity check
The normal V4 in A4 is the subgroup of double transpositions.
Hmm, I don't think they intersect at all, do they? The intersection would need to only involve $3$ of the elements, which makes a double transposition impossible.
Oh, right.
It's easy to see abstractly. If P and Q are 2-Sylows which intersect then N_G(P \cap Q) contains both P and Q since they are both abelian, so has to have order more than 4. The only possibility is 12 (can't be 20, index is 4, that'd give a left reg rep A5 -> S4 but A5 is simple).
Hm.
Maybe not so easy.
Anyhow, it's direct, like you said. Thanks.
16:51
Hey @CaptainAmerica16 how is it going?
@AbdullahUYU If that's your definition of the rationals, then explaining that there are inverses as I described is the obvious way to go.
@Fargle If you are referring to Aluffi's Algebra Chapter 0, a similar book is MacLane and Birkhoff's Algebra, which also has lots of categories. It's in the AMS Chelsea series.
Is it correct to say that while many say naturals, rationals, and reals, not many say complexes? If so, might it be because a complex can mean so many other things?
@WillHunting I think "the rationals" and "the reals" are fairly used, but "naturals" and "complexes" much less so
17:06
Aha! You are right!
Maybe all this just happened naturally and there isn't really a reason.
What would we do without AMS and Springer publishing math books?
Milne wrote something on his website I saw long ago: If you want to see new books, buy books. Good advice. I hope publishers never stop publishing.
@WillHunting We don't have any equivalent for "the integers" so that probably read to usage like "the reals" and "the rationals"
Aha! I was thinking integrals but that means something else!
I think it is also uncommon for someone to say this number is integral, rather than this number is an integer.
One does sometimes say "integral ideal" to mean "a fractional ideal that is actually an ideal", which is definitely in the "integer" sense
Also, "integral solutions to <equation>".
OK, got it @rschwieb. So, when we're trying to show that $\langle\mathbb{Q},+\rangle$ is a group, we refer to integers. On the other hand, "A first course in abstract algebra" gives $\langle\mathbb{Z},+\rangle$ as an example to group (abelian). We accept it as a group and don't show anything.
No proof.
17:17
Counting, -
Whole, whole
Naturals, natural
Integers, integral
Rationals, rational
Irrationals, irrational
Reals, real
Complex, complex
(anything else is too rare to be of significance)
@AbdullahUYU Well, I don't know how you define the integers, but quite often you define it as a list of axioms stating that it has the properties of a group.
@Secret Transcendentals, transcendental; Algebraics, algebraic
@AbdullahUYU I don't know what you mean about $(\mathbb Q,+)$ "referring to the integers." It is true that the addition in $\mathbb Q$ is defined in terms of addition and multiplication in $\mathbb Z$, if that's what you mean.
@Secret What's the game going on here?
Yeah, that's what I mean.
May I ask a quick question about vectors?
17:19
Noun, adjectives
and the adjectives often refer to things that are not the set of numbers themselves
I also learnt that the terms integer part / fraction part or integral part / fractional part are all used in the literature when I posted an answer to an English question long ago.
e.g. natural morphisms has nothing to do with the natural numbers
@Imago In this room, you should just ask and not ask to ask.
Sometimes there are intense, meaningful conversations going on and then I am kinda too shy / afraid to interrupt / ask.
e.g. Integral can mean integral operators, or something where division is well defined always
17:22
Well.
@Imago Sometimes you may get chided but don't let that frighten you.
and I am not going to go into how many meaning the word "complex" has in mathematics
The word complex has a complex meaning.
What would be a good notation to denote the space the transpose of a vector lives in?
For example one may write $v \in \mathbb{R}^n$
If you have an inner product, it is usually the dual vector space, otherwise, it is the space of linear functionals
17:25
However, if one then looks at the transpose of $v$, how would one denote it's space? I would like to not use matrix notations etc. (keeping it simple)
Ok, assume further: My target group up are freshers at university, engineers.
@WillHunting It's going alright. You?
@CaptainAmerica16 It's going as it usually goes, which is not alright, but never mind me. Have you made any progress on the set theory study?
Anyone able to explain why my question was marked as off-topic? It has all the essential details very clearly spelled out with the example.
