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3:00 AM
@beginner crazy shit.. avoid pdes please
 
By fluke :) you're writing $X$ and $Y$ as sums of indicator functions. By linearity, you get a sum of probabilities of two things happening. :)
They're for turning analog into digital @beginner
 
Hey @TedShifrin, @beginner
 
@TedShifrin ill learn the proper way tonight hehe, ill write it up and message you
hey @kaj how did you do
waived harvard fees?
 
Heya @Kaj ... Happy Putnam.
 
I definitely got one of the problems without a doubt. I got partial answers for 2 or 3 more. Should've had another.
 
3:02 AM
how many questions?
 
It sounds like Doug did very well though. Maybe a 40 or higher.
 
with the median $[0,1]$ out of $[0,120]$
 
12 @beginner
 
Well, @Doug is not known for writing solutions that get points ...
 
so you got more than 10marks, you beat major majority!
 
3:03 AM
anrgh
 
That's true @TedShifrin. Though he did get 21 points last year.
 
@usukidoll ... that phrasing is not correct. just do the substitutions and see what happens.
 
He infuriates me :D
 
I'll send word to my people to give you 120, @Kaj
 
3:03 AM
do you just take putnam for fun, or does it go to your grades for uni? does it cost money to enter?
 
Yes, no, no
 
It's for fun @beginner
Free too
 
wow i wish i lived in the US or canada!!
 
ok let's start again...

$\frac{1}{\sqrt{2 \pi}} \int^{-\infty}_{\infty}f(\frac{x+b}{a})e^{i \xi x} dx$

then if we let u=x+b

we get this guy

$\frac{1}{\sqrt{2 \pi}} \int^{-\infty}_{\infty}f(\frac{u}{a})e^{i \xi x} dx$

so now we need to let $v= u/a$ which I can subsitute for the u/a in the f @anon
 
@usukidoll you have more work to do before you involve v
rewrite $e^{i\xi x}dx$ in terms of $u$ and $du$
 
3:06 AM
$\frac{1}{\sqrt{2 \pi}} \int^{-\infty}_{\infty}f(\frac{u}{a})e^{i \xi x} dx$

u= $e^{i \xi x} dx$
du = $i\xi e^{i \xi x} dx$
 
no
 
?
rewrite it in terms of u and du?
like take derivatives?
 
no
 
or something else @anon
 
express it using only u and du
 
3:08 AM
$e^{i\xi u}du$
 
for instance if u=x^2, then (x^3)dx=udu/2
@usukidoll no
you can't just replace x with u
 
do I let u = $e^{i \xi x} dx$?
 
@PedroTamaroff made a stupid error in my comment above, it should be easy enough to spot
 
$u=x^2, x^3dx=\frac{u}{2}du$ is the example provided
 
@usukidoll no
did you lose your memory? what did we decide u is?
 
3:11 AM
damn it... u = x+b
du = x if b is a constant
 
and how do you rewrite $e^{i\xi x}$ in terms of $u$?
 
$e^{i \xi u}$
 
no
5 mins ago, by anon
you can't just replace x with u
 
$u = e^{i \xi x}$?
 
3 mins ago, by anon
did you lose your memory? what did we decide u is?
 
3:14 AM
u = x+b
du = x if b is a constant
 
let's try this. we decided $u=x+b$. so then what is $x$ in terms of $u$?
 
u -b=x?
 
yes, $x$ is $u-b$. now rewrite $e^{i\xi x}$ in terms of $u$.
 
$u = e^{i \xi x}+b$?
 
actually wait isn't it $e^{\color{Red}{-}i\xi x}$ in the fourier transform?
 
3:16 AM
yeah
 
okay, so $$\int_{\Bbb R}f(\frac{x+b}{a})e^{-i\xi x}dx.$$ we need to do the substitution $u=x+b$
 
what is the point of $inf S$ and $sup S$ i get the point of upper and lower bounds, but i dont know why we need to know the first to reach the bound
 
@usukidoll no!
 
