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17:02
@robjohn I think they started driving here at 730 despite my warning.... so it could be that they're just going to be very late.
@MikeMiller Could be. Where were they coming from?
730 mph... ohwait
Someplace in or nearby SD
@MikeMiller OMG, yes, they will be late. I left the West SFV at 7:30 and didn't get here until 9:06
I probably should have been more stern in my warning.
17:04
@MikeMiller Obama's in town and the freeways will be very slow through town
17:15
@robjohn Do you have any thoughts on 'TAOCP' D.E knuth?
Ok I back
I drive to hospital. Grandmother needs doctor
When does it stand that $f_{xy}=f_{yx}$ ??
But I brang my notebook
So I continue to solve in car.
Do you know what condition $f$ should satisfy so that $f_{xy}=f_{yx}$ ?? @robjohn @ThomasAndrews @DavidWheeler
Do $f_{xy}$ and $f_{yx}$ be continuous??
17:31
@AlexW: You survived your whirlwind visitation days?
You can prove a slightly stronger theorem, @MaryStar, but the standard hypothesis is that both second partial derivatives are continuous, yes.
So far, so good @Ted :)
I enjoyed meeting Bryan at Wisconsin! Very nice guy.
Good thing you're young and energetic, @AlexW :)
Did you find the visits interesting/enjoyable?
Yes, @AlexW, very nice, despite taking his 5th class from me :P
Can someone check? imgur.com/99fOo6I
I have little data left
Haha, well, I will admit the travel tired me out quite a bit. It's also a lot to meet so many new people at once. But my experiences these past couple months have been devoid of this kind of math oriented social interaction, so I really enjoyed it nonetheless.
I did, @Ted. There's one (possibly two) visit to go, but I think I have a clear answer in front of me after the last two days. :)
Rooting for UCLA ahead of Wisconsin?
17:38
I am looking at a pde and I want to write it into the canonical form.

So we have set $u(x, t)=V(\xi , \eta )$ where $\xi=at+bx, \eta=ct+dx$.

Does it stand in this case that $u_{xt}=u_{tx}$?? @TedShifrin
Owatch to control, is anyone receiving?
Yessir. I definitely enjoyed my visit to Wisconsin, but UCLA really blew me away.
Sure, in solving such PDE you ordinarily assume as differentiable as you need, @MaryStar, unless the course is very advanced.
It seems like the best fit for me both personally and mathematically.
Except for having to put up with @Mike, @AlexW.
17:40
Haha, one must put up with some difficulties, @Ted... ;)
Well, you must ... not I.
Anonymous
Hello!I am new to this chat room!
Anonymous
Can someone introduce
Help
True, hahaha. In seriousness, @Mike was fantastic. We had a fun time, and he was able to show me around a bit during my downtime, which was nice.
17:40
Ok.. Do I have to mention that we assume it?? @TedShifrin
@Owatch: We honestly are not here to check each and every one of your homework problems.
You need to ask your professor, @MaryStar.
Anonymous
So what happens here?Can we discuss maths problems here?
Sorry. I will check when I get back home.
Hello @SanchayanDutta :)
@SanchayanDutta Are you a student?
Ok... Thank you!! :-) @TedShifrin
Anonymous
17:42
Hello Everyone!I new to mse!I am from india....high school student...preparing for iitjee...and i love solving mathematical problems
When do you take main exams?
Anonymous
introduce yourselves please?
Anonymous
main exams?
Anonymous
meaning?
India has main and advanced examinations no?
Anonymous
17:44
oh well in 2016
Anonymous
march
You pass main, you can take advanced. Best schools take students who passed advanced?
I see. Ok.
Anonymous
well not schools...we call them engineering colleges here
Anonymous
anyway where are you from?
I am from France/Usa
Anonymous
17:46
and what do you do?
Study.
@Owatch France or USA ?
France, but I live in USA. I also have passport for the US
Bonjour
Vous êtes français?
17:49
Oui :D
Vraiment?
oui oui vraiment
J'espère que vous n'utilise pas Google Translate.
Bien sur que non xD
Vous habite ou en France?
17:51
Paris
J'ai visité Paris l'été dernier.
Mais j'étais plutôt à Fontainebleau
gtg dinner, afk
Malheureusement, mon Français n'est pas au niveau d'un étudiant mon âge. J'ai grandi avec l'anglais pour longtemps et maintenant mon vocabulaire et grammaire souffre beaucoup.
@PiDay
I'll be right back.
17:57
I hope you don't use Google Translate.
I do not.
C'est*
(too lazy to look for the cedille)
You said: This good.
18:02
Colloquial, formally, yes-ca c'est bien.
Approximately, in English: 's good.
C'bien would be that
Whatever
It is hot in the car
Right. J'oublie
back
It's has been so longtemps, lol. oh, excusez-moi, mdr.
18:29
Is someone of you familiar with PDE's??
a bit. Not much.
I have to solve the following pde:

