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7:00 PM
Something else... when we have that $f(0)=0$ and $f'(0)=0$, can we deduce something from that?? For example, can say something about the sign of $f'(x)$ ??
Hello @evinda @KajHansen !! Do you maybe have an idea??
 
C = -4, B = -8, A = 9
 
Your value for A is wrong
 
Well.
A+B = 1
B = -8
A - 8 = 1
A = 1 + 8
That is how I got it. .
 
No, the equation to be working with is $x^2+1=A(x-2)^2+B(x-3)(x-2)+C(x-3)$ for $\frac{A}{x-3}+\frac{B}{x-2}+\frac{C}{(x-2)^2}$
 
I was working with that.
 
7:07 PM
Then how did you get to $A+B=1$??
 
$A(x^{2}-4x+4)+B(x^{2}-5x+6)+Cx-3C$
= $Ax^{2}+Bx^{2} - 4Ax - 5Bx + Cx + 4A + 6B -3C$
 
Leave the equation in its factored form, it's easier to work with that way
 
Oh.
$x^{2}+1 = x^{2}(A+B)-x(4A+5B-C)+4A+6B-3C$
Well, that is what I was doing at the time.
 
Not like THAT
Leave the equation as I stated, you're making it more complicated than it has to be
 
You did ask how I got those answers, so I was trying to show you. I can just stop though.
 
7:11 PM
Continue, I suppose
 
In any case, A+B was matched to the coeffecient of $X^2$, which is 1.
-x has no coeffecient. So I made $4A + 5B -C = 0$
 
No.... That's not how to use partial fraction decomposition at all...
 
And then $4A+6B-3C$ = 1
 
Maybe it's valid, I dunno.
I've never seen it done that way before.
 
I would do it your way.
But it's all done on the notebook I have not gotten back, so I could not recall exactly what it was.
So I did it the way I was taught. . .
Which is annoying.
In any case, those three equations are to be solved for A, B, and C.
Which is what I was trying to do at the time.
But let me go back to the point where you do it differently and start there.
Okay I am back.
@teadawg1337 To this point
 
7:17 PM
 
Now I think you told me to find whatever would make the other two equations = to zero
And then solve for the remaining one
For A: x = 2 will make it zero. For B: x = 2, and x = 3, will make it zero. And for C, x = 3 will make it zero.
I cannot solve for B though? Because nothing will make both A and C zero.
 
In some cases, you'll have to use the other values obtained and plug in a random number for $x$ and solve for the last coefficient
 
What do you mean? I have B(x-3)(x-2) as the remaining piece to solve.
So I should plug in anything for x?
I will choose zero then.
B(-3)(-2) = 6B
Now how could I use A and C to solve for B?
Oh wait
Perhaps I can choose one of the ones that makes A or C zero
Then plug in A or C depending on which remains?
 
Use the equation $x^2+1=A(x-2)^2+B(x-2)(x-3)+C(x-3)$. Since $A$ and $B$ are now known, plugging a value in for $x$ will turn this into an equation with only one variable to solve for
And no, using either $x=2$ or $x=3$ will make $B(x-2)(x-3)=0$
 
You're right.
 
7:28 PM
My first message should read $A$ and $C$ instead of $A$ and $B$
 
I got -6.5 for B
 
sighs That means either or both of your values for $A$ and $C$ are wrong
 
I got A = 10
 
And $C=?$
 
C = -5 . .
Must be C then
 
7:29 PM
No, both of those are correct.
 
$x^{2}+1 = 10 + B(x-3)(x-2) - 5$
 
No...
$A\ne A(x-2)^2$....
Likewise with $C$...
 
Oh my bad.
$x^{2}+1 = 10(x-2)^{2} + B(x-3)(x-2) - 5(x-3)$
 
... Do you really need my approval before continuing?
 
I'm doing it
B = -9
 
7:36 PM
Correct
 
Standby
 
Something else... when we have that $f(x) \leq 0$, $f(0)=0$ and $f'(0)=0$, can we deduce something from that?? For example, can say something about the sign of $f'(x)$ ?? @DanielFischer @DavidWheeler
 
@MaryStar It can be deduced that $f(x)$ has a global maximum at $x=0$
Well, no, the function could have other zeros as well...
 
