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00:01
maybe? setting-related stuff is usually the beginning, not the end, of an investigation. having a group that acts on something isn't an end in itself
tons of open and maybe unsolvable problems relating to group actions
One thing that's interesting to me is acting on each point with the same group action. To me this might constitute a sort of global symmetry of the space.

Also you could act on different points with different group actions.
i hesitate to do this, but a lot of people who study 'symmetries' find slightly more general structures to be more appropriate to their need than groups. which is not to say that groups are not already the site for tons of difficult problems, some of whom will not be solved in our lifetimes
see e.g. this survey article on groupoids for a taste ams.org/notices/199607/weinstein.pdf
from almost 30 years ago. just knowing "oh there is some group[oid] that encodes some of this information" it isn't particularly useful or helpful
math is hard
groupoids are good, but you can usually already go quite far with groups
very interesting
Yeah I definitely have more structure than just group for my space I'm trying to understand. What's more difficult is figuring out how to make these different structures talk in non-trivial ways
00:21
Maybe someone can set me straight on this. I'm reading this question, where apparently $$\mathcal{M} \otimes \mathcal{N}:= \{A \times B: A\in \mathcal{M}, B\in \mathcal{N}\}$$is not always a $\sigma$-algebra despite $\mathcal{M},\mathcal{N}$ being ones. Now consider the following Proposition in Folland:
I feel like this is saying the very opposite. Hmm.
Oh wait.
I think I see the difference.
Folland is saying the $\bigotimes_{\alpha\in A}\mathcal M_\alpha$ is generated by the set above.
Hope I'm making sense :)
yes. it is very common for the simplest description, maybe the only description worth retaining mentally, of a sigma algebra, is in terms of a generating set
the sigma algebra itself can be quite complicated to describe. like the borel hierarchy, what the hell is that
like shooting fish in a borel
or the completion of borel measure on R, thats a whole lot of very goofy sets of measure zero
indeed
@BenSteffan that's usually a description of something too easy, isn't it
00:30
yes
26 mins ago, by leslie townes
math is hard
00:45
math is hard but sometimes its easy
making it look easy is part of the hard part
01:00
> Therefore we may call that art true art which does not seem to be art.
01:23
Math is the easiest when its complicated
 
3 hours later…
04:04
Hi everyone. Could anyone provide a link/reference to anything related to this generic problem: In how many ways can we get $a_1,a_2,a_3,….a_n$ so that their sum is $k$ and $0\leq a_i\leq x$? Basically the problem of putting balls in bins with a max capacity of each bin
04:42
In number theory and combinatorics, a partition of a non-negative integer n, also called an integer partition, is a way of writing n as a sum of positive integers. Two sums that differ only in the order of their summands are considered the same partition. (If order matters, the sum becomes a composition.) For example, 4 can be partitioned in five distinct ways: 4 3 + 1 2 + 2 2 + 1 + 1 1 + 1 + 1 + 1 The only partition of zero is the empty sum, having no parts. The order-dependent composition 1 + 3 is the same partition as 3 + 1, and the two distinct compositions 1 + 2 + 1 and 1 + 1 + 2 represent...
it isn't clear from the embedded link but there is a subsection of that page entitled "restricted part size" which is relevant to your query and the link points to that subsection
In the number theory of integer partitions, the numbers p k ( n ) {\displaystyle p_{k}(n)} denote both the number of partitions of n {\displaystyle n} into exactly k {\displaystyle k} parts (that is, sums of k {\displaystyle k} positive integers that add to n {\displaystyle n} ), and the number of partitions...
a common way of approaching this type of problem is to consider the desired number as a function of the inputs (here k and x) and to look for recursive relationships between the values of this function for different inputs
it is often possible to identify a small number of such relationships that allow a computer (or a patient person computing by hand) to compute the desired quantity for any input
05:14
I have never read Folland before, it is pretty nice, at least for the measure stuff.
