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5:00 PM
@AkivaWeinberger There are many... The first is a Taylor series about $0$. The second is a Taylor series about $\infty$. Probably not what the teacher would mean on an exam.
 
Oh, right, Taylor series about infinity. Laurent series about 0. Gotcha.
 
@arctictern yes, the last few lines of 2nd slide does tell, however, in slide 1, it also implies $h(x)$ takes another expression
so im confused
 
Nice to see you @robjohn back in action :-)
In this room, I mean.
 
@skillpatrol Oops... I didn't know anyone could see me!
 
@LittleRookie huh? I don't see h(x) anywhere on slide 1.
 
5:05 PM
lol
The mean square is the most visible stat @robjohn
imho
 
$h(x) = \frac{(\beta -x)^{n+1}}{(n+1)!}$
 
@robjohn do you still correspond with chris's sister?
 
@LittleRookie I'm not sure how you deduce that from the given formulas on slide 1 and 2, f(beta) and f(x) are not the same thing, those terms do not cancel
 
ah sorry, i forgotten to add $P$ beside the right hand side
Perhaps, can you help me with this, why is $h(\alpha)=0$?
 
5:19 PM
plugging alpha in for x in the eqn on slide 2, and then substituting f(beta), THEN stuff cancels
 
which $f(\beta)$ do you use?
 
the expression on slide 1
 
@skillpatrol I got a message near the end of March that I just saw, so not much
 
thats wrong! that is what you want to show is true!
you cant use that
 
@robjohn Okay, just wondering when the book is coming out.
 
5:29 PM
@skillpatrol I have no idea
 
@LittleRookie youre totally right lol
 
im really stuck =/ how is $h(\alpha)=0$ ??
 
well that's what i was using the theorem for
the thing you said i can't use
if we want h(0) we can use the expression on slide 1
 
it was mentioned $h(\alpha)=h(\beta)=0$ in the proof
the proof is by using rolle's theorem
so you cant use the theorem..
 
well the way i reason through why that could be true is because h(x) is kind of an error term, and the error at the endpoints can always be set to zero
equivalent to shifting the origin around
h isn't the error term, to be clear
on slide one, it is the final term with the (n+1)'th power
 
5:36 PM
the original expression is the taylor series, here in taylor theorem, u are proving that u can change the infinite series to a summation of finite number of terms, with the expression of taylor polynomial + remainder.
unless you are using the taylor series, its fine
 
Indeed
I'm digging through some notes right now, did this a bunch in Numerical Analysis
 
This is actually Real analysis
 
finding the exact $\gamma$ is basically impossible so you maximize the error term every time to get an upper bound on the actual error
 
i dont think you learn the proof of taylor theorem in numerical analysis
you just simply apply it
 
I sure did lol
 
5:39 PM
yep its true that u cant find $\gamma$
but at least we know it exist
and the bounds for it
 
using the fact that you can always build a polynomial that stays in a 'tube'
another way of saying rolle's theorem
 
@arctictern can you help me on why is $h(\alpha)=0$?
 
@robjohn How does that make sense with regards to $\frac1{1-x}+\frac1{2-x}$, which has three Laurent series around zero?
 
@AkivaWeinberger There is the Taylor series, then there are two Laurent series that converge either in an anulus between $|z|=1$ and $|z|=2$ or outside of $|z|=2$
 
Hi, @robjohn, DogAteMy :)
 
5:47 PM
@AkivaWeinberger The last can be thought of as a Taylor series about $\infty$
@TedShifrin Howdy! How are things in SD?
We might have rain here tonight.
 
Reasonably peaceful, @robjohn, but I've got this respiratory/sinus thing that's been nagging at people for months.
 
@TedShifrin yeah, I had that for all of January
 
How to make sense of complex differentiable functions at a point z_0 , i mean in real it was tangent line what is the case in complex intuitivly speaking
 
I already had to postpone oral surgery (dental implant), and I can't postpone again, because I'm leaving for a month of travel in Europe for June. Ugh.
 