$10ln(x-3)-9ln(x-2)-\frac{5}{2-x}$
 
Incorrect @Owatch, but close, try again
 
7:40 PM
That means that $f(0)$ is the maximum value of $f$. Since $f \leq 0$ can we say that $f=0$?? @teadawg1337
 
@MaryStar No. For example, $f(x)=-x^2$ satisfies those conditions.
And it has a global maximum at $(0,\,0)$
 
Oh sorry...it is $f(x) \geq 0$ not $f(x) \leq 0$. @teadawg1337
 
I can't copy paste it
But was it + $\frac{5}{2-x}$
Not -
 
Then it has a global minimum at $(0,\,0)$ @MaryStar
 
That's the only thing I think I caught wrong.
 
7:44 PM
@Owatch You're still missing one tiny detal
 
What do get from the fact that $0$ is the minimum value of $f$ ?? @teadawg1337
 
$f(x)$ must be an even function @MaryStar
 
Goddamnit Tee dog
That doesn't Count
+c
 
It doesn't count without the constant of integration @Owatch
 
Is that a question?
 
7:46 PM
??
It was a statement, implying that you will get points deducted for not including $+C$
 
@KajHansen You know anything about Rado's theorem?
 
It does count.
+C, never forget. o7
 
So, can we not conclude somehow that $f(x)=0$ or something about the derivative $f'(x)$, for example the sign?? @teadawg1337
 
@MaryStar Using the information given, no.
 
I have put together a sheet to bring to the test.
 
7:48 PM
Well... Let me think on it for a second
 
He has permitted us to bring trigonometric identities, and so forth.
 
@Owatch What does calcII consist of?
 
It consists of pain.
 
Definitely cannot conclude that $f(x)=0$, I haven't thought about the derivative...
 
@Owatch Calculi hurts, definitely, but what's the syllabus?
Basic differentiation/integration/series etc?
 
7:50 PM
A little @BalarkaSen
 
Ok... @teadawg1337
 
7.1: Integration by parts. 7.2: Integration of trig. 7.3: Integration using Trig substitution. 7.4: Integration of rational functions by partial functions
We are now doing approximate integration.
 
@MaryStar, not for functions $\mathbb{R} \rightarrow \mathbb{R}$. E.g. $f(x) = x^2$ and $f(x)=x^3$ both satisfy those properties.
 
@KajHansen But $f(x)=x^3$ does not have a global minimum at $(0,\,0)$
 
oh, I didn't know that needed to be a stipulation
Well x^2 and x^4 then
 
7:52 PM
Time to practise trig substitution.
 
@KajHansen The number theorist guy I met a few days ago is working on something called monochromatic number theory, and said a lot of stuff about arithmetic combinatorics that went above my head. He mentioned this one Rado's theorem, though. What's this stuff about?
 
The idea is that you color (partition) subsets of certain sets and see if they have a given property.
So let's start with Schur's theorem, a specific case of Rado's theorem.
 
The function that I am looking for is $E(t)$ of the answer of math.stackexchange.com/questions/1205836/… @teadawg1337 @KajHansen
 
Disregard my statement that $f(x)$ must be even, I didn't think it through properly @MaryStar
 
@BalarkaSen, you can see my discussion of Schur's theorem here: youtube.com/watch?v=VO4EFKctSFE
 
7:56 PM
let me have a look
 
I have yet to study differential equations @MaryStar
 
Solved one!
Another now.
 
Ok... @teadawg1337
 
If I'm not mistaken, Rado's theorem generalizes Schur's theorem to give sufficient conditions for when that holds for polynomials in $\mathbb{Z}[x_1, x_2, ..., x_n]$. Schur's theorem is just one of these w/ the polynomial being x+y-z @BalarkaSen
 
this looks like a very interesting claim. i am gonna watch this video. rubs hands
 
8:02 PM
I don't know how much Ramsey theory you've seen @BalarkaSen, but you may need to watch earlier videos. In particular, you need that $R(x_1, x_2, ..., x_n)$ is finite for $x_i \in \mathbb{N}$.
 
ah, i see you talk about a proof in the video. that's ok, i am not planning to read a proof since i am not familiar with Ramsey theory, but the theorem is surprising.
wonder how possibly can this interact with number theory...
 
