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### Messages - Jinchao Lin

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1
##### Final Exam / Re: problem 5
« on: December 20, 2012, 02:50:21 PM »
Hopeful solution for problem 5

2
##### Home Assignment 9 / Re: Problem 1
« on: December 06, 2012, 02:28:07 PM »
sorry the original image is vertical, I don't know why after upload it rotate to the left

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##### Home Assignment 9 / Re: Problem 1
« on: December 05, 2012, 09:31:22 PM »
http://www.math.toronto.edu/courses/apm346h1/20129/HA9.html#problem-9.1

WTH: Can you rotate before posting?!!!! - V.I.

Hopeful solution for problem 1:

4
##### Home Assignment 9 / Re: Problem 2
« on: December 05, 2012, 09:31:09 PM »
Hopeful solution for Question 2:

5
##### Home Assignment 9 / Problem 1
« on: December 05, 2012, 08:37:46 PM »
Solution post after 21:30

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##### Home Assignment 8 / Re: Problem1
« on: November 28, 2012, 10:37:51 PM »
part(d)

Jinchao--are we supposed to rotate a monitor? Sure one can download and rotate an image but methinks it would be simpler for you to rotate it before posting. Also, why all these weird colours--it is a home work in math, not in visual arts V.I.

7
##### Home Assignment 8 / question a-c
« on: November 28, 2012, 10:33:54 PM »
Question (a)-(c)

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##### Term Test 2 / Re: TT2--Problem 2
« on: November 15, 2012, 10:30:12 PM »
Solutions for part (a) and part(b)

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##### Term Test 2 / Re: TT2--Problem 1
« on: November 15, 2012, 10:30:03 PM »
Solution for part(a)

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##### Term Test 2 / Re: Scope of Term Test 2
« on: November 15, 2012, 01:45:42 AM »
Dear professor, I am wondering that will you provide basic Fourier transform table in the exam?

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##### Home Assignment 5 / Re: Problem 1
« on: October 31, 2012, 02:07:42 AM »
in part a, it seems that if we want the expansion to be real domain, the only way we can do is first manipulate in complex domain by the exponential expansion, then use Euler formula to convert into real domain? Because there is no integral formula for $\int sin(x)exp(x)$

12
##### Term Test 1 / Re: TT1 = Problem 5
« on: October 16, 2012, 09:20:20 PM »
We are only asked to draw the graph but not the functional form, don't we?

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##### Term Test 1 / Re: TT1 = Problem 4
« on: October 16, 2012, 09:11:35 PM »
From part (a) we have
$u(x,t) = \frac{1}{\sqrt{4\pi kt}} \int_{-10}^{10} e^{ \frac{-(x-y)^2}{4kt} } f(y) dy$

Rewrite the formula as:
$u(x,t) = \frac{1}{\sqrt{2\pi } \sqrt{2kt}} \int_{-10}^{10} e^{ \frac{-(x-y)^2}{2 \sqrt{2kt}^2 } } f(y) dy$

We see this is a normal density of random variable x centered at y with standard error $\sqrt{2kt}$ .  When $t \to 0$, the standard error approach to 0 as well. The random variable x approaches to a deterministic form. So $lim_{t \to 0} u(x,t) = f(y)$.

I think the argument can be written more accurate if we take the functional form of f(x) in the whole real line. But then we are not directly prove the result based on the formula of u(x,t) we get from part (a).

14
##### Term Test 1 / Re: TT1 = Problem 1
« on: October 16, 2012, 08:52:58 PM »
Subqueston (d):

$\frac{dt}{1} = \frac{dx}{x^2}$
$t = -x^{-1}+c$
so the general solution is $u(t,x)=f(t+x^{-1})$.
$u(0,x)=f(x^{-1})=g(x)$
$f(y)=g(y^{-1})$
Since $y=x^{-1}$, so when $x>0$ we have $y>0$ as well.
$u(t,x)=f(t+x^{-1})$
We need $t+x^{-1}>0$
Since $x>0$,
therefore $tx+1>0$
so the domain be defined is \$\{(t,x) | tx>-1 \}.

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