A weighted least action principle for dispersive waves

We extend the least action principle to continuum systems. The data for the new principle consist of the intensity of the wave (or rather the wave action) at two instances of time. We define an appropriate Lagrangian, and formulate a variational problem in terms of it. The critical points of the functional are used to determine the wave's phase. The theory is applicable to the semiclassical limit of a large class of dispersive wave equations. Associating the wave equation with a Liouville equation for the Wigner distribution function, we are able to extend the theory to include singular solutions such as caustics.