Another solved!
 
yeah, so i just looked at the paper and what they prove is that given an r-coloring of the integer lattice points in $\Bbb R^2$, there is always a triangle of area $T_r$ with monochromatic vertices @Kaj
 
Sounds about right. Link?
 
actually, it seems it's a known result but they've given a different proof.
ah, in here they prove something related about trapezoids.
ah, i recall : the number theoretic thing comes from van der waerden's theorem. it states that given an r-coloring of {1, 2, ..., n} there always exists a monochromatic k-tuple the elements of which are in arithmetic progression.
fun stuff.
 
8:20 PM
Green-Tao theorem
Such wut
 
And another done.
 
@KajHansen indeed. i never knew that it comes from such a combinatorial background.
 
I will go back and do more partial function problems.
 
8:32 PM
$\int{\frac{1}{x^{3}-1}}dx$
How should I approach this problem?
 
$x^3-1=(x-1)(x^2+x+1)$
 
I tried factoring it on my calculator
Is just gave me: x1 = 1
 
Is someone familiar with computability, and especially with automata??
 
Which I guess is (x-1) = 0
Could you explain how you factored that?
I guess it makes sense if you distribute it.. but
 
It's a simple property of polynomials of higher order
$x^n-1=(x-1)(x^{n-1}+x^{n-2}+\dots+x^{n-k}+\dots+x^2+x+1)$
 
8:37 PM
can someone help me on functional analysis ?
 
Factoring on your calculator will just give you the zeroes of the function @Owatch...
 
Yes it did not give me much.
Because there was only one zero.
The rest has no roots I guess.
 
$x^2+x+1$ has two complex roots @Owatch.
 
No real roots. ..
 
Anyways.... Solve $1=A(x^2+x+1)+B(x-1)$
 
8:39 PM
Isn't it Bx+c(x-1)?
 
@Owatch Yes, my mistake.
 
I was starting to distribute the Bx+C a bit, but I suppose I shouldn't do that right?
 
I told you before, leave the equation in its factored form...
 
I had changed it to $A(x^{2}+x+1) + B(x^{2}-x) + C(x-1)$ But I will not go that way.
 
There is a famous "bogus proof" that goes: Suppose $a^2 + a + 1 = 0$. Then $(a - 1)(a^2 + a + 1) = a^3 - 1 = (a - 1)0 = 0$, so that $a^3 = 1$. Thus $a = 1$, and thus $a^2 + a + 1 = 3 = 0$.
 
8:43 PM
Hi @DavidWheeler
Long time no see
 
@DavidWheeler hello, can you help me in functional analysis please ?
 
I cannot make A zero.
 
That is true, so plug a value in for $x$ once you have found $B$ and $C$.
 
What should be done, is to solve $(Ax + B)(x - 1) + C(x^2 + x + 1) = 1$. This can be re-written as: $(A + C)x^2 + (B - A + C)x + (C - B) = 0x^2 + 0x + 1$.
 
I got: A = 1/3, B = -1/3, C = -2/3
 
8:51 PM
@DavidWheeler That works as well, I suppose it comes down to personal preference
@Owatch correct
Now integrate.
 
I prefer to write the numerator of a quadratic factor as linear, and the numerator of the linear factor as constant.
 
What a terrible integral.
 
@Owatch I agree, it's not pretty by any means.
 
$\int{\frac{1}{x-1}}dx + \int{\frac{\frac{-1}{3}x+\frac{-2}{3}}{x^{2}+x+1}}dx$
what did you say about my integral???
 
I said it was incomprehensible, that was before you fixed the former into a fraction
 
8:56 PM
Oh.
 
@Owatch in all fairness he redacted his statement haha
But I still think he wants to fight.
 
