Nondispersive waves: interpretation and causality

It is proved that the focus wave modes are only a subclass of a larger class of nondispersive waves. Then these nondispersive waves are interpreted in the framework of Goursat's theory of boundary value problems where data are given on the characteristics of the wave equation. Finally, from the remark that causality requires only that no effect precede the cause, it is proved that there is no casual dilemma for the nondispersive waves.<
>