The students will understand $v \in \mathbb{R}^n$, and $v^T$ however I would also like to formally denote then space for $v^T$ and it feels like, I should not simply write $\mathbb{R}^n$
@WillHunting Lol. I've been a bit swamped with school, but I do think I have the very first sentence figured out. I'm trying to see what I can derive from that. I feel like I need a more thorough understanding of the properties/axioms of functions and why they are.
@WillHunting I'm using a few different sources at the moment.
17:31
@CaptainAmerica16 Have you done functions in school?
@WillHunting Yes
0
Q: Prove that a finite vector space and its dual space are isomorphic.

NormalsNotFarLet $V$ be a finite dimensional vector space over a field $k$ with basis $B=\{v_i\}$. And let $V^*$ be its dual space with basis $B^*=\{\beta_i\}$, with $\beta_j(b_k)=\delta_{kj}$. Show that $V^*$ is isomorphic with $V$. We haven't been shown that two vector spaces are isomorphic if the have t...

@CaptainAmerica16 Then you already know what they are, and this is just to make the informal and intuitive idea completely formal and rigorous.
This might be waaaay higher level than necessary, but it justifies why you can just write $\Bbb{R}^n$
I don't know of anymore specific answers
@WillHunting It was very surface though, at least compared to set-theoretic definitions. We learned domain/co-domain and graphing, not much beyond that.
@WillHunting It seems like I always almost have the answer, but then I start overthinking.
17:33
@DavidCoffron I don't like people excessively downvoting and closevoting questions on this site myself, but if you want to know why they did it, you can read the explanation in the closing box.
@WillHunting I say co-domain, but I think I really mean range. Lol, I just learned recently that they're not always the same thing.
@Secret, yes that is total overkill. I am a bit surprised there is not real notation for that, cause normally you can't define $v + v^T$ properly..
@CaptainAmerica16 In high school, a function will yield a graph, which can be thought of as a set of ordered pairs. Now formally, we just define the function to be this set of ordered pairs. Hope that gives you a good idea.
I would like to keep confusion/ambiguity at a minimum..
The trouble is they live in $(\Bbb{R}^n)^*$ but that is isomorphic to $\Bbb{R}^n$ for finite n
17:35
@WillHunting I read it. I tried to add more details; all they say is ""This question is missing context or other details"
@CaptainAmerica16 Well, some authors use range to mean codomain and image to mean range. Like I said, different books will use the same word to mean different things or different words to mean the same things. A cat can be defined as a dog, and a dog can be defined as a cat.
@DavidCoffron Read the words in smaller font. There are lots of words in that box.
@WillHunting I read them and that is the context I've added (what I know so far and what the source of the problem is)
@DavidCoffron Good, then you have done your best. Now you can wait patiently for the question to be reopened, but I don't know if or when. I am frustrated with the current state of things as well.
@WillHunting Is it at all commonplace to ask on the meta site for an additional explanation. I know some sites encourage this and some sites forbid it.
@DavidCoffron You can, but I don't know the reaction. I don't even want to comment too much because some users may think I am attacking them if I do so. I only replied to you because I wanted to help you.
17:40
@WillHunting Sounds like a healthy condition for the site... I'll try my luck perusing the meta and ask a question if it seems like others have in the past
@CaptainAmerica16 A word about the subset notation. Some people use the notation without an extra line at the bottom to mean subset, but some use that to mean proper subset.
@WillHunting I was just reading a Math.SE post on that yesterday. It threw me off a bit when I looking at examples from different sources. From now on I just assume it means subset unless otherwise stated.
@CaptainAmerica16 And of course, for your school exams, you need to use the notation that the exam prescribes. Don't forget that.
@WillHunting True. When it comes to math, I need to get better at utilizing exactly what's presented to me. Not necessarily with tests, but with problems in general. I've found I overthink things to the point where I miss obvious details.
@WillHunting You point about functions a few comments ago is very helpful. That's the thought pattern I'm following. I'm just trying to find a way to express it.
@CaptainAmerica16 Yes, just be careful if you have the tendency to overthink. Don't let that lead to anxiety problems or mental illnesses.
@CaptainAmerica16 Yes, once you truly understand some things, you can use a few words to express the idea beautifully. That is one test of understanding.