Move the factor of $\frac{1}{3}$ in front of both integrals, and solve
@Jaken Nah
 
hi guys
 
Hello @Karim
 
Hey @KarimMansour
 
8:58 PM
I made it $\int{\frac{\frac{-1}{3}(x+2)}{x^{2}+x+1}}dx$
Shouldn't I move a three out front
 
So $\int\dfrac{1}{x^3 - 1}\ dx = \dfrac{1}{3}\left(\int\dfrac{1}{x - 1}\ dx - \int\dfrac{x+2}{x^2 + x + 1}\ dx\right)$
 
Thank you dahvid. But tee dog has already assisted me with this part.
 
He's helping me explain, it's alright
Recall that $A=\frac{1}{3}$ @Owatch
 
@Owatch I was explicitly illustrating a 1/3 comes "out in front", not a 3
 
Ohh
 
9:02 PM
A mistake like that can mean your answer is off by a factor of 9, not so good.
 
So how did you move out the 1/3 from the second integral?
Isn't $\frac{\frac{1}{3}}{1}$
1/3 . . . . .. .
 
=3? HELL no
 
what is ANYTHING divided by 1?
 
Oh my,
 
$a/1 = a$ (even if $a$ is a fraction)
 
9:04 PM
Edited
That was such a silly mistake, by bad.
 
You can easily check: $\dfrac{1}{3}\left(\dfrac{1}{x-1} - \dfrac{x + 2}{x^2 + x + 1}\right) = \dfrac{1}{3}\left(\dfrac{x^2 + x + 1}{x^3 - 1} - \dfrac{(x + 2)(x-1)}{x^3 - 1}\right)$
$ = \dfrac{1}{3}\left(\dfrac{x^2 + x + 1 - x^2 - x + 2}{x^3 - 1}\right) = \dfrac{1}{3}\dfrac{3}{x^3 - 1} = \dfrac{1}{x^3 - 1}$
 
Down to $\int{\frac{1}{x^{2}+x+1}}dx$
 
I always "back-substitute" to test for errors.
Where did the x + 2 part go?!?
 
I split it.
That is the second piece, the 2 was moved out in front
$\int{\frac{x+2}{x^{2}+x+1}}dx$ = $\int{\frac{x}{x^{2}+x+1}} + \int{\frac{2}{x^{2}+x+1}}dx$
 
If you split it, you should have $-\dfrac{1}{3}\left(\int\dfrac{x}{x^2 + x + 1}\ dx + \int \dfrac{2}{x^2 + x + 1}\ dx\right)$
But I don't really recommend splitting them up
 
9:13 PM
I wouldn't recommend splitting them either
 
Oh
I have to go.
Goodbye.. :<
Thanks for all the help today
 
Ok... Be sure to finish that integral
 
So long and thanks for all the fish
 
I'm off to play some Torchlight II. I need to blow off some steam by killing hordes of monsters
 
9:26 PM
@DavidWheeler hello
 
9:38 PM
A wild meer appeared! What will you do?
 
about the same as I was before
 
David uses: charm!
 
@DavidWheeler It was super effective!
 
Huy
@MikeMiller: Basic geometry question, if I enlarge a straight pyramid by some factor $\lambda$, does the volume correspondingly increase by $\lambda^3$?
 
9:51 PM
Hello
 
Yes, @Huy. This is linear algebra.
 
Can we post problems here?
 
Huy
@MikeMiller: I thought so. Can you give me a pointer? I've been thinking about it for a bit but don't quite see it.
 
Given a matrix $A$ and an "object" $S \subset \Bbb R^n$ (a closed $n$-manifold should be good enough), then $\text{vol}(AS) = \det(A)\text{vol}(S)$. The idea is to approximate $S$ by cubes and then apply $A$ to this approximation; by taking a limit as the cubes get smaller we obtain the Riemann integral.
 
Huy
@MikeMiller: I need a more basic argument without the determinant / integral.
 
9:54 PM
Bah humbug.
 
Huy
(I know what you just said though)
 
The answer is yes, I don't know how one of the ancient Greeks would prove it.
 
