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00:14
Hiya @mixedmath
 
2 hours later…
01:46
Hi I have other question which is easy I think
are there someone here?
well I'll post it Hi I´m continue working in group theory and I´m again stuck, this problem says: suppose $N_i$ for $1\le i \le k$ normal subgroups of $G$ and define $N$ as the intersection of the normal subgroups. Then there is an isomorphism between $G/N$ to a subgruop of $\prod G/N_i$.
FIrst is almost trivial to show that $N$ is a normal subgroup. And I define the map as follows: $gN\mapsto \prod G/N_i$ so is not too difficul to show that the map is well defined and is injective and also is an homomorphism, so is sufficient to say something like is an isomorphism to the image of the map?
@VibhavPant hi
So I was trying to get c omplex plot of $e^{2/3z\pi i} -1$, the plot should be zero (black) at integer values of $z$, right?
@JoseAntonio hi
02:10
@JoseAntonio Remember that an ismorphism is just a bijective homomorphism - and it's a bijective homomorphism onto its image, so yup
First is almost trivial to show that $N$ is a normal subgroup. I define the map $f$ as follows: $gN\mapsto (gN_1, \ldots gN_k)$.

$f$ is **well-defined**. For is $hN=kN\iff h^{-1}k \in N$ and so $h^{-1}k \in N_i$ for all $i$. Hence $(hN_1, \ldots hN_k)=(kN_1, \ldots kN_k)$ as desired.

$f$ is **one to one**. If $(hN_1, \ldots hN_k)=(kN_1, \ldots kN_k)$, it follows that $\forall i (h^{-1}k \in N_i)$ i.e. $h^{-1}k\in N$ as desired.


$f$ is **homomorphism** Clearly we have a structure of a group. Then $f(gN kN)=f(gkN)$, so $(gkN_1, \ldots gk_k)=(gN_1kN_1, \ldots gN_kkN_k)=f(gN)f(hN)$
@MikeMiller thank as usual
You got it!
@MikeMiller Thanks :)
 
2 hours later…
04:26
@TedShifrin This answer is incorrect, isn't it? Null-homotopy of pairs generally means that, for any $f_t$ in the homotopy, the second term maps to the second term. But there's no reason AFAICT that $M-D$ should map to $x_0$ in some potential null-homotopy.
Alas, you are but a ghost
afloat in the ether of the internet...
04:42
@MikeMiller hiya! (only a few hours late)
hi @mixedmath
@skullpatrol hello
@mixedmath and hello back
Greetings @Chris'ssis
@Mike: AFAICT, he also confuses open and closed disks. He also doesn't relate $H_*(X/A)$ to $H_*(X,A)$, which can be done. I dunno. Way past my bedtime. Generally, I don't trust him, but this approach might work. I personally prefer mod 2 degree.
04:54
Hi Professor @TedShifrin
@TedShifrin Yup. He answered a tangent bundle question coherently, just now (showed $S^1 \times S^n$ was parallelizable by using your approach... embedding $S^n \times [0,1]$ in $\Bbb R^{n+1}$, used the constant vector fields in each direction, and pushed forward to $S^n \times S^1$
05:11
Hi
Again I need some hint with other problem of algebra... the entire day was dedicated to solve a lot of problems, abusing of this place do you think is possible give me a hint?
I'm stuck with the following. Let $N$ a normal subgroup of $G$. Show that if $[G:N]=4$, exists a normal subgroup $M$ of $G$ s.t. $[G:N]=2$. Since $G/N$ has orden 4, either is isomorphic to $\mathbb{Z}/4$ or $\mathbb{Z}/2 \times\mathbb{Z}/2$ and in any case we have a subgroup of order $2$ and for instance the same applies to $G/N$ but from here I'm not sure how to get the normal subgroup $M$ of $G$
sometimes I feel so stupid, hehe
05:40
lattice correspondence
subgroups between G and N correspond to subgroups of G/N
the correspondence preserves indices; a subgroup of G/N of index k corresponds to a subgroup of G containing N of the same index k
note that the correspondence is as follows: given a subgroup of G containing N, take its image under the projection G->G/N to get the subgroup of G/N; given a subgroup of G/N, take its preimage under G->G/N (equivalently, take the union of all of the cosets of N it contains) to get a subgroup of G containing N.
06:25
Hello, help please :math.stackexchange.com/questions/909005/… if you can
@Vrouvrou have you tried the homotopy room?
there is a homotopy room ?
@Vrouvrou MO
06:46
Greetings @robjohn @r9m
Here is a cute limit $$\lim_{x\to1} (2-x) ^{\tan(\pi x/2)}$$
to be done elementarily
@skullpatrol Hi
@Chris'ssis Hi
07:07
@r9m @robjohn have you seen this one? $$\lim_{x\to\infty} \sum_{k=0}^{n} (-1)^k \binom{n}{k} \log(x+k)$$
@Chris'ssis $$\lim_{u\to0}(1+u)^{1/\tan(\pi u/2)}=e^{2/\pi}$$
@robjohn Indeed.
@Chris'ssis is that not $0$?
@robjohn Sorry, I was away. No, it's not $0$.
For $n=1$, it is $\lim\limits_{x\to\infty}\log\left(\frac{x}{x+1}\right)=0$ and I don't see it being different for higher forward differences.
07:24
@robjohn Sorry, it's $0$. I mixed $2$ limits.
@Chris'ssis If it were multiplied by $x^n$, it might yield something interesting.
@robjohn Really? Let me see ...
$-(n-1)!$ I think
Is it possible to take differential equations course by knowing calculus I and a little bit of calculus II ?
@FractalHand I don't know how your calculus classes are divided up, but the more calculus, the better for DE.
07:35
@robjohn How did you get that?
@Chris'ssis The formula you have there is $(-1)^n$ times the $n^\text{th}$ forward difference, which goes like the $n^\text{th}$ derivative. The $n^\text{th}$ derivative is $\frac{(-1)^{n-1}(n-1)!}{x^n}$
@robjohn Ah, I see now.
08:00
Let $f$ be a continuous real-valued function on $[-1,1]$ such that

