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20:00
[that's the hint]
Anyway I have to leave now
I'll be back later
alright
so we have $L-x_{p_n}$ and $L+x_{p_n}$?
@GFauxPas not sure how that helps
i meant
the intervals can be, maybe, $[x_{p_n} - |L| .. x_{p_n} + |L|$
20:01
@GFauxPas no, that wouldn't make sense
or, rather
ive never proved anything like this before :( hm
draw a graph
of xpn only
you don't need xn anymore
what about $a_n = -|x_{p_n} - L|,b_n = |x_{p_n} - L|$?
@GFauxPas it wouldn't work
draw a graph
you're close
you just need to think more carefully
$\inf S$ and $\sup S$?
not the whole thing
20:08
Hello :)
of $S$ for $n> p_n$?
@GFauxPas draw a graph
I did
i have a bunch of dots asymptoting and a horizontal line $L$
What a limit $ \lim_{n \to \infty} \sqrt[n]{1+x^n} $
20:11
If $x \in [0,1)$ we have 1.
try setting $y = (1+x^n)^{1/n}$ and taking $\ln$ of both sides
is my first inclination
i dont see why $L\pm x_p$ wouldnt work, and if it doesnt work i dont see how to make it better :(
oh, i see why it doesnt work
but still dont see how to fix it
So, I have $f_n(x)=\sqrt[n]{1+x^n}$ and I looking for area convergence..
@TobiasKildetoft here?
20:27
Don't forget to participate in the election!
Oh yes, I have a question I meant to ask
Let $a_0,r\in(0,1)$
Define $a_{n+1}=r\cdot a_n^2$
I've found that $a_n<a_0^{2^n}$, but are there any significantly better bounds?
@SimplyBeautifulArt $a_3 = ra_2^2 = r(ra_1^2)^2 = r(r(ra_0^2)^2)^2 = r^7 a_0^8$
So by inspection $a_n = r^{2^n-1} a_0^{2^n}$
good enough bound?
@LeakyNun Wow, I'm stupid xD
Well...
XD
@SimplyBeautifulArt no, you obviously aren't
@LeakyNun :P
@SchrodingersCat Well, as a replacement problem, you may be interested in finding a function such that every nth derivative evaluated at zero is (n!)^2
Okay.. thats nice.. lets think..
20:39
@SimplyBeautifulArt oh for the love of god
@SchrodingersCat Lol, actually I already posted my solution in here. @LeakyNun can testify. Though it is a fairly interesting problem IMO.
@LeakyNun xD
Another problem, the one I was originally working on, is somewhat interesting
The problem is to compute the solution to x = -2^x
@SimplyBeautifulArt ?
This solution is most nearly x ≈ -0.641185744505
@SimplyBeautifulArt Well, we can obviously use Taylor expansion and plug that value... We get an awkward inf series.. you want something better as an answer...
@SchrodingersCat it wouldn't converge
20:42
And as I was working, I found a decent numerical iteration method that converges at the rate of ≈ O(0.376^(2^n))
$$\begin{array}{rcl} x &=& -2^x \\ -(x \ln 2)e^{-x \ln 2} &=& \ln 2 \\ -x \ln 2 &=& W(\ln 2) \\ x &=& - \dfrac {W(\ln 2)} {\ln 2} \end{array}$$
And 0.376^(2^n) is extremely fast, in case you didn't notice
Just find an approximation method of $W$
@SimplyBeautifulArt it's just quadratic convergence
@LeakyNun Yeah I realize
But it was relatively simple for quadratic convergence I guess
I think Newton should be enough to do quadratic convergence
20:44
@SchrodingersCat You get a series with zero radius of convergence. The terms happen to be n! x^n
@LeakyNun Hm, I haven't checked if my solution is equivalent to Newton's yet...