!!!
 
you know how to write $x$ in terms of $u$, so how do you write $e^{-i\xi x}$ in terms of $u$?
you shouldn't be doing Fourier theory if you don't remember Calculus 2...
 
3:17 AM
$x=u-b$
 
You want the points that will be max/min if there are such, @beginner
 
@beginner, inf and sup are pretty important. For one, the existence of infs and sups of sets of real numbers is important for many results.
 
$e^{-i\xi (u-b)$
 
$e^{-i\xi (u-b)}$
 
@usukidoll yes!
 
3:18 AM
yay i got it @anon
 
@usukidoll which equals $e^{i\xi b}e^{-i\xi u}$ right?
 
dear gawd...must be past my munchie time which is why I can't do math without a meal x.x
oh yeah I see it coming now

$e^{-i \xi u +b \xi i}$
 
@TedShifrin but that is the upper bound, where inf is the first one in a sequence? i dont need an application but they are nice :)
 
mhm
 
Doing math tired, hungry, depressed ... Doesn't work.
 
3:19 AM
I know but I'm like trying to get this crap done by monday
I've done some of it yesterday
so it's just this...this other problem I know and I'm just going to wing 6.4...
 
applications everywhere, @beginner. What is $\sup\{x\in \Bbb Q: x^2<2\}$?
 
is rearranging that in terms of u really calculus 2? i havent done any calculus??
 
one uses all sorts of algebra and trig in calculus, @beginner
 
there isnt a sup for that
right?
you always get closer and closer
 
in $\Bbb R$ there is!
 
3:22 AM
really?
 
so do I take the fourier transform for $e^{-i \xi(u-b)}$? @anon
 
wouldnt it happen if you add a digit each time only at infinity??
 
@usukidoll no, taking the integral of blah and taking the fourier transform of blah are two different things, don't confuse them
 
Huh? @beginner
 
oh ._. so after we have $e^{-i \xi(u-b)}$ then what occurs next @anon
 
3:23 AM
you now have $$e^{ib\xi}\int_{-\infty}^\infty f(u/a)e^{-i\xi u}du.$$ now do the substitution $v=u/a$
 
ok
 
is there a $sup\{x\in\mathbb{R}:x^2\lt 2\}$?
 
$e^{ib\xi}\int_{-\infty}^\infty f(v)e^{-i\xi u}du.$
 
Sure!
 
but it has infinite digits, how could we find the sup?
 
3:24 AM
@usukidoll continue. keep writing everything in terms of $v$.
 
Give me an upper bound.
 
$e^{ib\xi}\int_{-\infty}^\infty f(v)e^{-i\xi v}dv.$
 
$x=\sqrt{2}-0.000000000001$
 
x.x fml not even close X)_!@(
 
and then we can just keep adding $0$'s and always beat the last upperbound
 
3:25 AM
No, that's in the set, not above it!
 
@usukidoll nope. you can't just replace u with v! stop cheating!
 
so there is more than one thing in terms of v
besides f(v)
$v = \frac{u}{a}$ so where else could we write in terms of v @anon?
 
Write $e^{i\xi u}du$ in terms of $v$
Later you should go back and refresh your memory of substitution.
 
^ it's probably because I'm doing this and I'm hungry at the same time... of course my mind goes in a blitz
 
The sup isn't always contained in your set @beginner
 
3:28 AM
oh the sup is $\sqrt{2}$
 
Anonymous
@KajHansen How did Putnam go?
 
$e^{i \xi \frac{u}{a}}$? @anon
 
is that right then @kaj, it is $\sqrt{2}$
 
Righto @beginner
 
And likewise for inf. For example: What is $\operatorname{inf} \{ \frac{1}{n} : n \in \mathbb{N} \}$?
 
3:29 AM
oh i thought it had to be in set woops, that makes sense
$0$?
 
