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Heat equation problem

  1. Mar 13, 2015 #1


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    1. The problem statement, all variables and given/known data
    Find the solution ##u(x, t)## to the semi-infinite interval problem

    $$ u_t = u_{xx} - 4u, \hspace{2 mm} 0 < x < \infty, \hspace{2 mm} t>0\\
    u_x(0,t) = -1, \hspace{2 mm} t>0\\
    \lim_{x \to \infty}u(x,t) = 0, \hspace{2 mm} t>0\\
    u(x,0) = e^{-x}, \hspace{2 mm} 0 \leq x < \infty.$$
    2. Relevant equations
    fourier cosine series, which I shall denote ##C(u)## when in variable ##x## operating over some function ##u##.
    3. The attempt at a solution
    after taking fourier cosine series of the governing PDE i arrive at $$
    \frac{\partial}{\partial t}C(u) = \frac{2}{\pi} - \omega^2 C(u) -4C(u) \implies\\
    C(u) = \frac{2}{\pi(\omega^2 + 4)} + C_1e^{-(\omega^2 +4)t}$$
    Notice ##C(u(x,0)) = C(e^{-x}) = \frac{2}{\pi(1+\omega^2)}##. This implies ##C_1 = \frac{2}{\pi(1+\omega^2)} - \frac{2}{\pi(\omega^2 + 4)} ##. Thus we arrive at $$
    C(u) = \frac{2}{\pi(\omega^2 + 4)} + \frac{2}{\pi}\left[ \frac{1}{(1+\omega^2)} - \frac{1}{(\omega^2 + 4)} \right] e^{-(\omega^2 +4)t}$$
    I have skipped some in-between steps, but I think the idea is here. My question is, how do I undo this fourier cosine transform, if I have arrived to a correct solution?

    Thanks so much!

  2. jcsd
  3. Mar 14, 2015 #2
    Use the inverse fourier cosine transform
  4. Mar 14, 2015 #3


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    Cool, that's what I did! Just wanted to make sure my work was correct. Thanks!
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