Zakharov–Schulman system

In mathematics, the Zakharov–Schulman system is a system of nonlinear partial differential equations introduced in to describe the interactions of small amplitude, high frequency waves with acoustic waves. The equations are
 * $$i\partial_t^{} u + L_1u = \phi u$$
 * $$L_2 \phi = L_3( | u |^2)$$

where L1, L2, and L3, are constant coefficient differential operators.