Yep. But notice that $0$ isn't in our set.
 
yay that makes sense!
 
You'll see that sups/infs are crucial for the theoretical underpinnings of calculus @beginner
 
@usukidoll you think $e^{i\xi u/a}$ and $e^{i\xi u}$ are the same thing?
 
@anon nope
 
3:30 AM
@TedShifrin awesome i am happy now
 
3 mins ago, by anon
Write $e^{i\xi u}du$ in terms of $v$
 
@Ashwin, it went alright for my first time. I definitely got one problem. Should've gotten a second but I made a stupid mistake. And of course some partial answers here and there.
 
but we have $ v = \frac{u}{a}$ right?
 
yes
 
Anonymous
@KajHansen Were there any perfect scorers in the Putnam till datE?
 
3:31 AM
hmm $va = u$?
 
$u=va$, $e^{i\xi (va)}$
 
I guess I can't complain too much without having prepared whatsoever.
 
@beginner stop doing usuki's work
 
@anon sorry!
 
^ yeah I was getting there thanks @beginner
let me erase that from my memory
 
3:32 AM
Not sure @Ashwin
 
so we have $e^{i \xi u}$ if I have to write it in terms of v, yet I have $v=\frac{u}{a}$ then I need to have $va=u$ to have a good subsitution for this $e^{i \xi u}$, so in the end I have $e^{i \xi va}$
 
Anonymous
I just created a blog where I will keep track of my (slow) progress:mathematicsautodidact.wordpress.com
 
$e^{ib\xi}\int_{-\infty}^\infty f(v)e^{-i\xi va}dva.$ @anon
 
@usukidoll correct. now what's du in terms of dv?
 
if u = av wouldn't du be dav?
 
3:35 AM
correct. now put the a out in front of the integral.
since d(av)=a*dv
(we're assuming a>0 here)
 
What trick am I missing? This problem is too nasty to solve the normal way

"Let $\textbf{n}$ be the outer unit normal of the elliptical half-shell $S: 4x^2 + 9y^2 + 36 z^2 = 36, z \geq 0$, and let
$\textbf{F} = 5 y \textbf{i} + 5 x^2 \textbf{j} + 5 (x^2 + y^4)^{3/2} \sin e^{\sqrt{xyz}}\textbf{k}$.
Find the value of $\mathop{\iint}_S \big(\nabla \times F) \cdot \textbf{n}~\textrm{d}\sigma$

Hint: One parametrization of the ellipse at the base of the shell is $x = 3 \cos t, y = 2 \sin t, 0 \leq t \leq 2 \pi$. But you can avoid a line integral altogether."
 
ok! $ae^{ib\xi}\int_{-\infty}^\infty f(v)e^{-i\xi va}dv.$ @anon
 
@kaj $\operatorname{sup} \{ \frac{1}{n} : n \in \mathbb{N} \}=1$ i guess, because $n=0\pm.0000000000000001=\inf$ but $n=0$ is undefined and i can only take whole numbers in $\mathbb{N}$
 
Yes, @Ashwin, a few before, and Kaj this year.
 
Anonymous
@MikeMiller LOL
 
3:37 AM
Well it's a strictly decreasing sequence. So the largest term will be the first term. The sup is actually in the set in this case.
The inf isn't though.
 
@usukidoll right. if $\hat{f}(\omega)=\int_{-\infty}^{\infty}f(x)e^{-i\omega x}dx$, then what is $\int_{-\infty}^\infty f(v)e^{-i(a\xi) v}dv$?
 
So inf=0 and sup=1
 
@anon whoa now we're taking inverse fourier transform?
 
yeah i thought so, is $0$ a natural number, or only an integer?
 
@usukidoll no
what makes you bring up the inverse fourier transform?
 