$$u_{tt}(x, t)-u_{xt}(x, t)=0, x \in \mathbb{R}, t>0 \\ u(x, 0)=f(x), x \in \mathbb{R} \\ u_t(x, 0)=g(x), x \in \mathbb{R}$$

I have done the following:

$$\Delta(x, t)=1>0$$

so it is an hyperbolic equation.

We want to write the equation into the canonical form, which is of the form $u_{\xi \eta}=D(\xi, \eta, u, u_{\xi}, u_{\eta})$.

$u(x, t)=U(\xi, \eta)$

$\xi=at+bx \\ \eta=ct+dx$

$$u_t=aU_{\xi}+cU_{\eta} \\ u_{tt}=a^2U_{\xi \xi}+2acU_{\xi \eta}+c^2U_{\eta \eta} \\ u_{xt}=u_{tx}abU_{\xi \xi}+adU_{\xi \eta}++cbU_{\eta \xi}+cdU_{\eta \eta}$$
@MaryStar could you define your notation ? What's $u_{ab}$ ?
$u_{ab}=\frac{\partial^2{u}}{\partial{a} \partial{b}}$ @PiDay
@MaryStar is $u\in C^2$ ?
(twice derivable with continuous second derivative)
18:36
I think so but I am not sure... @PiDay
@MaryStar Also, what's $\Delta$ ?? I know the $\Delta$ operator, but you're not applying it to any function here, so ...
It is the disciminant.

Let the differential equation be :

$A(x,y) U_{xx}(x, y)+2B(x, y)u_{xy}(x, y)+C(x, y)u_{yy}(x, y)+Du_x(x, y)+Eu_y(x, y)+Zu(x, y)=0$

then the discriminant is defined as followed:

$\Delta(x, y)=4(B^2(x, y)-A(x, y)C(x, y))$

If $\Delta>0$ then it is an hyperbolic equation,
if $\Delta=0$ then it is an parabolic equation,
if $\Delta<0$ then it is an elliptic equation. @PiDay
I believe it is the discriminant of the characteristic conic.
@MaryStar I don't know much about hyperbolic equations, sorry
Instead of having one linear operator $D$, one has two: $D_1$ and $D_2$.
18:46
Ok... no problem... @PiDay
Do you have an idea @DavidWheeler ?? Have I done something wrong??
Hallo @Theorem Wie war dein Tag heute?
@evinda Es war Ok .
Was hast du so gemacht? @Theorem
Why is $\cup_{i\in\mathbb{N}}$ a good notation for both finite and (countably) infinite unions? An author uses it and I'm not sure why
Because any finite set can be indexed by a finite set of natural numbers
18:54
But the infinite case?
@evinda Ich hab mountainbiking .
If you have a countably infinite set, it is in one-to-one correspondence with the natural numbers, and this correspondence can be used to index the set.
@Theorem Also hast du doch ein Hobby :)
@evinda Auf jeden fall ! laufen und Philosophie bucher lesen sind meine Hobby .
:D
@DavidWheeler but for it to work (in the finite case) I need to have some "neutral element" (eg $\emptyset$ in $\cup$ and $0$ in $\sum$) - well I suppose I do so it makes sense.
18:56
I must learn German.
@Owatch What languages do you speak?
@Theorem Gut!
French/English
I'm learning! :) But it is very hard :/
One more question, is there a way I can write a $\cup$ but with a dot in the middle? (I don't know how I'd search for this)
and it proves brouwers fixed point thm
18:57
@AlecTeal Yes, that is one of the reasons why the empty set is so indispensable.
@Hermine Will you take exams?
No, I just do it for fun
@Hermine Interessant!
@DavidWheeler yes, good point, as for the dot inside the cup?
@robjohn Hi :) good morning
19:00
Are you a native german speaker @evi
@evinda
Haven't seen that one-I have seen the one with a + inside
@Nick Where have you been? Haven't seen you in a long while? How's life going?
@DavidWheeler it's a start, how do you do it?
wait- \bigcupdot will work-let me see, $\bigcupdot$
How do I provide Mr.Wolf with boundaries for integrals?
19:02
hmm, mathjax must not support the MnSymbol package
$\biguplus$ (did that work? I need someone to confirm this because chatjax wont work)
$\bigudot$ - did this work?
that did the plus one, ya
2nd one did not
@Hermine I have 2 native languages, that is one of them. What's with you? What's your native language?
@TedShifrin the center is a fixed point no ?
@evinda mine is spanish, which others do you speak?
19:08
@Hermine Greek, english, french.
Spanish has words similar to French.
Many, in fact.
@evinda Nice, je parle francais aussi!
@Hermine Bien!
@Owatch, yes, that's the reason it was so easy for me to lern french, but not so easy to lern german :P
learn*
Tout a fois tout le monde parles francais.
19:11
@Hermine Now you used a german word ^_^
I would like to learn either German or Russian.
I think German would be easier.
Wow, so many languages :D
@Owatch I want to learn russian too
@Owatch Yes, I think so too
@evinda What use?
@Sawarnik What languages do you speak?
19:13
I downloaded duolingo, and used it a bit with German.
@evinda Hindi and English.
only :D
@Sawarnik What do you mean with "what use"?
But I think a instructor and courses would be better, if not living in a country that speaks the language.
What is the use of learning 10 different languages?
What is the use of learning any other languages?
With that outlook.
19:14
@Sawarnik I like to learn foreign languages.
ok :)
learn some non-european languages too then :D
Other languages are useful for travelling, communicating, and understanding what people say.
Russian is non-european, kind of.
It's Cyrillic based I believe.
But Eastern Europe speaks those types too.
I think
Greetings
19:18
My Calculus instructor is Russian
$$\int_0^{\phi} \frac{d \theta}{\sqrt{1-\cos{\phi}\cos\theta}}=\int_0^{\pi/2} \frac{d \theta}{\sqrt{1+\cos{\phi}\cos\theta}}$$
$0<\phi<\pi$
This was a proposed problem ...
Oh my, $ \frac{dy}{dx} secx$ is no simple derivative.