@SimplyBeautifulArt it doesn't matter
@LeakyNun sure it does
Oh, nifty
$$W_0(x) = \sum_{n=0}^\infty \frac{(-n)^{n-1}}{n!} x^n$$
This should converge fast (nvm, it shouldn't)
Newton's method always over approximates, and my method always under approximates
Well, yes... I just made a mess... took the inverse and was getting radius of convergence infinity.. @SimplyBeautifulArt
20:46
@LeakyNun Lmao, $\ln(2)>1/e$
@SchrodingersCat but it wouldn't have the required properties
@SchrodingersCat Haha, :P
@SimplyBeautifulArt rip
Yes they dont @LeakyNun
$-x=2^x$
$\ln(-x) = x \ln 2$
does iterating that work?
@SimplyBeautifulArt $x_{n+1} = \log_2(-x_n)$
20:48
@LeakyNun How are you iterating it?
Fails to converge
Simply iterating $x=-2^x$ will converge
@SimplyBeautifulArt speed?
Btw, if you can't see MathJax in chat: tinyurl.com/cfqcvpc
@LeakyNun $\mathcal O((x\ln(2))^n)$
rip
wait, $x$?
20:50
Yes, where $x=-2^x$
You do realize I just took the derivative of $-2^x$ at it's fixed point?
@SchrodingersCat Want some hints on how we solved that crazy differential thing?
(or solution if you don't wanna do it yourself)
Give me some hints man... i m running out of options..
lol
Suppose that $$f(x)=\sum_{n=0}^\infty n!x^n$$
And that in some crazy world this converged
Compute $1+x\frac d{dx}(xf(x))$
via termwise differentiation
I think I came up with a fun problem, although it might be too easy
20:55
@LeakyNun Well, you'll notice that $(-2^x)'<0$, and so $a_{n+1}=-2^{a_n}$ is alternating around $x$
You get 1+\sum_{n=0} n.n! x^n @SimplyBeautifulArt
Hence why $\frac{pa_n+qa_{n+1}}{p+q}$ might converge faster than $a_n$
No you don't.

1 + x D (x * f(x))
where D means derivative
Wow missed x...
Geez
Let $k$ be a field and $X \subset k$ an infinite subset. Show that $X^n \subset k^n$ is dense in the Zariski topology
@LeakyNun And so from there you would want to choose weights $p,q$ such that $p+qx\ln(2)=0$
I ended up making $p,q$ functions of $a_n$
and the result was quadratic convergence.
21:00
Okay I was careful.... You get \sum_{n=0} n! x^n =exp(x)
@SimplyBeautifulArt
@SchrodingersCat x'D No, it should be $\sum_{n=0}^\infty n! x^n = f(x)$
exp(x) has n! in the denominator
@SchrodingersCat exp(x) is sum x^n/n!, not multiply
sniped
:P
So you get a horrendous second order differential equation
which obviously has a horribly complicated solution
I should be off to sleep ... I have no right to stay awake anymore...
Lmao, good night @SchrodingersCat
If you are interested, the second solution requires taking the Borel sum of the divergent series @SchrodingersCat
21:03
Still okay... lets finish this first... i want to know how u did it
Oh ... okay...
Well, you get something along the lines of: 1 + x f(x) + x^2 f'(x) = f(x)
ah! divergent series
And with the condition that f^{(n)}(0) = (n!)^2, this boils down to...
@SimplyBeautifulArt: are you not proud that you're living in such a glorious time? you're freely discussing divergent series without any getting embroiled in any deeply heated controversy
f(1/x) = x e^(-x) Ei(x)
Where Ei(x) is the exponential integral
@Fine Lmao!
@SchrodingersCat My solution basically goes as follows:
\begin{align} f(x)&={\small\mathcal B}\sum_{n\ge0}n! x^n \\&= \int_0^\infty\sum_{n\ge0} x^nt^ne^{-t}~ \rm dt\\ &=\int_0^\infty \frac{e^{-t}}{1-xt}~\rm dt\\&=\dots\end{align}
(note it still requires another regularization of the series inside the integral)
If x < 0, this integral converges to the claimed solution above
where the exponential integrals are almost naturally apparent.
