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03:00 - 18:0018:00 - 23:00

6:12 PM
welcome back, pal
 
Can someone help me with the last part of this question?
T = [[a,-b],[b,a]]
I just don't know what that represents
 
try to act upon a few vectors with that matrix
 
I have
 
for example, how about the unit vectors in the directions of the x- and y-axes to start?
 
I can't figure out the pattern
I've always only dealt with 1 variable in a transformation, but this has 2 so it has 2 "layers of freedom"
 
6:18 PM
okay
 
that I don't understand
 
If you look at $f:\Bbb R^2 \to \Bbb C,(x,y) \mapsto x+iy$, what is $f\circ T\circ f^-1(z)$ ?
 
i'd make it a bit easier upon yourself then and look at a special case where $a = f(t)$ and $b = g(t)$, so you reduce the "two degrees of freedom" to one
there is a particular choice of $f$ and $g$ that will make a lot of sense (it's should hopefully not be very hard to figure out)
 
Can anyone give an example of a function $f$ defined on the real line such that $1-f(x)=f^{-1}(x)$?
 
looking at $x = f(1 - f(x))$ made me think of $f(x) = c - x$, which gives one such function
 
6:26 PM
Examlpe:
Just solve it lol
 
Alright, then how about one such that $\lim_{x\to -\infty}f(x)=0$ and $\lim_{x\to\infty}f(x)=1$?
 
@Rithaniel sigmoid functions
 
this is a start
 
Ah, I had never heard the term "sigmoid function" before. It's always good to have a little bit more vollllcabulary
 
@Rithaniel: volcanic volcabulary?
 
6:34 PM
Maybe vivacious vocabulary
Oh wait, I typoed, I understand now
 
LOL ... you have to account for my mean, spiteful nature.
 
nothing's changed has it
hi Ted, wonder if you remember me
 
nope, @Soham
and nope, Soham
 
Nonsense, you're hardly mean and spiteful at all.
 
He's an old timer...
 
6:36 PM
it's okay, Ted isn't a real 90s kid
 
LOL, more like 50s and 60s.
 
:)
does Mike still swing by here
 
Yes, occasionally.
 
@Astyx Not sure what kind of answer you want, I'm only getting (a+ib)f(x,y)
 
So you multiply $z$ by $a+ib$. Do you know what such an operation does ?
 
6:41 PM
@SohamChowdhury I'm going to have to confess, that hint is a bit advanced for me
@Astyx I do not
 
Salut @Astyx
 
Salut @TedShifrin
 
stands up
 
@Typo Ok then you can look at specific examples. What if a=1 and b=0 ?
 
Just z(?)
 
6:44 PM
You're too short to stand up, @skull.
 
Forget about complex numebr and just look at $\Bbb R^2$
But the result is the same, you get the same thing
Now what if $a=0$ and $b=1$ ?
 
Reflection about y=x(?)
wait no
90* anticlockwise rotation
 
Right
What if $a=2$ and $b=0$ ?
 
@TedShifrin was that "name calling"?
:-)
 
Scaling the plane by 2
 
6:47 PM
I don't know — you're the one who stood up, purportedly.
 
Exactly
 
you all were saluting
 
Now what if $a=b=1$ ?
 
but not to you, skull :D
 
that makes the line y=x
 
6:49 PM
:D
 
What do you mean ?
 
sorry
i keep forgetting a minus sign
That gives 45* anticlockwise rotation
 
Almost
 
what happens to length?
 
huh, length gets scaled up $\sqrt2$
 
6:53 PM
Right
 
assuming the pattern is that it gets scaled by $\sqrt{a^2+b^2}$
 
Yes!
Now if you look at $T/\sqrt{a^2+b^2}$, you'll find you can rewrite it as $[[sin{\theta}, \cos{\theta}], [-cos{\theta}, \sin{\theta}]]$ for some $\theta$
 
I was just about write that
This ties in with the hint that Soham gave
I just looked up the rotation matrix and it matches T
 
Cool
So what does $T$ represent ?
 