17:55
@WillHunting Lol, not to get too deep, but I've been doing a lot better with "overthinking" this year. I used to have bad tics, but now it's only when I'm stressed. Thanks for the advice.
18:06
@WillHunting ...And I think I just realized something big about the proof I'm doing.
So for the integral $\int_0^1 \sqrt{x}\sqrt{1-x}dx$ I started off by doing u-sub, setting $u=\sqrt{x}$. I'm a little stuck after this
I also found $du=1/{2\sqrt{x}}$
18:25
$$-\int_0^1 u(-2u) \sqrt{1-u^2} du$$
You should be able to do an IBP with u differentiated and then $(-2u)\sqrt{1-u^2}$ integrated with inverse chain rule I think...
We are currently offline for maintenance.
damn it
@WillHunting Jasper is that you ?
@GabrielRomon Yes bro.
how are you doing ?
@Ultradark there's an easier way: do a linear substitution: make a linear substitution $u=\frac{x}{2}+\frac{1}{2}$ do you know the integral $\int_{-1}^{1}\sqrt{1-x^2}\mathrm{d}x$? this has a clear geometric interpretation
18:32
@GabrielRomon Bad. =(
@MatheinBoulomenos yeah the integral is related to arcsine, how did you decide to do that linear substitution though?
hi @MatheinBoulomenos
$\sqrt{x}\sqrt{1-x}=\sqrt{x-x^2}$, then it's just basically completing the square and multiplying by a constant to get it into the form $a^2-ax^2$ (in this case with $a=\frac{1}{4}$)
@WillHunting if $A = B$ can you also say $A \subseteq B$?
and vice versa
Hi @LeakyNun
18:39
@LeakyNun Hi.
@CaptainAmerica16 Yes. In fact, two sets are equal if and only if each set is a subset of the other.
$$\int_{-1}^{1}\sqrt{1-x^2}\mathrm{d}x$$
@WillHunting :D
@Ultradark yes, you can compute $\int_{-1}^{1}\sqrt{1-x^2}\mathrm{d}x$ with an arcsin, but it's really just the area of a half-disk, one learns how to compute that before you learn integrals
I never understood how to predict which definite integrals are easy without seeing pictures
18:40
@LeakyNun How is it going?
@CaptainAmerica16 Of course, if one set is a subset of the other, they may not be equal, because the second set might contain elements the first one does not.
or more generally, predict what substitution will transform a given integral into the form:
@WillHunting True, I think I have a way to prove that the sets are equal.
$$\int f(x) dx = \int \text{easy integral} + \int \text{integral with a geometrically obvious meaning}$$
what really is geometry anyway...
@CaptainAmerica16 Yes, very often to prove two sets are equal, we try to prove both inclusions, that each is a subset of the other.
18:42
@MatheinBoulomenos I just had my first lectures today
they were slow
@WillHunting That's perfect. Not saying I have it completely figured out, but I think I'm finally on the right track. It literally all stemmed from your comment earlier.
Why is Math SE down? What kind of maintenance are they doing?
@CaptainAmerica16 Thanks. You make me feel very smart.
@Secret I think circles are universally agreed upon to be geometry
@WillHunting lol
@CaptainAmerica16 Routine maintenance.
18:44
We're investigating a database overload and working to resolve it ASAP.
@Secret Geometry is the study of shapes.
@MatheinBoulomenos True. What is less clear is what integrands and what integration region will give a geometrically straightforward integral
I don't think there is a systematic relation between the two
More rigorously, Define the class of all geometric integrals as the ordered pair $(f,\mathcal{D})$ such that $\int_{\mathcal{D}} f d\mu$ is computable without series methods
I just read the news about the sexual assault allegation against Cristiano Ronaldo...
The site is working now.
Cool beans
they probably solved the problem by checking stackoverflow
18:51
@Secret @MatheinBoulomenos have either of you heard of systolic geometry? It's a branch of differential geometry
Systolic geometry is a branch of differential geometry, a field within mathematics, studying problems such as the relationship between the area inside a closed curve C, and the length or perimeter of C. Since the area A may be small while the length l is large, when C looks elongated, the relationship can only take the form of an inequality. What is more, such an inequality would be an upper bound for A: there is no interesting lower bound just in terms of the length. Mikhail Gromov once voiced the opinion that the isoperimetric inequality was known already to the Ancient Greeks. The mythological...
so mathein
the whole idea of quotient contruction is that we work with smaller group / object and keep the same structure
did I get that right ?