@TedShifrin Hi Ted I hope you are good :)
 
5:50 PM
@Kasmir: Still best linear approximation. You have to draw the same picture in $\Bbb C\times\Bbb C$.
 
Oh okay just wanted to make sure , sometimes we do alot of things , the minor ideas dont stick =p
 
@logical123 which $\gamma$ are you looking for?
 
@Kasmir: If you ever watch my multivariable lectures, you'll see multivariable calculus done from this viewpoint (using linear algebra).
 
As I got confused before sir Ted with TED lectures,Am I confusing Robjohn with sir Robjohn of UCLA ?
 
glares at BAYMAX
 
5:54 PM
like in mathjax links ??
 
Yes.
 
so I am confusing
 
this is the robjohn of the mathjax links.
 
oh hi @robjohn , nice to meet you.
 
5:56 PM
@BAYMAX: So, based on what we did a few days ago, can't you give me a continuous function on the square missing the origin that won't extend to a continuous function on the entire square?
 
still thinking of the sin(1/x) function
will that do ?
also its a unit square
and sin(1/x) is in x-y plae only
 
You need a function of x and y that's continuous except at $x=0, y=0$.
It can be unbounded. Don't worry about that.
Or you can use the idea with sin.
 
sin(1/x)*sin(1/y)
 
No, that is undefined along all of $x=0$ and along all of $y=0$.
 
yup,thinking now!
Just guess - space filling curve?
 
6:01 PM
What is the command to create a blunt arrow in latex ?
 
Yikes. Calm down!
 
I would be in favor of a spelling reform that pluralized "shark" as "sharx"
 
What's a blunt arrow?
Makes sense to me, DogAteMy, like the sharx brothers.
 
A arrow that has its back end "flat".
 
@TedShifrin on the cauchy integral formula , what is the roles of z,z_0 and greek zeta ? f(z_0) = 1/2pi*i * line integral
 
6:02 PM
@Ted "Karl Sharks."
 
@BAYMAX. Can you give me a simple function of $x$ and $y$ that is $0$ precisely at $x=y=0$?
 
@A---B Try Detexify
 
@A---B: You mean \mapsto?
 
you can use detexify @A---B
 
$\mapsto$
 
Sorry, I edited, @A---B.
There.
That?
@Kasmir: That's too many variables. $z_0$ is a fixed point and you're integral has $dz$ or $d\zeta$, not both.
 
Yes I know but why use zeta ?
 
$f(x,y) = y.sin(x)$
 
6:04 PM
does zeta live on the boundry ?
 
@TedShifrin
 
I know that z_0 must be inside the countour
 
Yes, can I get that flat end a little bit ahead of its current position, I want a little bit of the arrow to be sticking out.
 
$f = x.y$
 
Since no one else ever wants that, @A---B, you'll have to build it yourself using \rlap or \llap.
@BAYMAX: You're not paying attention. You're writing down continuous functions, first of all, and they're zero lots of places.
 
6:06 PM
@BAYMAX how are you today?
 
sorry@TedShifrin ,wiil think
 
Since it is in my chemistry book, I think I am not the first one to discover that.
Basically I want to create a dipole arrow.
 
@robjohn yes fine ,did a lot of calculations in numerical analysis
today
 
@BAYMAX what were you computing?
 
@BAYMAX: What polynomial is $0$ at $0$ and positive everywhere else?
 
6:07 PM
@TedShifrin does zeta and z have the same "roll" in that formula ?
 
@robjohn: He's trying to construct (with only mild hints from me) a continuous function on $[0,1]^2-\{0\}$ that does not extend to the closure.
 
Hey everyone!
 
Hi chat
 
@TedShifrin i mean dz and d zeta , is it just change of name of variable ?
 
Hi @Daminark
 
6:08 PM
@Kasmir: You can put any "dummy variable" you want in an integral. If I write $\int_a^b f(t)\,dt$ it's the same as $\int_a^b f(u)\,du$.
Hi Demonark, Astyx.
 