Ark
Apparently no one can solve this problem. Would anyone like to give it a shot? math.stackexchange.com/questions/1206376/…
 
Huy
I have an exercise for high schoolers where a certain pyramid and its volume is given and I need to find the pyramid with a fraction of the other pyramid's volume. And I don't quite know how to do it with the tools of a high schooler.
(the height would suffice)
 
I don't understand. You're scaling the pyramid by $\lambda$. Is it not obvious that the height also scales by $\lambda$?
 
Huy
9:57 PM
@MikeMiller: It is somewhat obvious to me but I doubt it will be to them which is why I'll need an argument they understand.
 
I mean, you're going to have to trade off "too wordy" for "not rigorous enough". You could rephrase my previous sentence as "Compose the embedding $\Delta^3 \hookrightarrow \Bbb R^3$ with the projection $\Bbb R^3 \to \Bbb R$. That's the height map. Fitting in $\lambda I: \Bbb R^3 \to \Bbb R^3$ in the middle, we obtain the map $\lambda h$, and thus the height scales by $\lambda$."
 
Huy
lol
Now you're just making fun of me.
 
No. I just think that if you have a scale to balance here. There are two ends: the above, which is a proof, and the previous thing, which is "well, it's obvious".
I dunno what the best balance is. You'd know better than me.
 
Ark
The function $f:\mathbb{Q}\rightarrow \{0,1\}$ is defined as $f(a)=0$ if there do not exist integers $x$, $y$ and $z$ for which $a$ is expressible in the form $$\frac{xy+yz+zx}{x^2+y^2+z^2},$$ and $f(a)=1$ otherwise. Show that $$f(a)=f(0.5-a).$$
 
Huy
I have a proof from my years in high school where I used the theorem on intersecting lines but it's not so nice.
I thought there would be an obvious proof using some ratios.
 
Ark
10:04 PM
@Huy What is a straight pyramid. I only know high school level math so any solution I can come up with is what you're looking for.
 
If $V = \dfrac{lwh}{3}$ and all of those factors scale by $\lambda$ (that is, we have simiiar pyramids) then $V_1 = \dfrac{\lambda l\lambda w\lambda h}{3} = \lambda^3 V$
 
Huy
@Ark: The axis of the pyramid is perpendicular to its base area.
 
He seems unconvinced that those factors scale by $\lambda$, @David.
 
Ark
@Huy Thanks, I'll give it a go
 
Huy
@MikeMiller: I'm not unconvinced but I doubt any of my students would be convinced.
 
10:05 PM
Hi guys, I'm looking for a name of a professor that I'm trying to remember
 
I know, @Huy. Bad phrasing on my part.
 
It's a Japanese guy who, recently (at least post-2000) developed an entirely new (branch of?) Teichmuller theory
He says that it can be applied to solve a very famous problem---forget the name :(
But he refuses to really travel to spread his ideas
Someone must know who I'm talking about
 
Ark
@Huy Wait by pyramid do you mean square-based?
 
Huy
@MikeMiller: Something else entirely: Do you know which contributions of Nash lead to his Abel prize? Would be fun to tell my students in their Game theory course tomorrow.
@Ark: No, it's actually triangle based, but should work for any base.
 
You're thinking of Shinichi Mochizuki. It's related to part of Grothendieck's anabelian geometry program. I think it's wildly unfair to say that he refuses to really travel to spread his ideas; he prefers to stay in his home country, yes, but he is working with mathematicians there to help them better understand it.
@Huy: Nothing related to game theory.
 
10:07 PM
Nash's paper that established the concept of "Nash equilibrium" is VERY funnily short :D
(less than a page)
@MikeMiller Yes, awesome, thanks :)
 
Huy
@MikeMiller: I know, some connection between PDEs and analytic geometry or so?
 
I'll find the excerpt the AMS sent me.
Oops; my game theory claim was wrong.
The Norwegian Academy of Sciences and Letters will award the 2015 Abel Prize to John F. Nash Jr. (above left), Princeton University, and Louis Nirenberg (above right), Courant Institute, New York University, "for striking and seminal contributions to the theory of nonlinear partial differential equations and its applications to geometric analysis."