3:38 AM
@anon f hat in my book is inverse fourier transform
 
jeesh
well, it's my fourier transform, so deal with it
 
do we use the fourier transform table @anon ?
 
no
 
oh wait no no scratch that
 
stop relying on tables
 
3:40 AM
$\hat{f}(v)=\int_{-\infty}^{\infty}f(v)e^{-i\xi av}dv$ ??? @anon
 
nope
 
:/
 
look at how I put parentheses in it
3 mins ago, by anon
@usukidoll right. if $\hat{f}(\omega)=\int_{-\infty}^{\infty}f(x)e^{-i\omega x}dx$, then what is $\int_{-\infty}^\infty f(v)e^{-i(a\xi) v}dv$?
I did that on purpose
 
did you let omega be $a \xi$
 
also look at what you're trying to prove
1 hour ago, by usukidoll
1. Show that if $f(x)$ is absolutely integrable on $(-\infty, \infty)$ then \\


1b. $[f(\frac{x+b}{a})]^{\xi}=ae^{ib \xi}\bar {f}(a \xi),a,b$ real, $a \neq 0$\\

A real of complex-valued function defined on $(-\infty, \infty)$ is said to be absolutely integrable on $(-\infty, \infty)$ if $\int_{-R}^{R} \mid f(x) \mid dx$ exists for all $R>0$ and\\

$\int_{-\infty}^{\infty} \mid f(x) \mid \equiv $$\lim_{R \to\infty} $$ \int_{-R}^{R} \mid f(x) \mid dx < \infty$\\
 
3:42 AM
it's getting close to the right hand side of the equation!
 
$ae^{ib\xi}\hat{f}(\color{Red}{a}\xi)$
 
balarka had me spend 30 min trying to prove that $\mathbb{R}^2$ isnt ordered yesterday and then worked out i cant solve it if i dont know what a field is @ted @kaj lol nooo
 
so we need that f hat to equal something... like $\int_{-\infty}^\infty f(v)e^{-i(a\xi) v}dv$
$\hat{f}(\color{Red}{a}\xi) = \int_{-\infty}^\infty f(v)e^{-i(a\xi) v}dv$ @anon
 
yep
 
woo!
alright now all that's left is the ode problem that has a good example in the book and that section is done...
 
3:55 AM
Well, it's not a field, @beginner, unless you think of it as $\Bbb C$.
 
complex ^ it's in a complex set
 
@TedShifrin i was trying to show $\Bbb{R}^2$ wasnt ordered and then after 30 min he said i couldnt do it without knowing what a field was
@TedShifrin i said you can't make it ordered because you cant for example order $(2,0),(0,2)$ as they are equally far from the origin in two positive directions
 
What a ring is ...
 
what?
i dont think you can order a 2-tuple or any tuple
 
An easier exercise is showing that $\mathbb{C}$ isn't ordered.
 
4:00 AM
i think you could only order it if you had a mapping $\mathbb{R}^n\to \mathbb{R}$
 
There are ordered rings (like $\Bbb Z$)
 
where each element was unique in that mapping
but Z is a 1-tuple so it makes sense, but 2-tuples dont
i will do your exercise thanks @kaj
 
Anonymous
@Committingtoachallenge my blog is all set to go!
 
4:23 AM
@balarka is a loop a geometric thing, or just a name like a ring
 
@Ashwin Awesome. I'm following it on wordpress so I'll check everything you put up out
@beginner You should start every sentence with a capital letter :)
 
 
2 hours later…
6:18 AM
@KajHansen that's what i gave him to do :P
@beginner pretty geometric, in the context i said it
loop can also mean noncommutative groups, i think. dunno.
 