How are you ?
 
@TedShifrin sort of like...
 
@TedShifrin so z_0 is inside the contour but z and zeta can be any point right ?
 
:p
 
How's it going @Ted? And I'm alright @Astyx, how about you?
 
6:09 PM
The letter $z$ or the letter $\zeta$ is used as the dummy variable for the point moving around the curve in the line integral, @Kasmir.
 
chat.stackexchange.com/transcript/message/36577165#36577165
 
Yes, $z_0$ is inside the contour.
 
$x^{2}$
 
@TedShifrin thanks alot Ted ! you are the best! :D made things clear now !
 
Fine I guess
 
6:09 PM
Two variables, @BAYMAX, but right idea.
 
$x^{2} + y^{2}$
 
Perfect!
 
sorry
 
Now can you write down a function that will answer your original question? Combine what we did with one variable with what you just did here.
 
How much time do you have until exams @Astyx?
 
6:10 PM
10 days or so
 
@AkivaWeinberger @BAYMAX @TedShifrin Thanks for the link. I think I have to use $\mapsto$ anyway.
 
Ah, good luck!
 
@A---B: I don't remember what's used for a dipole. Of course you can do \leftrightarrow and such things ...
 
Thanks
 
I have designed my own things putting together symbols and moving them left or right or up and down.
 
6:12 PM
$(x^{2} + y^{2}).sin(1/x)$
 
I have good news everybody
 
again not defined on $x = 0$
 
What is it Zach?
 
@A---B there is also
 
My math teacher is letting me use a computer for independent study during math.
 
6:13 PM
Ooh, @A---B: There's also \rightarrowtail :P
 
' \longmapsto '
 
Instead of sleeping
 
@TedShifrin so the whole idea of cauchy integral formula is that we can find the value of any point z inside the contour by knowning the integral arount the countour yeah ? :D
 
@MeowMix Wee
 
No, @BAYMAX. That has problems everywhere on $x=0$.
 
6:13 PM
So I can bug Ted more >:)
 
Don't misuse that by hanging around this chat though!
 
Nice, you can do Tedcercises!
 
Zach: I think "independent study" means actual study.
 
zack is the strangest dude I seen online or in real life
 
I agree with @BalarkaSen though. If you have the hour, you're required to get actual work done.
 
6:14 PM
I've learned more in this chat than half of undergrad, so maybe it could be actual study.
:P
 
@KasmirKhaan Not a chance.
 
$\rightarrowtail $
 
anyone know where toshiba kindletoft gone, everything about him seemed to have vanished from the site?
 
@MikeMiller how is that sir ?
 
$\longmapsto$
 
6:15 PM
Tobias, you mean?
I saw him about a week or so ago.
 
oops (yeah misremembered his name that's why I cannot find hiim, all good now, thanks)
 
google.co.in/… close but not quite it.
 
Balarka is 28x weirder.
 
bows
 
$\longmapsto$
 
6:16 PM
$\longrightarrow$
 
$\longrightarrow$
 
@A---B: You will have to design that yourself. It doesn't exist in LaTeX, so far as I know. You need to design a vertical line of the right length and then shift it on top of the rightarrow.
 
ohhh i see which one you're talking about now
 
If you pay me for my time, I'll do it for you in 15 minutes or so.
6
 
ohh
TIME - Inception
 
6:18 PM
@TedShifrin I want to look into it myself.
 
$(x^{2} + y^{2}) $
 
Sure thing, @A---B.
@BAYMAX: So how to use that to get something not continuous at $x=y=0$?
 
If it takes 15 mins for you to do then I am fine with that tail arrow.
 
LOL, to get it just right takes some time. Plus I haven't played with this sort of thing in 15 years or so.
 
What are we trying to do
 
6:22 PM
$x^2 + y^2 + cos(x)$
 
@BAYMAX: You keep writing down things that are continuous everywhere.
 