7:17 PM
Scales the plane by $|z|$ and rotates anticlockwise by $\arctan{\frac{b}{a}}$
although I got the angle by experimentation, not by algebra
nevermind, the derivation is simple
 
Yeah, it's simple
 
@Astyx Thanks for the patience and the step by step walkthrough
 
@Typo Glad to help
 
Also thanks to @SohamChowdhury for the hint that I would've gotten if I'd just thought about it a bit more
 
7:24 PM
:D
 
Yo
just wanted to point out that my MSE rep
is on 15,420
420
:D
 
54k to go.
 
Maybe
I have to get 200k or else I won't be in the club
Could you imagine some peoples reps must have skyrocketed to a million
since the latest global rep changes
Those are the math wizards
 
I just realized I speak both the native languages of the two people that helped me :o
French for Astyx and Bangla for Soham
 
7:44 PM
that's neat
 
ekdom
 
:-)
 
i remember most of the names i used to see back in the day, and yours is no exception
 
 
1 hour later…
9:02 PM
Hi everybody
 
hey
 
Can you answer a quick functional-analysis question?
 
ahah not at all
😅
 
you should just ask the question, someone with the knowledge might come later and help you
 
9:13 PM
I should, gonna type it up
Is the uniform norm $\|f\|_\infty\leq \|f\|_2$ (the $L_2$ norm) for all $f\in C[0,1]$? The space of continous functions from $[0,1]$ to $\mathbb{R}$
 
What do you think?
 
Yes, but I'm not sure
 
Hmmm ok let's see
What if $f$ is $1$ on $[0,1/2]$ and $0$ otherwise?
 
Oh god I wrote it backwards
I meant $\|f\|_2 \leq \|f\|_\infty$, sorry
 
9:28 PM
Ok now that seems plausible
 
Yeah it obviously wouldn't work the other way with your example
 
C 0,1 is a BR with the sup-norm, with the l_2 norm it is not a BR.
 
Sorry for disturbing, could somebody suggest me math books like 'calculus made easy'.
 
@Ludoss What is BR?
 
banach space
 
9:33 PM
@TheTerriblePuddle Well what's the definition of the $L^2$ norm? Can you find an easy upper bound for it in terms of $\sup f$?
 
C[0,1] is a banach space with the supreme norm.
 
@Ludoss Yes, does that imply $\|f\|_2 \leq \|f\|_\infty$?
 
No, why should it?
 
@Ludoss Just wanted to know why one being a complete space and the other not would help with my question
 
9:36 PM
with $\|f\|_2$ do you mean the $L^2$ norm?
 
@Ludoss Yes, I usually write $L_2$
@AlessandroCodenotti Hmm.. maybe, I don't know
 
Well write down the definition of the $L_2$ norm to begin with
 
Which axis of the complex plane should the locus $\mathbf{Im}(z)>1$ be plotted on? $\mathbf{Im}(z)$ itself is just going to be a real number, that's where my confusion lies
 
Say C[0,1] with $\|.\|_2$ and $ t \rightarrow t^n $. It is a Cauchy series in this room, but doesn't have a lim.
$\|f\|_2 = \sqrt \int_0^1 \|f(x)\|^2dx$
 
$(\int_0^1 |f|^2dm)^{1/2}$
 
9:47 PM
Nevermind my question, I was thinking about this the wrong way
 
alright, but $\int |f|^2 dm\leq \int |\sup f|^2 dm$ so...
 
yeeeeaaaa
 
$|\sup f|^2$ is a constant, what happens if you pull it out of the integral?
 
Hello Everybody
Can someone help me about my question please?
0
Q: What is the graph of $f(\theta)=\frac{\sin\theta}{\cos\theta+C}, 0<C<1$?