@SchrodingersCat And if you want another challenge, try having the nth derivative equal (n!)^3
21:08
Hello, how to prove that $\inf{2+\frac1n, n\in\mathbb{N}}=2$ without using limit, just the cararterisation of the if that is : $\forall \varepsilon>0, 2+\frac1n< \inf\{2+\frac1n\}+\varepsilon$
(we're still sorta working on that one)
I think doing Mittag-Lefler with a=2 should work for that
@Semiclassical so messy though x.x
No disagreement there =P
I found using the approach of turning it into a differential equation to work half-decently with heavy WA assistance.
(also very messy)
Well, I'm off.
Good night! @SchrodingersCat
21:13
@LeakyNun I'm not confident that that statement is correct, Leaky. You ordinarily need a pair of complex conjugate roots to get a transposition in the Galois group.
I thought you said 5 real roots in what I just cited.
Hi @TedShifrin
heya @Alessandro
oh you were citing an incorrect version
I corrected it later
21:14
Have you run over me yet, Alessandro?
Well, it's still there, @Leaky.
hi @MatheiBoulomenos.
3 hours ago, by Leaky Nun
Theorem (Galois theory): any irreducible rational polynomial of degree 5 with exactly 2 non-real roots is not solvable by radical.
@TedShifrin here
Well, I only looked at your idiotic "Any polynomial of degree 5 has no solutions" and what was near that. I'm not going to read the whole transcript.
You can replace 5 with any odd prime
greater than 3
21:15
Most cubics can be solved by radicals. Most. :D
@LeakyNun =P Pretty sure any polynomial of degree 5 has at least one complex solution, and at most 5 solutions.
@TedShifrin that's my fault
@SimplyBeautifulArt and exactly 5 solutions counting multiplicity
I think Leaky is earning a reputation for being a troll.
@LeakyNun =P
Probably
21:17
Well, I can see that this visit is a waste of my time. I'm going to cook dessert for guests tonight.
@Faust: Did you figure out the angles on your pentagon? You saw my comment/hint?
What I can write : $\sqrt[n]{1} \le \sqrt[n]{1+x^n} \le .... $ ?
for $ x \in [0, \infty)$
@PawełKusz it probably isn't the right lower bound
wait, it depends on the value of $x$
I wrote interval :)
I know, but it still depends
@Semiclassical :o
21:22
@TedShifrin do you have any problem for me regarding Galois theory?
So, I am looking for area convergence for $f_n(x)= \sqrt[n]{1+x^n}$...
@PawełKusz it converges everywhere
What function is a limit?
max(1,x)
I know you didn't ask me, but you could try to compute the Galois group of $X^6-2tX^3+1$ over $\mathbb{Q}(t)$
21:27
@MatheiBoulomenos thanks
what is t?
Well, it's a variable
hmm
Let $u=X^3$. Then, $u^2-2tu+1=0$ gives $u=\dfrac{2t\pm\sqrt{4t^2-4}}{2}$
Maybe I should warn you
My computation of that Galois group is super messy
lol
do I need to do casework?
but maybe you can figure out something more beautiful than I did
21:31
Let $r_1^3=r_2^3=r_3^3=u_1$ and $r_4^3=r_5^3=r_6^3=u_2$
@MatheiBoulomenos I kind of learnt them just today
so don't expect much
@SimplyBeautifulArt the suggestion to write the reciprocal binomial coefficients as beta functions and then pass to the integral representation seems particularly smart
where $r_2=r_1\omega$ and etc
Okay, it might be to advanced if you just learned about Galois groups today
so basically you have $p$, $p\omega$, $p\omega^2$, $q$, $q\omega$, $q\omega^2$
oh and also $u=t\pm\sqrt{t^2-1}$
where $p^3+q^3 = 2t$
So we need $\Bbb Q(t,p,\omega)$ amirite
Yes, that description of the splitting field is a good starting point
21:34
thanks
I feel like we would swap $p$ and $q$
@LeakyNun $f_n(x)$ will be uniformly convergent?
as well as complex conjugation which swaps $\omega$ and $\omega^2$
@PawełKusz I believe so, but I have no idea
$(14)(25)(36)$ and $(23)(56)$
oh I hate $S_6$ you know
@MatheiBoulomenos I kind of don't believe that $x^3-1$ has Galois group $S_3$
but it does seem to include the transposition
oh, lol
it is $S_2$
x^3-1 is not irreducible
are my generators correct?
