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Homework Help: Harmonic function

  1. Dec 4, 2005 #1
    I am to find a function U, harmonic on the disk [tex] x^2 + y^2 < 6 [/tex] and satisfying
    [tex] u(x, y) = y + y^2 [/tex] on the disk's boundary. I am not sure where to start. Hints, help, anything?
  2. jcsd
  3. Dec 4, 2005 #2

    matt grime

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    Use the integral formula.
  4. Dec 4, 2005 #3


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    Gold Member

    I would think Cauchy's integral formual would be useful here: you have the value of a function on a boudry and want the value in the interior.
  5. Dec 4, 2005 #4
    You are trying to solve the Laplace equation on a disk. Try seperation of variables, then break it down to 2 ODE's. Here is a start for you..

    You will probably need to solve the PDE in polar coordinates.

    - harsh
  6. Dec 4, 2005 #5
    Then is [tex] u(\sqrt{6}, \theta) = \sqrt{6} \sin(\theta) + 6\sin^2(\theta) [/tex] a boundary condition?
  7. Dec 4, 2005 #6
    Looks right. Make sure you solve the correct PDE, the laplacian in r,theta is not as simple as U_rr and U_theta*theta

    - harsh
  8. Dec 5, 2005 #7
    I know. In an earlier problem I had to compute the laplacian in polar. Oh and one more thing, is there anything else I need to know about [tex] \theta [/tex]? Other than [tex] 0 < \theta < 2\pi [/tex] ?
  9. Dec 5, 2005 #8
    The theta condition that you are going to use, I believe, will be that theta is 2pi periodic.

    - harsh
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