@MatheinBoulomenos @LeakyNun Hi
The site is still not working well.
@Kasmir hi
Hi Will
18:56
We are offline for maintenance.
I hope this systolic geometry has nothing to do with that riemannian hypothesis question. A month full of chat conversations that does not line up is enough for my poor brain
repetition is sooooooooooooooooooooooooooooooo boring
@Kasmir I'm not sure about "keep the same structure", but that intuition is in the right direction
I wonder if the database overload has anything to do with recurring formulas on this site.
on the other hand, seems like it has close relationship with fluid mechanics
Yes, next we will be talking about the Navier-Stokes problem.
18:58
@MatheinBoulomenos I meant inharite the homomorphism =p
@Secret I'm just exploring different topics in math for fun to see what interests me the most
ok that really clears it up
quotienting certainly means that make the group/object smaller by grouping different things together. For example in $\Bbb Z/2\Bbb Z$ we group all even numbers and all odd numbers together, thus dividing the group into two objects
Because the quotient operation is defined on the group and the image inside the target group =p
Yes this part of algebra i did not get properly and it went down hill for there =p
now am watching ben gross lectures and they are very very good :D
might give it a quick read while on the train as I always lack of time except when on the train
19:00
@KasmirKhaan I think working a bit with groups in terms of generators and relations is a good way to get used to what it means to take a quotient.
@KasmirKhaan intuition does not always come right away, sometimes you need to work with something as a purely formal thing and you get some intuition later with experience (at least that's how it is for me)
one question now that might occur hmm
Because there is becomes very clear how it really means to take some set of elements and throwing them away.
@MatheinBoulomenos that is what I had to understand the hard way, i was fighting to get an intuition without much experience
@MatheinBoulomenos @TobiasKildetoft Thank you guys ! :)
@MatheinBoulomenos Can really relate on this when studying point set topology
19:02
@TobiasKildetoft yeah I agree
algebra is very beatiful subject once one really understand it properly ><
Suddenly everything mostly clicks when the concept of nets is introduced
generators and relation ?
like in D_2n
We did not even mention that last semester i took algebra
I personally like to study algebra with actions and sometimes morphisms
makes it more like an animation
and one can code these to visualise the orbits
Secret no offence but you are a very strange guy so I cant relate to how you do your thing :D
I never understood one thing you put on chat, exept very rarely like now when you talk normal ):D
19:04
Yes, it is very hard to understand Secret.
@KasmirKhaan I don't think anyone understands most of what Secret says here
@TobiasKildetoft Tobi ! do you have somethign about relation and generators ? or should i use wat is written in DF
haha true that
I think Dummit and Foote will serve all your algebra needs for now @KasmirKhaan
I am very strange, it takes a while to understand what I am doing, because as it turns out, according to artists, I actually think more like an artist than a scientist
Mathematics is an art, not a science, even though it is the language of science.
19:06
@KasmirKhaan There is a short bit about free groups and presentations in home.math.au.dk/sorsted/persp_algebra.pdf which I referred to when I went over it in lectures
If something I want to brought attention to you guys, I often will spend some time to explain what I am doing
e.g. when I talked about integral symmetries back then and the semiclassical expanded on my hypothesis with galois theory considerations
@TobiasKildetoft Perfect thanks Tobi !
@WillHunting it is good then i bought that book ! =p
okay Kasmir will keep his fight with algebra now =p thanks yall
or when those times Leaky and Alessandros elaborates on the infinite things I am trying to construct
I am currently taking some light dab on forcing in wikpedia because of the dedekind finite borel set article
cannot guarentee I will be able to understand it, but it is fun to try
Because these pathologies are really fun
@KasmirKhaan Note that that book defines rings without 1, while other books may define rings to include 1. And that book actually covers up to first year graduate level algebra as well. I like how it proves the cubic and quartic formulas when you do Galois theory.
I'm reading about projective space, might have some questions to ask as I'm reading
19:15
@WillHunting Thanks Will , Ill keep that in mind
@Ultradark You still here?
@CaptainAmerica16 yes
@Ultradark Looking at this question: Prove that, for a function f:X→Y, we always have A ⊂ B ⇒ f(A) ⊂ f(B).
