### Author Topic: Laplace Fourier Transform S5.3.P Q1  (Read 193 times)

#### Sebastian Lech

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##### Laplace Fourier Transform S5.3.P Q1
« on: March 12, 2019, 01:00:45 PM »
I was wondering if anyone could let me know how to move forward on problem one:

Consider Dirichlet problem:
$$$$u_{xx}+u_{yy}=0$$, -\infty<x<\infty, y>0$$
$$$$u|_{y=0}=f(x)$$$$
We need to make a Fourier Transform by x and leave the solution in the form of a Fourier Integral.
What I did first was make the Fourier transform:
$$$$\hat{u}_{yy}-\xi^2\hat{u}=0$$$$
$$$$\hat{u}|_{y=0}=\hat{f}(\xi)$$$$

Which has general solution:

$$$$\hat{u}(\xi, y)=A(\xi)e^{-|\xi|y}+B(\xi)e^{|\xi|y}$$$$
and using equation (4):
$$$$\hat{u}(\xi,0)=A(\xi)+B(\xi)=\hat{f}(\xi)$$$$
Which I am now stuck on, how do we solve for $A, B$ ?

#### Victor Ivrii

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##### Re: Laplace Fourier Transform S5.3.P Q1
« Reply #1 on: March 12, 2019, 06:25:23 PM »
There was an explanation why one of the solutions in half-plane should be rejected (does not satisfy condition at infinity)

#### Zengyue Lin

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##### Re: Laplace Fourier Transform S5.3.P Q1
« Reply #2 on: March 16, 2019, 10:22:00 PM »
So for this question why we don’t use cos and sin for the general solution?

#### MikeMorris

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##### Re: Laplace Fourier Transform S5.3.P Q1
« Reply #3 on: March 17, 2019, 12:27:26 PM »
What condition is given at infinity? I don't see any condition given in the question for the behaviour of $u$ at infinity.

#### Zengyue Lin

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##### Re: Laplace Fourier Transform S5.3.P Q1
« Reply #4 on: March 17, 2019, 01:59:15 PM »
I think we need to make assumption that the Fourier transformation u will be 0 as y goes to infinity,that’s what my TA did in tutorial

#### Victor Ivrii

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##### Re: Laplace Fourier Transform S5.3.P Q1
« Reply #5 on: March 19, 2019, 02:14:40 PM »
I think we need to make assumption that the Fourier transformation u will be 0 as y goes to infinity,that’s what my TA did in tutorial
Or, at least, does not grow exponentially as $|k|\to \infty$

#### shuxian

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##### Re: Laplace Fourier Transform S5.3.P Q1
« Reply #6 on: March 31, 2019, 05:19:13 PM »
Here is my solution for solving A.