# Solving the wave PDE using the Fourier method

I am trying to solve the wave PDE using the Fourier method and I need to animate the plots of the partial sum of order 45 -- U45(x,t) with initial conditions U45(x,t0), {t0, 0, 12, .2}.

I am confused: should I define the function f[x, t, k_] or should I do it as f[x, k_] and later access the 't'.

a[k_] :=
Piecewise[{
{2/(k*π) Integrate[Cos[(k*π*x)/6] (2 - x) Sin[π*x]^2, {x, 1, 3}], k != 0},
{1/6 Integrate[(2 - x) Sin[π*x]^2, {x, 1, 3}], k == 0}}]

f[x, t, k_] :=
a[0]*t + Sum[a[i] Cos[(i*π*x)/6] Sin[(i*π*t)/6], {i, k}]

sol = f[x, t, 45];


Anyway, the above is the piece of code I've written so far. Tried a dozen different Plot3D combinations, but the error I receive tells me I can not replace t. I am open to everything tutorials, papers, tips, plain solutions, everything.

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Please consider using an ASCII equivalent for your name if such a thing exists to make it easier for others to interact with you here. According to Google Translate that is Nikola Dimitrov. Is that acceptable to you? –  Mr.Wizard Jan 21 '13 at 18:12
Yes, I am sorry for the inconvenience. –  Sektor Jan 21 '13 at 18:49
No problem, and thank you. –  Mr.Wizard Jan 21 '13 at 19:57

In your definition of f you should have f[x_,t_,k_], x_ means "Match anything and let it be referenced by x" you might want to read about patterns in general in the documentation.

Clear[a, f];
a[k_] := Piecewise[{
{0, k == 12},
{(1728 (k (-144 + k^2) \[Pi] Cos[(k \[Pi])/6]
- 18 (-48 + k^2) Sin[(k \[Pi])/6]) Sin[(k \[Pi])/3])
/(k^3 (-144 + k^2)^2 \[Pi]^3), k != 0},
{0, k == 0}}]

f[x_, t_, k_] := a[0]*t + Sum[a[i] Cos[(i*\[Pi]*x)/6] Sin[(i*\[Pi]*t)/6], {i, k}]

Plot3D[f[x, t, 45], {x, -1, 1}, {t, 0, 2}]


I replaced the Integrate in your definition of a to not have to calculate it over and over again, thereby speeding up the plot.

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I heart NDSolve –  drN Jan 20 '13 at 20:31
Thank you for your explanation and reference ! :) –  Sektor Jan 20 '13 at 20:34
+1 for adding the speed hint. To be more pedagogical, it's worth pointing out that the definition of a[k_] was OK in principle, except for the little difference between := and =. In this case, all that was needed is to use the latter (Set) to get the same speedup that you achieved by copying and pasting into the definition of a[k_]. –  Jens Jan 20 '13 at 20:35
@Jens Yea, problem was that for k=12 the integral should be zero but the resulting expression was undefined (limit was still zero though) so had to do it manually anyway –  ssch Jan 20 '13 at 20:40
Right, that was probably an oversight in the original Piecewise... –  Jens Jan 20 '13 at 20:42