In how many lines would you say you could prove it?
Hello!
@Ultradark I just want to make sure I'm on the right track. I think it's turning out to be simpler than I originally thought.
19:28
I'm looking for a result in functional analysis that says if the difference of two functions is small in norm, then their integrals are also close. Does anyone know which theorem I'm talking about?
@CaptainAmerica16 I think that should be a fairly short proof. I think you could do it in less than 15 lines! You can always be more thorough with a proof and write out every little detail to understand it completely. That helped me when I took an intro proofs class. But if you really understand, you can make the proof pretty succinct and concise.
@Ultradark Ah, ok! I'm still trying to figure out what is necessary to define and what isn't, but what I have so far seems to be lining up with what you described. I'm finally making progress!
that's good!
19:47
@Anthony which two functions and which norm? Continuous functions under sup norm? And on a compact domain?
In particular, I'm considering approximating a step function with polynomials.
I'm looking for the formal statement that says "since these two functions are close a.e., then they should have similar integrals." It sounds like something that I learned in a functional analysis class, but that isn't important enough to have a name. I was looking in Reed and Simon, but couldn't find it.
@CaptainAmerica16 You can prove it using less than 5 sentences.
@WillHunting So far I have two.
@CaptainAmerica16 When you are done, show me your proof and I will comment on it.
@WillHunting Ok
 
1 hour later…
21:08
@AkivaWeinberger Hi Akiva
I got a question
21:19
@AlessandroCodenotti Hello !
22:01
Let K be a finite extension of F and $L_1, L_2$ are intermediate extensions.
How can we show that there isa basis of $L_1L_2$ over $L_1$ that consists of elements of $L_2$ ?
Could you give me a hint?
22:12
@TedShifrin Ted :D
@Ted ! I have bad news
22:32
Oh no @EricSilva!!! :(
hi @Kasmir
Finally Ted you are here :)
I got a small question
What's that?
So we W subspace of V
dim W = m and Dim V = n
when we take V/W
why is it the case that the vectors v_m+1 , .... v_n form a basis for that quotient ?
What are those vectors?
and how come it is also inside V
22:34
Yeah, totally wrong.
vectors past the first m vectors in that spann W
It is isomorphic to something inside $V$, but it is not.
are in the kernel
So write a coherent explanation. You take a basis $\{w_1,\dots,w_m\}$ for $W$ and you extend to a basis $\{w_1,\dots,w_m,v_{m+1},\dots,v_n\}$ for $V$.
hmm what I dont get is , we get a subspace of V mapping to V/W
Sorry I was not very orginazed
22:36
Then you must prove that $v_{m+1}+W,\dots,v_n+W$ forms a basis for $V/W$.
@TedShifrin Sid's course AND my backup math course got cancelled lmao
@EricS: Did they make any effort to recruit students?
i got couple of questions about this contruction
At UGA those of us teaching advanced grad courses always sent around emails giving propaganda.
22:37
first many vectors of W are mapped to 0
If we wanted to teach our courses, of course.
and the other cosets so to speak are only 1 element coset
What do you mean 1 element coset?
it's all very sad
@EricSilva: So can you not take Benson's course?
22:37
im scrambling trying to figure out if there's anything else interesting
i could but it's a very bad time for me
we just collapsed W to 0 and the remaining vectors that are in V but not in W, each form a coset in V/W no ?
id have to rearrange stuff
Oh, I thought it was the one conflicting with Sid's.
oh no there's 2 courses for benson
@EricSilva -.-
22:38
the one conflicting is like a seminar style thing that is too advanced for me
The cosets are planes parallel to $W$, @Kasmir, passing through the vector $v_j$, $j=m+1,\dots,n$.
Oh, @EricSilva.
You need to write out the proof that what I said is a basis for $V/W$, Kasmir.
hmm okay :D
@EricSilva: Is there a possibility of doing a reading course with Sid? He will have to teach another course, of course, but ...
Thanks yet again Ted my hero
@LeakyNun Leaky ! how are you ?
@TedShifrin im planning on asking
22:41
hi @KasmirKhaan
That ends up being more work for you, but it might still be fun. And I gave you plenty of exercises :P
(Gives you an excuse to tell Sid I say hi. :P)
indeed

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