How do i find domain of influnce?

[gtfitzpatrick]

Homework Statement

determine the char. curve and gen. sol. for x^{3} <br /> \frac{\partial{u}}{\partial{x}}<br /> - <br /> \frac{\partial{u}}{\partial{y}}<br /> = 0

Homework Equations

find sol and domain of influence when u(x,0) = \frac{1}{1+x^2}

show sol is not defined when y> \frac{1}{2x^2}

The Attempt at a Solution

so \frac{\partial{y}}{\partial{x}} = \frac{-1}{x^3}

and \frac{\partial{u}}{\partial{x}} = 0

which gives u(x,y) = F( \frac{1}{2x^2} - y) is the gen solution right?



then sol. at u(x,0) = \frac{1}{1+x^2}

\frac{1}{1+x^2} = F( \frac{1}{2x^2} - y)
= F( \frac{1}{2x^2})

which gives x = +/- 1 am i right in this? and how do i find domain of influnce?

 

[hunt_mat]

Much better way to to set:
<br /> \dot{x}=x^{3},\quad\dot{y}=-1\quad\dot{u}=0<br />
with the initial condition:
<br /> x(0)=r\quad u(0)=\frac{1}{1+r^{2}}<br />

 

[gtfitzpatrick]

i don't understand what you are doing?

 

[gtfitzpatrick]

<br /> <br /> \dot{x}=x^{3},\quad\dot{y}=-1\quad\dot{u}=0<br /> <br />

Is that not what i did, then solved <br /> \frac{\dot{y}}{\dot{x}} = \frac{-1}{x^3} <br />

but then...

 

[hunt_mat]

From my equations you have:
<br /> -\frac{1}{2x^{2}}=s+r,y=-s,u=\frac{1}{1+r^{2}}<br />
so substitute to get u=u(x,y)

 

[gtfitzpatrick]

im getting


<br /> <br /> u(x,y)=\frac{1}{1+(y - \frac{1}{2x^2})^{2}}<br /> <br />

but I am still not sure about this, but this is the particular solution right?

 

[hunt_mat]

This is the correct solution, well done.

what happens in first order PDE is that your initial condition is propagated along the characteristics.

 

[gtfitzpatrick]

Thanks hunt_mat for your help, and patiance!

Im still not sure of a few things,
so the characteristics are <br /> <br /> \frac{dx}{dt} = \dot{x}=x^{3}, \frac{dy}{dt}=\dot{y}=-1, \frac{du}{dt}=\dot{u}=0<br /> <br />

the general solution is
<br /> u(x,y)=y - \frac{1}{2x^2}<br />

and the particular solution is

<br /> u(x,y)=\frac{1}{1+(y - \frac{1}{2x^2})^{2}}<br />

but I am not sure to about the domain of influence or how to
show sol is not defined when y> <br /> \frac{1}{2x^2} <br />

 

[gtfitzpatrick]

also in the book it gives the solution as u = \frac{1-2x^2y}{1+x^2-2x^2y} which try as i might i cannot seem to make mine fit

 

[hunt_mat]

Hi, I wrote some notes on the method of characteristics which I have included in this post. It is possible that the book you have is giving you trhe wrong answer, this happens sometimes. I just checked the answer with mathematica and you have the right answer.

What d you understand about domain of dependance?

 

[gtfitzpatrick]

thanks Mat for the notes, there is a lot of stuff in there that our lecturer hasn't covered yet. I didnt see anything in there about the domain of dependance, and i have nothing in my notes on it either and the more i try to find about it on www the more confused i get...

 

[gtfitzpatrick]

is the dom. of influ. just?

<br /> <br /> u(x+t,y+t)=\frac{1}{1+((y+t) - \frac{1}{2(x+t)^2})^{2}}<br /> <br />