Wave equation

$$\frac{\partial \dot s}{\partial t} = c^2 ( \frac{\partial^2 s}{\... ...rac{\partial^2 s}{\partial {x_2}^2} + \frac{\partial^2 s}{\partial {x_3}^2} )$$

$$\frac{\partial s}{\partial t} = \dot s$$

The primary unknowns are the wave_scalar $s$ and its first time derivative wave_fscalar $\dot s$ ( as TOCHNOG only solves first order in time equations, the first time derivative of $s$ also becomes primary unknown in order to turn this second order in time equation into a set of first order in time equations). Further notation: $x$ space coordinate, $t$ time and $c$ speed of sound.