$$a_n=N(n)\int_{-1/n}^{1/n} f(x) \ dx $$
where $N(n)$ is the number of perfect squares between $n^3$ and $(n+1)^3$. Compute

$$\lim_{n\to\infty} \sqrt{n} a_n$$
08:11
Initially I wanted to write "compute it without pen and paper" but then I realized this might be annoying.
Ussing my immagination, I see that near $x=0$, we have that the integral can be approximated by a rectangle $$\int_{-1/n}^{1/n} f(x) \ dx \approx \frac{2}{n}\cdot f(0)$$
The number of perfect squares is $$N(n)\approx n^{3/2}$$
Q.E.D.
Ups, I did a mistake ...
I wanted to say that $$N(n)\approx \frac{3}{2} \sqrt{n}$$
Hence, we get that $$\lim_{n\to\infty} \sqrt{n} a_n=3 f(0)$$
This is a challenge I received this morning from a kid. It was pretty fun to attend it.
How about this version? $$\lim_{n\to\infty}n\left( \sqrt{n}N(n)\int_{-1/n}^{1/n} f(x) \ dx-3f(0)\right)$$
I need to buy a new keyboard ... some letters remain blocked here ...
08:41
@Chris'ssis Right. $N(n)$ is nowhere near $n^{3/2}$
@BalarkaSen I thought of the correct thing, but I wrote another thing.
haha, how that happen to us all.
@Chris'ssis Do you have any interesting series question? maybe Euler sums?
@BalarkaSen Isn't my limit above interesting? :-)
08:48
@Chris'ssis series
Not limit!
Especially a limit with an integral inscribed! Ughh
@BalarkaSen OKAY ...
Prove that

