“…Concerning nonlinear wave interactions, there exist several results (mostly three-wave mixing) in the context of strictly hyperbolic systems, see, e.g., [18,19,22,25], and in the case of microscopically discrete dynamical systems, cf. [11,12] and the references given therein. For spatially periodic systems we again refer to [27] and to [1], where a similar system of coupled-mode equations is derived from Maxwell's equations in photonic crystals.…”