@Semiclassical It's for a derivative problem. I'm supposed to use it in Trigonometric Substitution since it is one of the Expression substitutions: sqrt(x^2 - a^2), x = asec(theta)
I skipped that one for now.
ah, so you do want the derivative of secant
I checked what it was with Mr.Wolf. It's enormous.
what's the specific problem?
19:31
$\int \frac{1}{t^2 \sqrt{t^2 - 16}} dt$
ahh. you can make that a simpler by subbing $x=t/4$ in order to pull the 16 out, but it's still hairy
I will just pick another.
I have a topology question, why in R is this true: F is compact iff F is closed and bounded? Is it because R is hausdorff?
well
$\mathbb{R}$ is a metric space, hence hausdorff
thus every compact subset is closed and bounded
the hausdorff property alone doesnt guarantee that a compact subset is bounded
So in what other spaces is that true?
I know that for R and R^n
19:39
is the sphere is compact ?
My questions arises from the compactness of the unit square, because it is Hausdorff and compact
@Vrouvrou yes it is
Bou it does not have the same topology of RxR
(Unit square with dictionary topology)
@Owatch That integral comes out rather simply, believe it or not
@teadawg1337 I can return to it after.
19:42
what do you mean by dictionary topology? you topologize the unit square with the subspace topology of \mathbb{R}^2$ right?
Properties of a topological space depend more on the topology than the underlying set (with some differences for "small sets"-which consequently have fewer topologies to choose from)
I've solved it, tell me when you need some help with it @Owatch
The lexicographic ordering not the induce topology
Hi @Huy
Lange nichts mehr voneinander gehört...
Huy
Huy
19:43
jo, viel zu tun die Tage
In general topology, the lexicographic ordering on the unit square is a topology on the unit square S, i.e. on the set of points (x,y) in the plane such that 0 ≤ x ≤ 1 and 0 ≤ y ≤ 1. == Construction == As the name suggests, we use the lexicographical ordering on the square to define a topology. Given two points in the square, say (x,y) and (u,v), we say that (x,y) (u,v) if and only if either x < u or both x = u and y < v. Given the lexicographical ordering on the square, we use the order topology to define the topology on S. == Properties == The order topology makes S into a completely normal...
eww german
@iwriteonbananas Du bist ein Schlauberger :P
@Huy Was machst du so?
@Hermine Do you study algorithms?
@teadawg1337 disgusting
Huy
Huy
19:45
@evinda: Hauptsächlich fürs Gymnasium arbeiten. Nebenbei etwas Differentialgeometrie.
Besuchst du Vorlesungen diesen Semester? @Huy
Huy
Huy
@evinda: Paar fürs Lehrdiplom, und DiffGeo2.
@evinda I study math, and you?
Huy
Huy
und selber?
@evinda, why?
19:47
@Hermine Compactness is a topological property-"which" sets are compact depends on "which" topology you use.
@Hermine Because I saw that you posted something about lexicographic order.. I study applied mathematics and take the subject algorithms and complexity.
@Huy Ich nehme 5 Fächer diesen Semester
Huy
Huy
welche?
@teadawg1337 is $(\sqrt{2})^5 = 2^3$?
@evinda I'm studyng topolgy now! There is not applied math in my University :(
@Owatch No, it'd be $4\sqrt{2}$
19:49
@David I know, that's why I am asking what property is necessary for the theorem to be true
@teadawg1337 I see.
@Huy Algorithmen&Komplexität, Numerische Lösung von Differentialgleichungen, Kryptographie, Partielle Differentialgleichungen, Methoden der Angewandte Mathematik
Huy
Huy
ok, cool
For compact = closed + bounded?
@Owatch Need help with anything?
20:00
Soon. I am solving another.
Before I return to other.
I am near end
Alright, I'll be here
I have currently got $\int tanxsec^{5}x - 2tanxsec^{3}x + tanxsecx dx$
Does not look too promising.
Oh wait, I can split it
@Owatch Try substituting $u=\sec(x)$
No splitting?
You can split it
20:06
Better not to?
It'd be better to split
But apply $u=\sec(x)$ to all three
How's it coming along?
Not good
This will take several pages, should not.
Do you remember the derivative of $sec(x)$?
It shouldn't take more than a few lines
$\int tanxsec^{5}x dx$
u = secx
= $\int \frac{u^5}{secx}du$
Whatnow.
Hold on, $du=\sec(x)\tan(x)dx$
20:13
Yes
So it should be $\int u^4du$, should it not?
Oh yeah
My bad then
I didn't realize secx in the derivative was u