$$\sum_{n=2}^{\infty} \frac{n(1-x)+2}{n(n+1)(n+2)}x^{n+1} (n-\zeta(2)-\zeta(3)-\cdots -\zeta(n))$$ $$=\log\left(\Gamma(2-x)\right)+\frac{3-\log(2\pi)}{2}x +\left(\frac{\gamma}{2}-1\right)x^2+\frac{1}{6}x^3-x \log\left(\Gamma(2-x)\right)-\log(G(2-x))$$
No, no, that's too much for me.
=)
If you have something a little elementary, ping it to me.
Otherwise, leave it.
May I suggest the following: Calculate $$\sum_{k=1}^{\infty} \frac{1}{k^n(k+1)}$$
:)
08:52
@rehband Partial fractions?
@BalarkaSen I don't know yet
$$\frac1{k(k+1)} = \frac1{k} - \frac1{k+1}$$
Sorry there is a typo!
1 sec
@BalarkaSen Good point!
Wait, what?
08:54
No, there was no typo, I'm confusing problems :D
Sorry haha
@rehband =P
Well, there you have it.
True!
@Chris'ssis I am getting used to all this. Just a beginner.
=)
@BalarkaSen Well, there is always a beginning, isn't it?
Ah, true.
08:57
$$\frac{1}{k^n(k+1)} = \frac{1}{k^n} - \frac{1}{k^{n-1}(k+1)}$$
So, you have $$\frac1{k(k+1)} = \frac1{k} - \frac1{k+1}$$ What's next? :-)
@rehband Right, this is the next step. :-)
@rehband Keep partial factorizing.
Yes, keep splitting it up
@BalarkaSen What do you mean? It's correct.
08:59
You're right.
Wait, is it going to be $\zeta(n) - \zeta(n - 1) - \zeta(n - 2) - \cdots - \psi(1) - \gamma$?
@BalarkaSen What's the $\Psi$?
Digamma function
@BalarkaSen What's the Digamma function? :D
@BalarkaSen Thx
09:05
Well, it's $\zeta(n) - \zeta(n- 1) - \zeta(n - 2) - \cdots - \zeta(2) - 1$
@Chris'ssis
By the way, @rehband, you can handle your mistyped sum in a similar way
@BalarkaSen I don't help you this time ... :-)
@BalarkaSen Ah okay
@Chris'ssis No need to help, it's $\zeta(n) - \zeta(n- 1) - \zeta(n - 2) - \cdots - \zeta(2) - 1$
Just check the solution.
Nah
@BalarkaSen Well, consider the simple case, $n=1, n=2, n=3$.
Ah, right, I mistook.
It's alternating.
09:11
Yes, nice, I got that too now
$$\sum_{k = 1}^\infty \frac1{k^n(k + 1)} = \zeta(n) - \zeta(n - 1) + \zeta(n - 2) - \cdots + \zeta(2) - 1$$
Of course, signs are show-off. It depends on the parity.
@BalarkaSen There is a problem with the signs ...
@Chris'ssis Nothing, only that the sign I wrote holds for $n = 0 \bmod 2$
For $n = 1 \bmod 2$, it'd be $\zeta(n) - \zeta(n - 1) + \zeta(n - 2) - \cdots - \zeta(2) + 1$
@BalarkaSen For instance, look at the last term for first few cases.
@Chris'ssis Just read what I wrote
09:18
@BalarkaSen OK
Hello I am trying to solve this problem: "For each function f(n) and time t in the following table, determine the largest size n of a problem that can be solved in time t, assuming that the algorithm to solve the problem takes f(n) microseconds." This is the chart with answer : clrs.skanev.com/01/problems/01.html Can any one tell me how answer is found for (n Log n) and n! ..
@BalarkaSen I want a beautiful solution there ...
@Chris'ssis Of what?
@BalarkaSen $$\sum_{k = 1}^\infty \frac1{k^n(k + 1)} $$
@BalarkaSen Can you show me one?
@Chris'ssis Ah! I know! I know!
09:26
@BalarkaSen :D
@BalarkaSen Do you know to find that answer?
@IndrajithIndraprastham No.
@Chris'ssis Wait a sec. Let me think about it. I think I have an idea.
$$\text{Li}_n(x) = \sum_{k = 1}^\infty \frac{x^k}{k^n}$$
Integrating w.r.t. $x$. and then setting $x = 1$ gives the desired.
@BalarkaSen Horrible ...
Snap. Well, apparently that's not what you had in mind.
The horror, the horror,...
09:31
@Chris'ssis I'm out of ideas.
Show me what you have.
@BalarkaSen use $$a_n=\sum_{k = 1}^\infty \frac1{k^n(k + 1)}$$
combined with the difference above.
$$\frac{1}{k^n(k+1)} = \frac{1}{k^n} - \frac{1}{k^{n-1}(k+1)}$$
@Chris'ssis That looks almost the same as repeating the partial fraction method to me.
$$\zeta(n) - a_{n-1}$$
But good solution nevertheess
@rehband yeah that
Or $a_n + a_{n-1}$
@Chris'ssis Your proof uses almost the same machinaries, same ideas, same identity, just writes up stuffs a bit tidier. Why should I believe that it's of interest?
No, @rehband
@BalarkaSen It is the brick of some very hard series.
09:41
:) sry
$$a_n + a_{n - 1} = \zeta(n)$$
@Chris'ssis This series? Or the solution?
I am talking of the solution.
@BalarkaSen Not solution, but the series.
Your solution and the one by partial fractions are almost the same.
09:42
@BalarkaSen Well, writing things as a recurrence relation makes the whole thing so easy.
@Chris'ssis As I said, it writes stuff up a bit tidier, nothing more.
I need to run. Byes. Have fun with series.
OK. Bye. :-)
Take care!
Nice bird @FractalHand
@JasperLoy yeah
09:50
@FractalHand What brings you to this chat?
@JasperLoy finding some people with same interest, particularly, learning.
$$\sum_{k = 1}^\infty \frac{(-1)^{n+1}}{k^n(k + 1)}=1-\zeta(2)+\zeta(3)-\zeta(4)+\cdots+(-1)^{n+1} \zeta(n)$$
@Chris'ssis Nice
@rehband Thanks :-)
10:13
$$\lim_{n\to\infty} (1+1-\zeta(2)+1-\zeta(2)+\zeta(3)+1-\zeta(2)+\zeta(3)-\zeta(4)+\cdots +1-\zeta(2)+\zeta(3)-\zeta(4)+\cdots+(-1)^{n+1} \zeta(n))$$
