Abstract. A spectral mapping theorem is proved that resolves a key problem in applying invariant manifold theorems to nonlinear Schrödinger type equations. The theorem is applied to the operator that arises as the linearization of the equation around a standing wave solution. We cast the problem in the context of space-dependent nonlinearities that arise in optical waveguide problems. The result is, however, more generally applicable including to equations in higher dimensions and even systems. The consequence is that stable, unstable, and center manifolds exist in the neighborhood of a (stable or unstable) standing wave, such as a waveguide mode, under simple and commonly verifiable spectral conditions.
Main ResultsThe local behavior near some distinguished solution, such as a steady state, of an evolution equation, can be determined through a decomposition into invariant manifolds, that is, stable, unstable and center manifolds. These (locally invariant) manifolds are characterized by decay estimates. While the flows on the stable and unstable manifolds are determined by exponential decay in forward and backward time respectively, that on the center manifold is ambiguous. Nevertheless, a determination of the flow on the center manifold can lead to a complete characterization of the local flow and thus this decomposition, when possible, leads to a reduction of this problem to one of identifying the flow on the center manifold.This strategy has a long history for studying the local behavior near a critical point of an ordinary differential equation, or a fixed point of a map, and it has gained momentum in the last few decades in the context of nonlinear wave solutions of evolutionary partial differential equations. Extending the ideas to partial differential equations has, however, introduced a number of new issues. In infinite dimensions, the relation between the linearization and the full nonlinear equations is more delicate. This issue, however, turns out to be not so difficult for the invariant manifold decomposition and has largely been resolved, see, for instance, [2], [3]. A more subtle issue arises at the linear level. All of the known proofs for the existence of invariant manifolds are based upon the use of the group (or semigroup) generated by the linearization. The hypotheses of the relevant theorems are then formulated in terms of estimates on the appropriate projections of these groups onto stable, unstable and center subspaces. These amount to spectral estimates that come directly from a determination of the spectrum of the group. However, in any actual problem, the information available will, at best, be of the spectrum of the infinitesimal generator, that is, the linearized equation