yeah, i mostly like it as a text. it doesn't even try to be comprehensive, which is a plus.
you can feel a pretty strong authorial hand in pushing the reader away from technicalities and toward general results and broader themes, that i don't sense with books like the rudin books or other things in that vein
its not a replacement for a good functional analysis book, or a good PDE book, or a good probability book, but it treats those topics lightly enough that you wouldn't confuse it for something like that
i am surprised in my search for simple answers (after damaging the wall with Durrett, Doob, etc) that i never looked at Folland. maybe i can find a hard copy in Moe's to add to my pointless eclectic library.
the last place remaining in Berkeley that i wish to visit.
except for Arinell's perhaps.
and Bongo Burger.
maybe you'll find my copy. my copy of folland was stolen by or from the us postal service
still has my name in it :)
:-) your pseudonym?
sadly no, it has my government name in it
if your copy has characteristics identifying it as belonging to someone in or near berkeley, do try to reach out to the former owner and ask if they parted with it voluntarily
then we can get rich suing moe's for fencing stolen property
05:30
:-) lol. will check, should i be lucky enough to find said property. i miss the days that one would rub elbows with notables in the math section of Cody's.
no doubt i will be deported as soon as the orange fellow takes power
vague tinge of ethnic irony
The all blacks upset your boys today
my daughter and i visited berkeley a few months ago. almost everything i remembered was gone, but she liked it
we saw an AC transit bus get in a pretty expensive but harmless looking accident
great times
How did you explain the crash to her?
@leslietownes Thanks!
05:40
yeah, the lads must've had one too many the night before
Berkeley has done one of those inversions. all business, big buildings while pretending to be doing good. at least the gas ban failed to pass.
even Pettingells is gone. berkeleyside.org/2020/12/02/…
@leslietownes Actually, I think my scenario is simpler than partitions, because in my case (1,2,1) and (1,1,2) are both counted separately.
Like the permutation form of partitions instead of combinations
112 is the European emergency number. not sure what genius thought of that
Ireland used to be (still is) 999. never used it. the few times my mum collapsed we just drove her to the hospital, waiting for an ambulance could have taken all day.
And after 5pm, the police were a minimum of 30mins away.
we had some burglaries, including one with my frail mother in her 80's discovering folks had broken in downstairs at 2am. she confronted them, of course and they left.
Brave woman.
👏👏👏
05:53
we had other words to describe her actions. not in front of her, ofc
my sister channels her. growing up with 4 brothers probably helped
i have only been slapped in the face by two people. one by an Irish consular official in San Francisco and the other my sister who had given me fair warning.
glad to say that my last meeting with an Irish consular official in SF went off without slapping.
i must be mellowing.
Wow, never by your parents?
not in the face.
corporal punishment was de rigueur in those days
May I ask, have you carried on the tradition?
not really, i did slap both of my offspring on the tail end when the bit me. but that was it.
Cillian Murphy's granddad used to slap me.
as in Cillian the actor
I still haven't watched any of Peaky Blinders :(
06:08
neither have i. i like Schwarzenegger style violence, Peaky is too real for me
same with Sopranos
he was good in Anna, if you have seen it
even if you haven't.
i suppose i have a preference for violent women
:-) jk
Thanks for the suggestion, I'll check it out.
movie taste rarely transfers in my experience, but i hope you enjoy it
i;m still waiting for a good time to binge watch The Diplomat 2nd season
it was the first show that i binge watched ever
@leslietownes i am happy to report that Folland excludes zero from the natural numbers.
sensible man.
06:34
IIRC, the good doctor Ted Shiffrin agrees.
Dec 10, 2022 at 21:40, by Ted Shifrin
I just think it's obnoxious to think that only a trivial minority of mathematicians don't consider $0$ a natural number.
pie
pie
07:24
Is there a rigorous ODE book? I'm tired of books that focus on 'just doing this because it works' and am looking for one that provides a more proof-based approach to ordinary differential equations.