@BalarkaSen google.co.in/…
 
2
Q: code for arrow with a short vertical line in the middle of the shaft

Ittay WeissI'm trying to produce an arrow similar to $\nrightarrow $ only with the smaller line being vertical rather than horizontal. I went over the big list of latex symbols several times and could not find it. Any help would be appreciated (I'm using xypic, so extended codes are fine if xypic supports t...

that should do it
 
I would probably use | or \vrule rather than a thin + ...
 
$\mathrel{\mkern3mu \vcenter{\hbox{$+$}}%\mkern-12mu{\rightarrow}}$
 
6:24 PM
$\to \!\!\!\!\!\!\!\! +$
 
how if I add $tan(x- \pi/2)$ ,then at $x =0 $ they blow up
 
There.
 
@BAYMAX. We need blow up JUST at x=y=0.
 
How can you view the tex code after jax has been started?
 
Interesting that you don't reason well with analogies.
 
6:25 PM
been driving me nutty for a while
 
@Ted You pinged the wrong person.
 
Damn, because you both start with BA.
 
@BalarkaSen That was a nice hack. Thanks.
 
@A---B Whitespace manipulation always ever.
 
oh I see you are tired of saying that to me and I make the same mistake.
 
6:25 PM
@Ted I'm going to read section 5, finally.
 
OK, Zach. Promises, promises.
 
@logical123 Thank you for all your efforts that was really kind of you.
 
balarka, what does \! mean?
in tex
 
Shifting symbols back
 
negative thin space
 
6:28 PM
wow, that is a nice hack
 
You can also use \rlap and \llap, as I suggested.
 
way simpler than the nonsense i sent lol
 
$A B$, $A \! \! B$, $A \!\!\!\!\!\!\!\! B$
eg
 
coolio
 
I scrambled that all up, but whatever.
 
6:30 PM
You can also do \hspace*{} and specify it all yourself.
But this isn't a TeX programming class.
 
@TedShifrin how aboyut a piecewise function
 
I write commutative diagrams by whitespace manipulating :3
 
No, @BAYMAX. How did we take something that was 0 at x=0 with ONE variable and create something that blew up at x=0? Reason analogously.
Very ad hoc and unprofessional, Balarka. You shouldn't tell me such things.
 
lol
Sorry.
 
1/sin(x)
@TedShifrin
may be my head is not right now,I will think about this in detail
 
6:36 PM
OK, @BAYMAX. I leave you to figure it out.
 
will talk to you soon ,till then bye . bye all!
 
@logical123 You can refresh the page
I think right-clicking also works
 
Do I need integration to find the length of sine like curves ?
 
Yup. And you'll have to do that one numerically.
 
@TedShifrin Why no exact solution ?
 
6:41 PM
Just because. :)
 
:(
 
cries
 
Again, Demonark?
 
What? Non exact solutions are saddening
 
Oh, I knew you were emotional for some reason ...
 
6:47 PM
Lol
 
7:02 PM
hi chat
 
Hey @Eric!
 
what's up
 
Not too much, Marianna talked more about games today
 
@Ted So, how's your sickness?
 
Not Banach Mazur
 
7:06 PM
is she giving you the same problems she gave us or is she changing things up
 
So we had 5 problems this first pset
One of which was on your midterm, 2 of which were on your pset, 2 of which I couldn't find
But I only have 5 psets of yours
So it's possible (though I think unlikely) that she assigned the remaining 2 problems on one of the psets I don't have, unless she really did assign you only 5
 
hmm ok
was it the really hard midterm problem
there was one I still have no idea how to do
 
It was to show there's no finitely additive measure on $\ell^{\infty}$ for which the measure of every ball is positive and finite
 
oh that was one of the reasonable ones
 
Yeah
This was a pretty light pset to start with
Aside from the free group one, whose only reasonable solution was having it operate over $\mathbb{F}_5$
 
7:16 PM
my Riemannian geometry pset was very light too
the only problem I have left is showing that every geodesic in a lens space is closed, and that there are lens spaces with simple closed geodesics of different length
 
