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Fourier Method for $1$D Wave equation-Visualization"

Neumann-Neumann

For boundary value problem $$\begin{aligned} &u_{tt}-c^2u_{xx}=0,\\ &u_x|_{x=0}=u|_{x=l}=0 \end{aligned}$$ there are simple solutions in the form $$u(x,t)= \cos(\frac{n\pi ct}{l})\cos(\frac{n\pi x}{l})$$

and general solutionsin the form $$u(x,t)= \sum_{n=1}^{\infty}\bigl[A_n\cos(\frac{n\pi ct}{l})+B_n\sin (\frac{n\pi ct}{l})\bigr]\cos(\frac{n\pi x}{l})$$