Why don’t such books exist?
07:46
@pie One of my friends recommended the book by Arnold (Ordinary differential equations) for similar situation
I believe it is translated
But in my opinion it is very hard unless you already gone through a standard "just do this and it works" course
 
2 hours later…
09:45
Arnold's book is not easy
10:30
Yeah
11:19
@copper.hat I see you're a man of culture as well :)
11:56
is there a way to construct a linear operator on $V(\mathbb{C})$ from a monic polynomial so that this operator's minimal poly is this one?
assuming fdvs and all that
we want basically $ T(v)=-\frac{(c_0 I + c_2 T^2 \cdots T^m)(v)}{c_1}$.
@nickbros123 Companion matrix
wow nice
thanks
 
1 hour later…
general question: should I just look at the proof
exams coming up :cryingemoji:
general answer: review old exams
🧐
13:40
I just hate going through proofs of theorems without trying to prove them for a while. But i guess such is exam preparation.
I just had some great inspiration
nice
i shud stop procrastinating, byebye!
nicer than nice
actually no I still have no idea I retract everything
14:11
What's your opinion on Lang's Algebra?
I've read the first 15 pages or so and I like it, does it have any disadvantages?
It's not for everyone but if you like it
every book has its disadvantages, but I don't know enough about Lang's algebra to say
you should be aware that Lang was a terrible person, however
which is not to say that you shouldn't read his books but
@BenSteffan I saw Bonn has it in its recommended literature for an Algebra course too? I assume you could only cover less than a chapter in a single course though
It covers a lot of material
@ILikeMathematics I've completed first chapter of that book with exercises. I've enjoyed it, but its definitely a little bit terse
It has an entire chapter dedicated to linear algebra, is that really usual? Or would you just call that representation theory
14:15
It's unusual for an algebra book
It's truly linear algebra, not representation theory
Although it of course also contains some representation theory
@BenSteffan I don't know. Its not really needed to know that a given person in mathematics was a terrible person for doing mathematics
Historically it might be relevant, I guess
@Jakobian No, but it is something you should be aware of
Some people have strong feelings towards Lang's books because of this
@Jakobian Actually it's a lot more chatty than my linear algebra 2 lecture notes haha
Same with Lovecraft right, you don't need to know he was a horrible person to enjoy his cosmic horror literature
14:18
@Jakobian How did you feel about the exercises? I read some people think they are 'on another level'
I guess the takeaway from this is that horrible people can still produce something good
and that you shouldn't judge people only based on their expertise in a given area
there can be some discrepancy between the author and his works
@ILikeMathematics Its as if Lang was taking something advanced in abstract algebra and putting it as an exercise because you have all the tools to do it
They're not very memorable to me, but they are interesting, and you can certainly take away something from them
@Jakobian So he's letting you prove some more advanced stuff?
Sounds a bit like what people say about Hartshorne then
What I'm saying is that usually people like to give only elementary things that an undergraduate could come up with based on his current knowledge. Some of the exercises in Lang seems to come from his external knowledge and aren't something that you could come up with on your own based on the material presented
14:25
Interesting, thanks
They're advanced in the sense of being too advanced for the material, yet you still have the ability to complete them
They're not super hard - I've managed to complete them all. I saw harder exercises
Hi
@Jakobian How long did it take you to get through the first chapter?
I think it took me a month
220 pages in a month is pretty insane
That's 7.3 pages a day
14:30
I was very motivated at the time
@Jakobian Did you take notes?
No, but I did write up my solutions to the exercises
I mean what is there to take notes of - the book is the notes
Well I'm still trying to decide if it's worth it to take any, I've heard it helps even if you don't actively notice it, but it slows everything down
14:31
unless perhaps I wanted to add something, maybe venture in a weird direction
but algebra is not my field anyway
I like to experiment in fields that actually interest me
@Jakobian Do you feel like after having read the 220 pages and arriving at the exercises, you still know everything that was in that chapter? Even if you just read 220 pages without writing anything down?
