### Author Topic: TT1 = Problem 1  (Read 10805 times)

#### Victor Ivrii

• Elder Member
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##### TT1 = Problem 1
« on: October 16, 2012, 06:26:37 PM »
Consider the first order equation:

u_t + x u_x = 0.
\label{eq-1}

• (a) Find the characteristic curves and sketch them in the $(x,t)$ plane.
• (b) Write the general solution.
• (c) Solve  equation (\ref{eq-1})  with the initial condition $u(x,0)= \cos(2x)$.
Explain why the solution is fully  determined by the initial condition.
• (d) bonus Describe domain in which solution of

u_t + x^2 u_x = 0, \qquad x>0
\label{eq-2}

is fully determined by the initial condition $u(x,0)=g(x)$ ($x>0$)?

#### Djirar

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##### Re: TT1 = Problem 1
« Reply #1 on: October 16, 2012, 08:25:30 PM »
My solution. Please check there might be mistakes. Sorry for quality of scan and handwriting.
« Last Edit: October 17, 2012, 12:21:25 AM by Djirar »

#### Aida Razi

• Sr. Member
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##### Re: TT1 = Problem 1
« Reply #2 on: October 16, 2012, 08:25:40 PM »

#### Aida Razi

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• Karma: 15
##### Re: TT1 = Problem 1
« Reply #3 on: October 16, 2012, 08:27:12 PM »
My solution. Please check there might be mistakes. Sorry for quality of print and hand writing.

Djirar, I posted my solution 10 seconds after you

#### Djirar

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##### Re: TT1 = Problem 1
« Reply #4 on: October 16, 2012, 08:32:04 PM »
My solution. Please check there might be mistakes. Sorry for quality of print and hand writing.

Djirar, I posted my solution 10 seconds after you

I was scribbling my solutions as fast as I could

Edit: I forgot to write down the axes to part a.  The vertical is x-axis and the horizontal is y-axis. I hope I didn't forget this on the test
« Last Edit: October 16, 2012, 08:38:51 PM by Djirar »

#### Zarak Mahmud

• Sr. Member
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##### Re: TT1 = Problem 1
« Reply #5 on: October 16, 2012, 08:41:46 PM »
My solution. Please check there might be mistakes. Sorry for quality of print and hand writing.

Djirar, I posted my solution 10 seconds after you

I was scribbling my solutions as fast as I could

Edit: I forgot to write down the axes to part a.  The vertical is x-axis and the horizontal is y-axis. I hope I didn't forget this on the test

Do you mean the $t$ axis?

#### Qitan Cui

• Jr. Member
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##### Re: TT1 = Problem 1
« Reply #6 on: October 16, 2012, 08:42:02 PM »
I have a different solution for bonus

#### Djirar

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##### Re: TT1 = Problem 1
« Reply #7 on: October 16, 2012, 08:46:51 PM »
I think Qitan Cui is correct. In my haste I must have messed up the integration of the bonus part.

#### Jinchao Lin

• Jr. Member
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##### Re: TT1 = Problem 1
« Reply #8 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 $\{(t,x) | tx>-1 \}. #### Djirar • Full Member • Posts: 24 • Karma: 8 ##### Re: TT1 = Problem 1 « Reply #9 on: October 16, 2012, 08:54:19 PM » My solution. Please check there might be mistakes. Sorry for quality of print and hand writing. Djirar, I posted my solution 10 seconds after you I was scribbling my solutions as fast as I could Edit: I forgot to write down the axes to part a. The vertical is x-axis and the horizontal is y-axis. I hope I didn't forget this on the test Do you mean the$t$axis? Yes$t$not$y$. Thank you. #### Ian Kivlichan • Sr. Member • Posts: 51 • Karma: 17 ##### Re: TT1 = Problem 1 « Reply #10 on: October 16, 2012, 09:24:06 PM » Subqueston (d):$ \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 \}.
I think Jinchao has the most correct solution.

Qitan, is it possible to only have the one discontinuity in your solution - won't your characteristic curves be "blocked" by the discontinuity at tx=-1, and not able to go any further?

#### Victor Ivrii

Djirar solved (a), (c) correctly and made a mistake in (b), Aida solved (a)-(c) correctly. If we consider $x$ which could be negative and positive $\log |x|-t$ is not good enough  as it is the same on two disjoint curves, but $xe^{-t}$ works. If initial function was not even then Djirar would not be able honestly to satisfy it from the "general" solution.
In two attached pictures one can see vector fields and curves for (1) and (2) respectively; one can see that certain characteristics never intersect initial curve. Condition $x>0$ was given only for a sake of simplicity.