\magnification=\magstep1

$$
- \thinspace {{n^2 \pi^2}\over{L_x^2}} \thinspace \psi  \ - 
   {{n^2 \pi^2}\over{L_y^2}} \thinspace \psi  \ - 
     {{n^2 \pi^2}\over{L_z^2}} \thinspace \psi   \quad = \quad 
      - \thinspace {{1}\over{c^2}} \thinspace \omega^2 \thinspace \psi
$$


$$
{{\partial^2 \psi}\over{\partial x^2}} \ + \ 
  {{\partial^2 \psi}\over{\partial y^2}} \ + \ 
  {{\partial^2 \psi}\over{\partial z^2}} \ + \ 
     \quad = \quad 
         {{1}\over{c^2}} \thinspace 
   {{\partial^2 \psi}\over{\partial t^2}}
$$


$$
\psi(r, \theta, \phi, t)
$$

$$
{{\partial^2 \psi}\over{\partial r^2}} \ + \ 
   {{2}\over{r}} \thinspace {{\partial \psi}\over{\partial r}}  \quad = \quad 
         {{1}\over{v^2}} \thinspace 
   {{\partial^2 \psi}\over{\partial t^2}}
$$

$$
\psi(r) \quad = \quad A \thinspace \sin(k r - \omega t)
$$

$$
\psi(r) \quad = \quad A  \thinspace \left( {{1}\over{r}} \right) \thinspace \sin(k r - \omega t)
$$

$$
{{1}\over{r^2}} \thinspace {{\partial}\over{\partial r}} 
     \left( r^2 {{\partial \psi}\over{\partial r}} \right)
   \ + \   
  {{1}\over{r^2 \sin^2(\theta)}} {{\partial}\over{\partial \theta}} 
       \thinspace 
     \left( \sin(\theta) {{\partial \psi}\over{\partial \theta}} \right)
   \ + \   
  {{1}\over{r^2 \sin^2(\theta)}} 
         \thinspace {{\partial^2 \psi}\over{\partial \phi^2}} 
     \quad = \quad
    {{1}\over{v^2}} {{\partial^2 \psi}\over{\partial t^2}}
$$
      






\bye
