PDE: Nontrivial solution to the wave equation

[RJLiberator]

Homework Statement

Consider the wave equation:

<br /> u_{tt} - c^2u_{xx} = f(x,t),<br /> \hspace{1cm}<br /> for <br /> \hspace{1cm}<br /> 0 &lt; x &lt; l \\<br /> u(0,t) = 0 = u(l,t) \\<br /> u(x,0) = g(x), u_t(x,0) = f(x) \\<br />
Find a nontrivial solution.

Homework Equations

The Attempt at a Solution

Here's what I did, but I have little understanding of it other than I know that I am using boundary conditions and some previous material to get here:

We form a series solution:

<br /> u(x,t) = \sum_{n=1}^{\infty} u_n(x,t) = \sum_{n=1}^{\infty} sin{\frac{nπx} {l}}u_n(t) \\<br /> g(x) = \sum_{n=1}^{\infty}b_nsin{\frac{nπx}{l}} \\<br /> f(x) = \sum_{n=1}^{\infty} \frac{cnπb_n*}{l} sin{\frac{nπx}{l}} \\<br /> \\<br /> b_n = \frac{2}{l} \int_{0}^{l} g(x)sin{\frac{nπx}{l}}dx \\<br /> b_n* = \frac{2}{l} \int_{0}^{l} f(x)sin{\frac{nπx}{l}}dx \\<br />

All together:

<br /> u(x,t) = \sum_{n=1}^{\infty} sin{\frac{nπx} {l}} [b_n cos(\frac{cnπt}{l}) + b_n*sin(\frac{cnπt}{l})]<br />

From my understanding, inputting b_n and b_n* would give us the nontrivial solution. Yes?

 

[Orodruin]

Yes, although you should consider using the notation $b_n^*$ instead, it would be less confusing.

If you would like a better understanding of what is going on, I suggest reading up on function spaces and orthogonal functions.

 

[RJLiberator]

Thank you kindly for the confirmation and suggestion.