Although Nash and Nirenberg did not formally collaborate on any papers, they influenced each other greatly. The Abel committee notes that: "Their breakthroughs have developed into versatile and robust techniques that have become essential tools
The biography on the Abel Prize website should help with specifics.
 
Ark
@Huy I'm definitely missing something. For convenience let x denote lambda. If you let the volume of the initial pyramid be V=bh/3, where b=base area and h=length of altitude from vertex opposite said base to the same base, then scaling everything by x means the new base area is x^2b and new height is xh. So new volume is V_1=(x^2b)(xh)/3=x^3(bh/3)=x^3V.
 
Huy
@MikeMiller: Sorry for my ignorance but what exactly is geometric analysis about?
 
Hell if I know. The catchphrase I've heard is in two directions: using geometric ideas to study PDE (someone told me there was recent progress on a certain hyperbolic equation arising from physics that came from studying foliations?), and using PDE to study geometry (gauge theory, i.e. Yang-Mills and Seiberg-Witten theory, goes here).
 
Huy
10:15 PM
Wikipedia claims Geometric analysis is a mathematical discipline at the interface of differential geometry and differential equations.
I see.
Doesn't sound like anything I'd be good at.
:-(
 
Boo to that. Don't put yourself down.
 
Huy
Nah, I can safely say Differential Geometry is not my strength.
 
At present, the starboard has something with 0! stars, 1! stars, 2! stars, and 3! stars, and some random dude with 1 star at the bottom.
 
Huy
And PDEs, I don't even want to talk about.
 
I at least need to try to be good at it. I'm morphing into a low-dimensional topologist, and gauge theory is a key tool in the theory of 4-manifolds.
 
Huy
10:19 PM
Good luck.
 
Danke schoen.
 
Huy
Bitteschön.
 
Thanks for the stars
 
@MikeMiller This always "sounds" hilarious to me as a Dutchman, since "schoen" is an actual Dutch word: It means "shoe" :) Of course, I'm aware of the German umlaut alternative spelling thing
 
Pronounced the same?
 
10:28 PM
No, totally different
You can try typing "shoe" into google translate and using the audio option
 
I think it's actually incorrect to write schoen instead of putting an umlaut above the o. I just don't have that on my keyboard.
 
The pronunciation is pretty okay :)
@MikeMiller No, it's pretty widespread in Germany as well, don't worry about it
Also, it's easy to make it possible to switch between e.g. US and GER keyboards (I do it all the time, now that I live in Germany) using some settings on your computer
 
I use a chromebook, which is very lightweight; there are probaby ways to do it if I download extensions.
 
Why did you choose to go to school in Germany @Danu?
 
I would go to school in Germany if I could
 
10:33 PM
@infinitesimalsimplicio My school is significantly better than what I could've gotten in Holland, I think.
Also, I liked the prospect of an internationally oriented degree in mathematical physics.
 
someone for complex analysis ? :)
 
Huy
10:53 PM
@MikeMiller: A Chromebook? Really? I'm looking at different possibilities to replace my current laptop and I haven't heard good things about the Chromebook. Can you recommend it?
 
Yes, it's the best purchase I've made in a while. The only downsides are that it can't do high-power things (playing games, eg), and that to do most anything you need an internet connection, and that some native windows apps don't have chromebook counterparts. (The only ones of this last thing that has affected me is photoshop and gp-pari.)
 
Using apps on a computer is just the worst prospect for me.
 
You use apps on your computer every day...
 
Huy
@MikeMiller: Can you install MikTeX or alike?
 
Not as far as I know. I use online equivalents; one that turns google docs into a latex editor and compiler, and overleaf for compiling complicated documents.
This has not affected me.
 
Huy
10:58 PM
@MikeMiller: So no offline compiling.
 
No. If you're regularly in places without an internet connection it might be trouble.
 
Huy
Dropbox integration?
 
This is, by far, the biggest problem with a Chromebook. I have no problem with it.
Look at the google apps store. I bet there's a dropbox app.
 
Huy
How are they integrated then? Can you save the stuff locally or just view it in the cloud?
 

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