@Chris'ssis Weird limit! :P
@rehband YES!!! :-)
@Chris'ssis Reminds me a little bit of problem 1.31 in Furdui's book
@rehband Yeah, I know it. That's a very nice limit.
10:43
@rehband That one diverges.
@Chris'ssis Hmm okay, how did you find that? :)
@rehband $$\sum_{n=1}^{\infty} (-1)^{n+1} \underbrace{\sum_{k = 1}^\infty \frac{1}{k^n(k + 1)}}_{\displaystyle a_n}$$ and this fails the alternating series test.
@Chris'ssis Hmm okay
@rehband Do you think I'm wrong? Look at what happens with $a_n$ when $n$ is large, it's almost $1/2$. Since it's an alternating series, we have after a while something like $-1/2+1/2-1/2+ ...$.
@Chris'ssis No, I just don't fully understand. What do you mean when you say it fails the alternating series test?
Yes
10:51
The alternating series test is a method used to prove that infinite series of terms converge. It was discovered by Gottfried Leibniz and is sometimes known as Leibniz's test or the Leibniz criterion. == Formulation == A series of the form Or, where an are positive, is called an alternating series. The alternating series test then says if {an} decreases monotonically and goes to 0 in the limit then the alternating series converges. Moreover, let L denote the sum of the series, then the partial sum approximates L with error bounded by the next omitted term: == Proof == Suppose we are given a...
Right!
@rehband Swapping the summation order, one gets a wrong result. Actually this is what I wanted to point out.
@Chris'ssis Oh ok, got it
@rehband That would have been $$\zeta(2)-1$$
@Chris'ssis have you ever thought about practicing your writing skills on Wikipedia?
10:54
@skullpatrol No. I don't think I have writing skills. :-)
It would be a great place to develop them.
@skullpatrol Wiki removes contributions coming from personal calculations.
And, of course, learn from others
Just a suggestion :-)
Most of what @Chris'ssis do are her own discoveries. Wiki only takes stuffs with unbiased references.
The best thing would be to set up a blog or something.
@skull I just read Poe's Raven. Eerie.
11:11
@TedShifrin Hello!
11:29
@rehband leaving out the first term of the inner series, one gets $$\zeta(2)-\frac{5}{4}$$
I just figured out that I have to acknowledge pings on my iPhone by tapping on them :-)
@Chris'ssis Ah okay
Huy
Huy
Yay, finally through with all exams. Now I can finally buy some more furniture and plants and make this place look nice. ^_^'
11:44
How many did you write?
@Huy Congrats!
Huy
Huy
@skullpatrol: I just did two this semester, quantum mechanics and functional analysis, because I got a nice opportunity to work 50%.
12:23
@Chris'ssis Crazy trick: $$\sum_{i=1}^{n} \sum_{j=1}^{n} \frac{1}{i+j} = \sum_{i=1}^{n} \sum_{j=1}^{n} \int_0^1 x^{i+j-1}dx$$ I would've never thought of writing it that way :D
@rehband For computing the limit, when $n\to\infty$, just apply Stolz-Cesaro theorem.
@Chris'ssis Okay :) This is for proving that $$\sum_{i=1}^{n} \sum_{j=1}^{n} \frac{1}{i+j} = 1 + (2n+1)H_{2n+1}-(2n+2)H_{n+1}$$
@rehband This is from Ovidiu's book as well.
@robjohn I don't know if this has already been discussed, but I think that these two users are attempting to impersonate this user.
@Chris'ssis Yep:)
12:34
(Based on the comments in this question (10k) and this question, it seems that they're trying to discredit those usernames, which are commonly associated the user now known as Thursday.)
13:06
@robjohn have you seen this one? It's cool I think. $$\lim_{n\to\infty} (1/2+3/2-\zeta(2)+1/2-\zeta(2)+\zeta(3)+3/2-\zeta(2)+\zeta(3)-\zeta(4)+\cdots +(1+(-1)^{n}/2)-\zeta(2)+\zeta(3)-\zeta(4)+\cdots+(-1)^{n+1} \zeta(n))=\zeta(2)-\frac{5}{4}$$
lol, my limits don't fit the screen anymore. :-)
hi guys a stupid question. suppose $G$ is simple and let $f$ be an homomorphism between $G \to H$. If $\#G\ne2$, $A\lhd H$, and $[G:H]=2$. Then $\text{Im}f$ is a subgroup of $A$. I know that the kernel of the homomorphism is a normal subgroup of $G$ so either is the trivial subgroup or the entire group. The last case is trivial, because everything is send to $1$ in $H$, and clearly the trivial group is a subgroup of the $A$.
The case when the kernel is trivial is my problem. Here we can conclude that the orden of the image is not two, because is the same as $G$, and also divides the orden of $H$, i.e., $#H=[H:A]\#A=2\#A$ i.e., the image divides the even number. But from here I'm not sure of how proceed, any idea ? Thanks in advance
I just took an interesting test.
Wait, to give you the link, only 10 questions.
@Chris'ssis I think these tests are very silly, lol.
@JasperLoy No, they are not at all! Not this one!
Here is the test - bitecharge.com/play/career
I'm shocked by the precision of this result. I only heard that some time ago from a psychologist.
13:21
silly enough.
heya @blue
heya
Chef