@ILikeMathematics I haven't been revising my algebra knowledge, I definitely forgot almost most of it
Well I just mean after that one month, not now
@ILikeMathematics I like his exercise about homological algebra which was removed from the third edition
@LukasHeger Haha, take a book on homological algebra and prove every theorem in it
@Jakobian And one more thing: did you begin doing the exercises whilst in the middle of the chapter or did you first complete reading it?
14:39
@ILikeMathematics I mean yes. Moreover a lot of it was elementary stuff. But I still would recommend not pacing too much to remember it better in the long run, if that's the goal
My methods at the time definitely weren't perfect
@ILikeMathematics I did the exercises after reading the whole chapter, and I was coming back to the material occasionally to remind myself of some things
@ILikeMathematics my opinion is that it is great if you already know some algebra to deepen your understanding. I wouldn't recommend it to a beginner in abstract algebra. But if you enjoy it, you should keep going, you can certainly learn a lot from it.
From the first chapter, I'd say the most important to remember are the Sylow's theorems, because they are extremely useful when it comes to finite groups, but you don't use them a lot so they disappear from memory quickly
@Jakobian Alright, thanks. It just seems like a really big gap - 220 pages and only then doing any exercises
@LukasHeger Alright
I don't know whats weird about it. Its natural to do things in order, isn't it
One disadvantage might be its sheer size (which is not strictly a downside of course). But if you want to go deeper in algebraic subjects, there is a point where you want to switch from "general abstract algebra" books like Lang's to more specialized stuff like more advanced group theory, commutative algebra, non-commutative algebra, representation theory, homological algebra, algebraic NT, algebraic geometry etc.
The point I'd make is that it's by no means necessary to read a ~900 pages book on "general abstract algebra" before you make that switch to more specialized stuff
2
14:44
@Jakobian Yeah it is, I'm just saying it's different than how you would do it in a course, where you attend maybe 1-2 lectures of 90 minutes and afterwards solve exercises
@LukasHeger Yeah sounds reasonable
I don't know. If I did things out of order it would be very annoying to me
I know its largely in the mind, but I get stuck a lot in terms of decision-making skills for example
So if you feel particularly interested e.g. in rep theory, I wouldn't necessarily wait until you've finished Lang to start reading specifically about rep theory. You could pick up some introductory book on some more specific topic to read on the side
Besides given that Lang has no dedicated exercises to every section, it would feel way too inefficient to just go back and forth
moreover you'd have to decide which exercises you can do at a given moment - do I really have all that processing power to do so
I don't know, it just sounds like a bad idea
14:51
tbh I don't think it's actually accurate to call those topics more specialized or more specific. "more advanced" is more like it. All those subjects are huge fields of their own
Breadth vs depth
If you read Lang you'd be jack-of-all-trades in algebra, but that's most likely not what you actually want to do
It does seem like you just want to read one specific chapter though
@Jakobian You took a course in algebra before, right? Or did you actually learn it through Lang
I did took a course in abstract algebra before, but the idea was to just chill and take things slowly. Our professor was basically retiring so they did the course like that. It wasn't overburdened with information, so I didn't learn everything from it, but I did get more confident in abstract algebra because of it
Other than that, I think I did occasionally learn about abstract algebra from other sources as well, so its not like it was my only abstract algebra-related experience
For example, I read a lot about semigroup theory, and about universal algebra
because I was in a phase
nowadays I don't touch abstract algebra much
15:38
@Jakobian out of curiosity, what did you read about semigroup theory? I only read Steinberg's Representation Theory of Finite Monoids
16:06
@LukasHeger Clifford, Preston, Algebraic semigroup theory I
thanks
its a bit of an old book, I'd say there are better positions for a good overview
I believe Howie was a pretty good one from what I browsed
or maybe it was Higgins... one of the two
let me check actually
I think the one I liked was actually Nine Chapters on the Semigroup Art by Alan Cain
@LukasHeger
thanks
Howie is most likely more complete though
@Jakobian in that case you can't not know lol
you won't find Lang's AIDS denialism within his algebra textbook, but you will find Lovecraft's racism in his novels
16:21
@Thorgott oh? I didn't know he included it there
I only knew about his cat
fun fact: the cat also appears in one of his novels
anyway, yes, though of course it varies from work to work
if you read Call of Cthulhu, you will find like a handful passages with vaguely racist descriptions of foreign tribes, which you may not pay too much heed to, but then you can also read The Street, which is just thinly (if at all) veiled racism from front to back
(I do not recommend reading The Street)
ph'nglui mglw'nafh Cthulhu R'lyeh wgah'nagl fhtagn
pie
pie
@VladimirLysikov: "One of my friends recommended the book by Arnold (Ordinary differential equations) for similar situation
I believe it is translated"

But is it rigorous? I took a look at the book, and it includes physical examples and content related to physics. Are you sure it’s rigorous?