Muhammed Ç. TUFANI have tried to draw the graph of the $f(\theta)$ function. Firstly the domain of the function is, $$D(\sin\theta): -\infty<\sin\theta<\infty$$ $$D(\cos\theta+1): -\infty<\cos\theta<\infty \Rightarrow -\infty<\cos\theta + 1<\infty$$ The intersection of these domains is $-\infty<f(\theta)<\inf...

 
$|x|_\infty \geq |x|_2 $
 
9:52 PM
It's ablit graphing but the function has a term that varies in range $0<C<1$.
 
but not equal.
or equivalent.
 
Since it's a Lebesque measure...
 
@TheTerriblePuddle it is, but I don't know what you're trying to say
 
@AlessandroCodenotti Okay, I don't know what happens
 
$\int af =a\int f$ for a constant $a$, do you agree?
 
9:56 PM
Yes I do
 
So $\int |\sup f|^2 dm=|sup f|^2 \int dm=\|f\|_\infty^2 m([0,1])=\|f\|_\infty^2$
So in particular we get $\|f\|_2=(\int |f|^2 dm)^{1/2}\leq (\int |\sup f|^2 dm)^{1/2}=\|f\|_\infty$
Note that we actually got a sharper bound, as long as $m(X)$ is finite we have $\|f\|_2\leq m(X)\|f\|_\infty$ for all $f\in L^\infty(X)$
 
I don't know what to say. That's really nice
Thanks
 
Its less-equal. Not equal.
 
The general statement here is that if $(X,\Sigma,\mu)$ is a finite measure space, then for $1\leq p<q\leq\infty$ we have $L^q(X)\subseteq L^p(X)$ and $\|f\|_p\leq \mu(X)^{1/r}\|f\|_q$ where $r>0$ is such that $1/p=1/q+1/r$
 
Okay, love We Lost the Sea btw
 
10:06 PM
Finally somebody who got the image! They're great
 
Reading the story and then listening The Last Dive of David Shaw really hit me
 
Have you listened to the new album?
 
No, only Departure Songs
 
Check out the last one too, it's called "Triumph & Disaster" and it's really good, it came out a month ago or so
 
I'll do that
I've another question posted on the exchange if anybody is interested math.stackexchange.com/questions/3471576/…
 
10:15 PM
rehi, demonic @Alessandro
 
I heard you the first time ;P
 
Did it actually ping you the first time?
 
@TedShifrin Hi
 
Yes, Ted always pings me.
Hi, @Jacksoja.
 
10:22 PM
can you recommend a book
that helps me make sense of different types of inifinity ?
 
Do you mean cardinalities?
 
That is a difficult question. For the beginning math student, understanding the difference among finite, countably infinite, and uncountable is more than enough. But then there's a whole thing in the field of set theory, where you learn about (infinite) cardinal numbers and ordinal numbers.
I would recommend you not go there until you're much older.
 
I see
thanks !
 
"Naive Set Theory" by Halmos contains all you need to know and some more, it's extremely readable and it's aimed at non set theorists
 
and btw there is a second Ted on this chat
have you had the chance to meet him ?
@AlessandroCodenotti thanks!
 
10:28 PM
Yes, we two Teds are interchangeable now.
 
Man I wonder when Ted from TED-Ed will finally show up
 
haha
 
What's Ted-Ed? I know about Ted Talks.
 
the good part is , that he is way similar to you
I think they are the same thing
 
ah I meant TED talk, TED-Ed is their educational channel
 
10:32 PM
@AlessandroCodenotti is there a first order theory that has exactly aleph_1 isomorphism classes of models?
 
Bob
10:59 PM
I am offering a bounty on the following post:
1
A: The Correlation of two Random Variables

ThomasFor $E[XY]$, if I understood correctly, there is a mistake. You are first choosing $X$ and than $Y=h(X)$, with $h$ deterministic. So you have to evaluate: $E[XY]=\int f_X(x) xh(x)dx$ Instead it looks that you integrated over $y$. But consider when $h$ is invertible: $\int f_X(x) xh(x)dx \ne \i...

 
03:00 - 18:0018:00 - 23:00

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