You're naturally creative, imaginative, and you can instantly come up with new ideas in your head. More than anything you want to create something beautiful and enjoyable that will satisfy and make people happy. You have an uncanny ability to make masterpieces out of sheer nothingness.
@anorton Thanks. I've brought this up with the other mods earlier today. I hope no one is really fooled.
i won't even click it.
13:24
@Chris'ssis I don't know if I've seen that one exactly. I've seen some things that I think are similar. Neat :-)
@BalarkaSen Better than using representation theory to discover the meaning of life
@JasperLoy Well, I think there is much truth in what I read there (although I only know you a bit). Do you agree with that result?
@Chris'ssis Sort of. I once took another long test and the result is the same, lol.
@MatsGranvik Nice result! Do you agree with the result?
@Chris'ssis Well I like bananas with peanut butter, if that is what chef means.
13:29
@MatsGranvik lol :-)
First meaning is probably person in charge like a boss. That I don't know. At work I am lowest in rank.
@MatsGranvik This doesn't mean that one that you won't be a boss. This is just the beginning.
13:41
@BalarkaSen did you take the test?
nopes.
@Chris'ssis I showed the people in the Eng room the test too.
as i said, i didn't even click it.
@JasperLoy Nice. Do they take it?
@BalarkaSen OK
@Chris'ssis Yes, some of them.
I just realised that @blue almost never talks to me, lol.
13:43
:/
@robjohn Glad you like it! :D
@JasperLoy blue talks mostly about mathematics. you almost never talk of mathematics.
@BalarkaSen Yes. I am more interested in building relationships in this chat.
@blue You never saw my explanation of gram phenomenon.
le sigh
13:58
I form attachments to people quite easily.
@JasperLoy great, you build relationships so that we can mourn when you delete your account? :-p
@robjohn I think I have promised somewhere in this chat never to delete my account again!
@JasperLoy yes, several times, if I recall :-)
@robjohn This time, it is real, lol.
I'll believe it if I don't see it!
3
 
2 hours later…
15:55
Hello, people of math.SE
leo
leo
@Zachiel hello
I'm not really great at math but I've got a question opened on rpg.se that might use some research on my part, so if anybody here wants to talk about optimization problems I'd be happy to learn something from you people. I have some basics (knapsack problem, rectangular distances, system solving for maximums).
Of course, since I'm basically asking for free help, I don't really expect much, but as they say "never ask, never get".
I would link the question, but I guess there would be too many subculture assumptions for you to parse it.

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