Also how do you quote or reply to certain message in a room chat?
16:37
it might vary depending on your client (e.g. i don't know how it works on mobile devices). if i hover my mouse over a message there's a little arrow i can click to respond to that message
you didn't ask me, i have read some arnold in translation and it all seemed pretty rigorous to me, although it was definitely not written in the formalist style that has become the default in english-language mathematics through the pernicious influence of French people
it might be better to phrase things in terms of style rather than rigor. i personally encounter problems with arnold's style
pie
pie
@leslietownes @leslietownes Why there is no PURE math book on ODE?
By pure here I mean that doesn't mention or discuss anything outside pure math
i'm sure there are many such books, they just aren't popular or written for wide audiences of non specialists
i think the issue is that way more people want to use or learn aspects of ODE, than there are pure mathematicians
:)
and for better or worse a lot of motivating examples and problems in the subject do arise from outside of pure mathematics :)
pie
pie
@leslietownes I couldn't find any such book, questions like this have been asked on MSE and MO several times and I non of the recommendations are like this.
Calculus was primarily motivated by physics, yet +90% of real analysis books don't mention anything related to physics.

Why isn't there a 'real analysis' equivalent for ordinary differential equations (ODEs) that similarly removes all non-mathematical discussions, just as real analysis does?
well, i don't know of any that take ODE as the organizing principle. you do find rigorous development of at least some ODE theory in some real analysis books, and books on smooth manifolds or riemannian manifolds, or in functional analysis books
i may be repeating myself, but i suspect that the market for that type of book doesn't exist at a size that makes it a meaningful commercial proposition. people are getting enough of the theory that they need for what interests them in other kinds of books
i saw it mostly from a functional analysis point of view, there's the "basic" theory that is worked out everywhere, maybe not in the generality that you want it, and then there's things where people will develop miniature chunks of functional analysis specific to a single family of differential equations
I don't see how including examples from physics makes a book any less "pure". If it's just an example, then it's just an illustration of the theory. This doesn't make the theory less pure or rigorous
16:50
i think eventually, if you have enough curiosity about ODE as ODE, you get over this allergy to examples :)
i would love to read a bourbakist ODE book, just as a curio
Bourbaki himself would probably try to treat real, complex and p-adic ODEs simultanously (yes, p-adic ODEs are a thing...)
yes, and such ODE are special case of PDE, which are special cases of differential relations on a k-stack of jet stack scheme gerbes
can't get much more rigorous than that :)
I mean we already have nlab pages on differential equations
pie
pie
@LukasHeger So what is the prerequisites that I need?
but do we have enough nlab pages on differential equations
pie
pie
16:53
@leslietownes What is that books?
