H. Mohamad and M. Oliver,
A direct construction of a slow manifold for a semilinear wave equation of Klein-Gordon type,
J. Differential Equations 267 (2019), 1-14, doi:10.1016/j.jde.2019.01.001.

Abstract:

We prove that a semilinear wave equation, related to the semilinear Klein--Gordon equation in the non-relativistic limit, albeit with a nonlinearity that is Fréchet-differentiable over the complex numbers, possesses an almost invariant manifold in phase space that generalizes the slow manifold which is known to exist for finite-dimensional Galerkin truncations of the system. This manifold is shown to be almost invariant to any algebraic order and can be constructed in the \(H^{s-1}\times H^s\) phase space of the equation uniformly in the order of the approximation. In particular, we prove that the dynamics on this "slow manifold" shadows orbits of the full system over a finite interval of time.
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