I have to say that my computation of this Galois group avoided writing down any actual permutations of the roots, so I'm not sure
21:39
is your group bigenerated?
that looks good
my group looks like $V_4$. Am I correct?
@Faust hi
morning leaky
21:40
@MatheiBoulomenos hmm
@Faust as you see I'm stuck lol
im stuck on geometry too
Yeah, my computation of that thing seems more like a hack than an actual computation lol
@MatheiBoulomenos how can you know if you've missed any automorphisms?
Do you want to see my solution?
I don't
21:43
One way to see that it can't be $V_4$ is that the polynomial is irreducible, so the splitting field must have at least degree 6
$p(X) = u^2-2tu+1 = (u-p^3)(u-q^3) = (X^3-p^3)(X^3-q^3) = (X-p)(X-q)(X^2+Xp+p^2)(X^2+Xq+q^2)$
@MatheiBoulomenos hmm
oh, intermediate fields
we should consider $\Bbb Q(t,p)$ and $\Bbb Q(t,\omega)$
the latter is degree 3 of base
the former is, well
$\mathbb Q(t,\omega) / \mathbb Q(t)$ is degree 2
@MatheiBoulomenos facepalm
@Semiclassical Isn't it a standard approach?
21:45
Well, the minimal polynomial of $\omega$ is $x^2+x+1$
well, $\Bbb Q(t,p^3)/\Bbb Q(t)$ is degree 2 and $\Bbb Q(t,p)/\Bbb Q(t,p^3)$ is degree 3
@MatheiBoulomenos oh right
so $\Bbb Q(t,p)/\Bbb Q(t)$ is degree 6
Probably. Doesn’t make it any less smart
@LeakyNun Have you checked out the coursera course on Galois theory? There are also some pretty good problems as assignments
@MatheiBoulomenos is it behind paywall?
21:48
No, you can access all the materials for free
You only have to pay if you want a certificate
Say, what happens to the rate of convergence of Newton's method when we replace the derivative with a difference quotient?
But you have to register
@MatheiBoulomenos am I doing anything wrong?
No, it's just that the problem maybe isn't really well-suited if you just learned about Galois theory. I was hoping that maybe you would come up with a more elegant solution than what I did
@MatheiBoulomenos give me more time to think lol
21:51
Sure
we all deserve time to think
$p, p\omega, p\omega^2, 2t-p, 2t\omega-p\omega, 2t\omega^2-p\omega^2$
hoping that category theory will save the day
the degree is 12
in The h Bar, 1 min ago, by Secret
I have a clusterf*** of a last night dream: Imagine that, a Groundhog day style dream where it repeats more than 6 times and you try so hard to wake up!
This is no ordinary dream. details later
21:54
@Secret write a blog
Yes, degree 12 is correct
@MatheiBoulomenos sure but I should have come up with 12 much earlier
I found like 3 paths already
it has 3 2 and 2 3
I'm considering $\Bbb Q(t,p,\omega)/\Bbb Q(t,p^3)$
I found a path of $3 \times 2$ and another of $2 \times 3$
When numerically evaluating $x=f(x)$, does $$a_{n+1}=\frac{f(a_n)(a_n-a_{n-1})+ a_n(f(a_n)-f(a_{n-1}))}{a_n-a_{n-1}+1}$$ generally converge quadratically, given $a_0\ne a_1$ are close enough to the root?
@LeakyNun Oh, that's simpler than what I did to compute the degree

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