pie: a hypothetical construct, i don't think it exists
@leslietownes can there ever be enough nlab pages on anything
@pie perhaps one prerequisite would be a tolerance towards physics examples... I think in most ODE books you can take the physics as black box that explains why a certain equation comes up. If that kind of context doesn't interest you, you can just take the equation on its own and ignore the physics background. I'd bet most ODE books are still readable with that approach
@leslietownes All you need to know about differential equations is that for any scheme X there is an equivalence of categories between the Eilenberg-Moore category of the jet comonad over X and the category of partial differential equations with variables in X
pie
pie
@LukasHeger I wish I had more tolerance for physics examples. For some reason, they make everything more complicated for me. Each illustration leads me to more questions about physics, and those questions only bring up more questions, leaving me confused and with a headache. I’d love to study physics after I've mastered its mathematical foundations, but I don’t enjoy studying or discussing physics without a solid understanding of the math behind it.
@pie what I'm trying to say is that you should in your head replace any physics explanation with "this is some reason why other people care about this equation, I can just study the equation on its own", if you're uncomfortable with the physics. I don't think most ODE books actually require physics knowledge to understand the math
17:12
@pie I would say it is rigorous, yes, modulo very concise style. And of course everything Arnold writes on history of math and science should be disregarded
But it seems you want a book that doesn't mention physics rather than just a rigorous one
That would be hard, I think
Hi! I have a question. Suppose $f$ is a locally Lipschitz continuous function and $y_0$ be such that $f(y_0)\neq y_0$. Now if I consider the ODE $\frac{dy}{dt}=-y+f(y)$, $y(0)=y_0$, by Picard's theorem it's a unique solution. I want to know if it's possible for $y(t)$ to be a fixed point of $f$ for some $t>0$.
Does somthing like this work? ODE course was long ago for me.

Suppose $y(t_0)$ is a fixed point, then we can write an ODE for the function with reversed time $u(t) = y(t_0-t)$: $\frac{\mathrm{d}u}{\mathrm{d}t} = u - f(u)$, and the constant solution is unique up to $t = t_0$, so $y(0) = u(t_0) = u(0) = y(t_0)$.
I don't think TikZ works in MathJax
17:31
It doesn't :(
If the two rows are exact sequences, why can be sure that the vertical arrows are isomorphisms?
G -> G is clear
How come G' -> ker(G) is one?
@VladimirLysikov what do you mean "the constant solution is unique up to t=t_0"?
@ILikeMathematics what are the maps?
what are $G$, $G'$, $G''$?
$\ker G$ doesn't make sense unless $G$ is a map
groups
Oh wait I messed up a bit
you have to actually give us all the data, otherwise this is not answerable :)
17:34
May 11 at 12:37, by Sine of the Time
"Ambiguous questions receive ambiguous answers, clear questions receive clear answers"- Sun Tzu
I mean the solution $u(t) = u_0$ is locally the unique solution of $u' = u - f(u), u(p) = u_0$ for any $p$, and therefore the unique solution for $u' = u - f(u), u(0) = u_0$ on the segment $[0, t_0]$.
Ok now
Why can we be sure that we have these isomorphisms, if the rows are exact sequences?
G -> G is clear
I shouldn't use the first isomorphism theorem because at this point, it hasn't been introduced yet
We know that 1) f is injective, 2) g is surjective and 3) im(f) = ker(g) from the first exact sequence
G, G', G'' are groups
"Define a map $G' \to \ker g$ by [fill in this blank]"
@VladimirLysikov got it.
Thanks
@BenSteffan Oh, can't we just pick f? We know f is injective, if we now restrict the codomain to ker(g) then it must also be surjective since im(f) = ker(g)
17:43
Yes, very good :)
Ok, then only $G'' \to G/\ker g$ is left
@BenSteffan Well the problem with the last isomorphism is that I also can't go the $G'' \to G \to G \to G/\ker g$ route because $g$ is not necessarily invertible
No, this needs a little more work :)
Isn't this just the first isomorphism theorem?