Looking over Samir's notes from last year, it seems like Marianna is switching gears slightly with us
 
that's good, last year was kind of a mess
 
Like, today she talked about topologies on trees that correspond to games, so how the binary game was homeomorphic to the Cantor set, the countable one to $\mathbb{R}\setminus\mathbb{Q}$ (the verification of which would be on the homework)
Also today we proved that the Borel hierarchy does not terminate in countably many steps, using a method I only 30% understand
It involved constructing these "universal sets"
 
i recall she mentioned these things to us and i immediately forgot them
 
On the other hand, she hasn't (yet) talked at all about typical functions, which is what she seemed to spend more time on with you guys
 
7:21 PM
oh yeah that stuff
 
I'm not sure if she's gonna get into that stuff or not, but she did say that next week we'd start measure theory
 
@Daminark Measure theory? Just use a ruler for god's sake! /s
 
Nah Zach, we don't want monarchy in math
:P
 
How about a thermometer?
To see if it's a hot theory :P
 
@Daminark It's not even guaranteed true, it's only a theory /s
 
7:27 PM
The measure of any theory is done in the lab.
::puts away high school humour book::
 
Good lord
 
Hi - is anyone here familiar with the intersection of math and physics (specifically statistical mechanics)?
 
Askaway.
 
What is the necessity of having a local entropy definition and a classical entropy definition?
I guess this is a little open-ended, let me elaborate
Using the partition function $\int_{x'} exp(f(x'|\theta)) dx'$, we get that the local entropy is log(partition function)
We take the log to find the probability density around the $\theta$ parameters of our space, as opposed to $E[log(f(x'|\theta))]$ which is classical entropy.
 
@Daminark how did you prove that?
 
7:42 PM
The Borel thing?
 
Why is it that local entropy is not in the form $E[log(f)]$ instead of $log(E[f])$?
 
So say we're just working on $\mathbb{R}$
Then you define $A\subset \mathbb{R}^2$ to be universal with respect to a property $P$ if it has that property, and if for any $B\subset \mathbb{R}$, there's a horizontal level of the set which projects onto $B$
 
The idea is that you sorta take $\{(x,x):x\notin A\}$, which necessarily doesn't have that property
 
Hey @Akiva
 
7:51 PM
There's something weird here since $x$ is in $\Bbb R$ and $A\subseteq \Bbb R^2$, but I've seen this kind of argument before
 
Now, in the Borel hierarchy, if you're at some level and you can find a set in its complement which isn't there (e.g. an $F_{\sigma\delta}$ set which isn't $G_{\delta\sigma}$), it means that moving to this step must have added something new, because any set in the hierarchy contains all previous lines
Oh whoops
I meant $\{x:(x,x)\notin A\}$
 
Ok, I see
Did you also show that the construction terminates at $\omega_1$ (in metric spaces)?
 
Now, the idea is to show that you can construct a universal set $A$ such that $\{x:(x,x)\in A\}$ has this property $P$ if you've only gone countably many steps in the hierarchy, by indexing things through $\mathbb{N}^{\mathbb{N}}$
You do stuff I only partially followed and it works
 
@MeowMix Hey
 
Hm, ok, thanks
I have to run away now, I'll be back later
 
7:54 PM
No, Marianna said that it does, by saying that if you chose countably many levels of the hierarchy when you've reached $\omega_1$, there's still stuff afterwards
But she didn't want to go into the set theory
And alright, see you!
 
@MeowMix You know how you can make ${\cal P}(X)$ into a group using the symmetric difference thing, right?
And same thing for the set of finite subsets of $X$, which I might as well call ${\cal P}_{\rm fin}(X)$
There's a bunch of different notations for the symmetric difference. Let's use $\oplus$
 
Uhh what's symmetric difference?
 
@MeowMix Set of things that's in $A$ or $B$ but not both
 
$(A \cap B^c) \cup (A^c \cap B)$ I think
Or $(A \cup B) - (A \cap B)$ like Akiva said.
 

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