@BalarkaSen was the ordered field rule you wanted me to use that $x+y\lt x+z$ if $x,y,z\in \mathbb{F}$ and $y\lt z$
is that a better definition?
The the $5$ statements of ordered fields?
is multiplication on a two tuple defined by the dot product or by corresponding components normally?
because a field needs to be armed with $(\Bbb{F},+,\times)$
 
Yes, that stuff but you don't even know what a field is.
 
i know some example fields, and i know what is required to be a field now i think
 
you're jumping again. go do set theory.
 
ok
sorry teacher
 
6:34 AM
@beginner knowing the definitions is not equivalent to understanding it. if you can give me an example of a finite field, then i'll believe you.
but to do that, you need to know what finite groups are which you don't...
 
ok i will make one, give me three min
$\Bbb{Z}_3=\{|0|,|1|,|2|\}$ has additive and multiplication closures, and has zero
 
How is addition defined?
How is multiplication defined?
 
$0+0=0,0+1=1,0+2=2,1+1=2,1+2=0,2+2=1$ and the commutative reverse
 
Good. This is called modulo 3 addition.
 
and multiplication is $0*a=0$,$1*1=1,1*2=2,2*2=1$ and commutative reverse
and it has an inverse addition which is
 
6:39 AM
yes, so now you have to prove that inverse exists and x distributes over +
 
$0-0=0,1-1=0,2-2=0$
so $a-a=0$
 
multiplicative inverse?
 
$0$ cant be inversed i think, $1*1=1$ and $2*2=1$
 
indeed, 0 can't be reverted.
 
and $0+x=x$
 
6:41 AM
sure, sure, so this guy's a field.
 
yayyyy
 
@beginner is $\Bbb Z_4$ also a field?
 
give me one min
 
what about $\Bbb Z_n$?
don't google
 
not googling
or do you want me to do working here
 
6:42 AM
nah. just think about it and tell me if Z_4 is a field or not
 
no cause $2*x\ne 1$
so 2 has no inverse
 
right.
 
cause $2$ is a factor of my $n=4$
 
mmhmm
if n divides m, then n has no multiplicative inverse mod m
 
$n|m\implies n^{-1} \not\in \Bbb{Z}_m$
is that written right?
 
6:46 AM
sure
 
yayyy, so i am the master of fieldss
don't cut me down pls i joke :)
i spent hours learning that and i did well :)
but i will go do set theory now
 
7:13 AM
@ronjohn What's this?
 
8:06 AM
 
Did you watch it?
 
Any abstract algebra fans here? :D
 
Not a strong one here
*yet
 
What can you say about a group in which all non-identity elements are of the same order?
See my answer here "https://math.stackexchange.com/questions/1055370/number-of-non-isomorphic-group‌​s-of-order-p2/1055401#1055401"
 
8:32 AM
Do you think user @Bahaviour is truly 24 as listed, or it is part of his absorbed persona for the current name(I don't recognize the person in his picture).
 
9:23 AM
@DanielFischer I'm having trouble to understand this answer. Could you help me to explain it?
237
A: Is the following matrix invertible?

André NicolasFind the determinant. To make calculations easier, work modulo $2$! The diagonal is $1$'s, the rest are $0$'s. The determinant is odd, and therefore non-zero.

How to find determinant matrix using modulo 2? Could you enlighten me @DanielFischer? Thanks.
 
Huy
@Venus: Since the determinant is a polynomial in the matrix entries, reducing the entries modulo $n$ does not change the value of the determinant modulo $n$.
 
@Huy Ya I know that but how? Can you elaborate your explanation? We can use the example in that problem
Anyway, I am type of person who can understand something by example
 
9:44 AM
0
Q: Help me generalize what this divisor transform does.

Mats GranvikI have an algorithm which takes as input the series expansion of: $$\frac{-(1 + ax(-2 + x + ax))}{-1 + ax} \tag 1$$ or expressed differently: $$\left\{a^0,(-a)^1,a^1,a^2,a^3,a^4,a^5,a^6,a^7\right\}$$ and applies the convoluted divisor recurrence via this Mathematica program: Clear[t, n, k, i...

 
@KajHansen Oh yeah, B1 was super easy. I think I got full points for A1 as well. Perhaps more, I had a good A1 write-up
(more points than B1, I mean)
 

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