(well, which I can't use)
Should be.. I know it as $\text{im} \varphi \cong V/\ker \varphi$ for $\varphi: W \to V$ from linear algebra
This exercise is asking you to prove part of the first isomorphism theorem, in a sense
Hint: Let $X \twoheadrightarrow Y$ and $X \twoheadrightarrow Y'$ be two surjections (of sets). Then there is a unique function $Y \to Y'$ that makes the obvious triangle commute.
18:02
@BenSteffan Hm,
Again we can't go the Y -> X -> Y' route
In the setting of the hint the map is not going to be an isomorphism
think about elements
and where they have to go
Well surely we can define some map Y -> X
For each element of Y, just choose one element of X that maps to it
ah, I should have thought this through a little better
yes, you can do that
...I guess it yields the same thing, in the end
18:07
Then compose with g and we get Y -> Y'
Not sure if this is unique though
Ok but I think uniqueness doesn't even matter, does it?
I am taking a course on complex analysis. Our univ is following the book by Scahum Series. The concept of branch points and branch cuts are explained somewhat by solving problems and not written down explicitly. So, I think I only have a crude intuitive understanding about them.
@ILikeMathematics It might not
Our prof recently gave us some assignments to solve.
18:08
but you should know it
It jad questions like this for example,
I have no idea on how to tackle questions like this.
Can someone give me some suggestions regarding this issue?
So we can construct this $f$
So we have $G'' \to G \to G / \ker g$
the thing I wanted you to observe is that to define a map $h\colon Y \to Y'$ making the triangle commute, I need to give an element $h(y) \in Y'$ for all $y \in Y$. But since $f$ is surjective, there's some $x \in x$ with $f(x) = y$. Applying $g$ to $x$ gives some $y' \in Y'$, and commutativity of the diagram forces you to define $h(y) = y'$.
@ILikeMathematics you can, yes :)
@SoumikMukherjee Can you help me with this?
but think for a second about whether this is what you want
18:13
@ILikeMathematics I personally think a proof of the first isomorphism theorem should not rely on the axiom of choice...
I think it's easier to go $G/\ker(g) \to G''$ than in the other direction. Since you construct an isomorphism in the end, it doesn't matter if you use this other direction
@LukasHeger Are you suggesting to go $G/\ker g \to G \to G''$?
no
I'm basically suggesting the same thing as Ben
@ILikeMathematics what's the map from $G$ to $G$?
yes. thus, [?]
maybe this is too obtuse
@ILikeMathematics rethink this diagram
you already have a map $G \to G''$, namely $g$. The idea is that you can use $g$ also to define a map $G/\ker(g) \to G'$ that makes the necessary diagram commute, if you think about it
there's a unique way to do this
in this case, it's easier to first figure out what this map $G/\ker(g) \to G''$ is, if it exists (it turns out to be unique!) And then work out that it actually exists
18:39
@BenSteffan .. there is an isomorphism from G to G''
Wait or am I tripping
Can we know this diagonal arrow f is surjective?
@ILikeMathematics no, not unless $G' = 0$
@ILikeMathematics can we? you tell me :)
is $f$ going to be a group homomorphism?
We take an element of $G''$. We determine its preimage in $G$. Then we send that to $G/\ker g$.
Maybe we can do something with this?
yes, that looks better
We just need to show that this gives us an isomorphism
Yes
As Lukas says, you might want to reverse the roles of $G''$ and $G / \ker g$
18:49
We take something in $G/\ker g$. Map that exactly on the element of $G$. This map is injective, not surjective. Then we apply $g$ onto it, which is surjective
We arrive at $G''$. We just need to show that the composition is bijective
But the composition of a purely injective and a purely surjective map can't be bijective, I think?
Why not?
@ILikeMathematics More importantly, you need to show that you get a group homomorphism
I think it's best not to think of the step $G/\ker g \to G$ as applying a map. It can be made into "applying a map", but I think that's not helpful. The obvious "map" that you're using here is not well-defined. But it becomes well-defined when you "compose" it with $g$
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