You have a tendency to mix variables, be careful
Okay, next integral?
I have two more pieces to do.
Of the first
I know, I meant next integral in this particular problem
20:16
Same thing, just with sec^3
I do
Oh no
What's wrong?
It's actually $\int2tansec^{3}x$, my bad.
for which one?
second piece
It's fine, you can still apply the same change of variable
Wait, is it $\int 2\tan(\sec^3(x))dx$?
20:20
No
I missed an x after tan
Okay, so just apply $u=\sec(x)$ again
I have
What did you get?
$2[ \frac{sec^{3}x}{3}]$
Why are you dividing by 3?
20:24
My bad.
Ah, I see.
And the third one is straightforward
It became zero?
Or 1
Which becomes x
Well, it became $\int du$ after the substitution, so it becomes $u$
Yes
1 du
Okay, combine everything
20:26
I will, I must rewrite.
(On various pages)
It's spread out on various pages, I need to have it on one.
So I'll rewrite it there now, so I can combine it properly without sifting arund
It will take a minute to rewrite
Alright, I'll be waiting :3
Just wanna point out that it's been about 6-7 minutes, @Owatch :P
There is much to write. Just one minute.
I was just kidding around, take your time
20:45
It's very long
For an answer
It should only be three terms not counting the constant of integration, though
Yes it is 3 parts.
Constant not in there but I added it
Wait wait wait, where'd all the square roots come from?
Constants
I did not mention earlier
20:50
They are outside the integral so I thought not important for now.
Sorry, I mention next time,
You're fine, don't worry about it
... But please do warn me next time
Yes I will.
What about putting it back in terms of $t$?
Oh right.
Must I draw tri-angle?
$\theta=\arctan\left(\frac{t\sqrt{2}}{2}\right)$
I'd draw a triangle, yes
20:53
tan(theta) = $\frac{t}{\sqrt{2}}$
Triangle draw, with opposite t, Hypotenuse is $\sqrt{2}$
That should be opp/adj
No, the adjacent side would be $\sqrt{2}$
hyp = $\sqrt{2 + t^2}$
sec = 1/cos
sec = hyp/adj
20:57
sec = $\frac{\sqrt{2 + t^2}}{\sqrt{2}}$
There
Woah
Or $\frac12\sqrt{2t^2+4}$
@Vrouvrou Were you not in NSChat the other day?
-_-
Focus, @Owatch :)
20:59
I have to substitute
But I don't have space
I will write above.
By the way...Happy $\exp{\left ( 1+\pi+\frac1{e} \right )} \sin{(\pi e)} \pi^{-e} $ Day!
I got $\frac{4(\sqrt{2+t^2})^5}{5} - \frac{8(\sqrt{2+t^2})^3}{3} + 4 \sqrt{2+t^2} + c$
@Owatch let me check
Nope, that's not right :(
The whole answer, or the substitution?
The whole answer
Wait, let me check again
21:08
Took me an hour
What a waste of time,
@Owatch Wait, I'm not sure. Let me expand it and check
Nope, it's wrong :(
I don't know what work you did, so I have no idea where you went wrong
Everything was correct up until the substitution
of sec(theta) with it in terms of x?
I think you were correct until putting the final answer back in terms of $t$, but I can't say for sure because I don't have your work to check
I linked it earlier
Up to before substitution and solving for sec
I'm working it to see where you went wrong @Owatch, gimme about four minutes
21:17
ok
Thank you
I will be back in 10m
@Owatch wait
I'm almost done
@Owatch you didn't take into account that $\sec(\theta)=\frac{\sqrt{t^2+2}}{\sqrt{2}}$
You didn't cancel out the common factor of $\sqrt{2}$ when rearranging the equation to be in terms of $t$
Either that or you messed up with distributing powers
21:34
I will be back and forth fleetingly, thanks I will check in a bit.
Okay, no problem
No I think distribution was fine
I'll substitute, and not simplify/
WAIT
You forgot to raise $\sqrt{2}$ to the proper powers
Where?
I see
Nice
Stupid me
@DanielFischer You disappeared.
21:46
Please the sphere is convexe or not ?
One careless mistake does not make you stupid :)
Practice is key
Can someone help me at:
0
Q: How can we get a contradiction?

Mary StarHow could we show that the problem $$u_{tt}(x, t)-u_{xt}(x, t)=0, x \in \mathbb{R}, t \in \mathbb{R}, \\ u(x, -x)=0, x \in \mathbb{R}, \\ u_t(x, -x)=x, x \in \mathbb{R}$$ doesn't have a smooth solution?? Do we have to suppose that there is any smooth solution?? But how can we get a contradictio...

??
@Owatch Now, back to $\int \frac{dt}{t^2\sqrt{t^2-16}}$?

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