Exponential Dichotomy and Smooth Invariant Center Manifolds for Semilinear Hyperbolic Systems

A “spectral gap mapping theorem”, which characterizes exponential dichotomy, is proven for a general class of semilinear hyperbolic systems of PDEs in a Banach space X of continuous functions. This resolves a key problem on existence and smoothness of invariant manifolds for semilinear hyperbolic systems. The system is of the following form: For 0 < x < l and t > 0 (SH)  ∂ ∂t u(t, x) v(t, x) w(t, x) +K(x) ∂ ∂x u(t, x) v(t, x) w(t, x) +H(x, u(t, x), v(t, x), w(t, x)) = 0, d dt [v(t, l)−Du(t, l)] = F (u(t, ·), v(t, ·)), u(t, 0) = E v(t, 0), u(0, x) = u0(x), v(0, x) = v0(x), w(0, x) = w0(x), where u(t, x) ∈ R1 , v(t, x) ∈ R2 and w(t, x) ∈ R3 , K(x) = diag (ki(x))i=1,...,n is a diagonal matrix of functions ki ∈ C ([0, l],R), ki(x) > 0 for i = 1, . . . , n1 and ki(x) < 0 for i = n1 + 1, . . . n1 + n2, ki ≡ 0 for i = n1 + n2 + 1, . . . , n1 + n2 + n3 = n, and D and E are matrices. It is shown that weak solutions to (SH) form a smooth semiflow in X under natural conditions on H and F . For linearizations of (SH) high frequency estimates of spectra and resolvents in terms of reduced diagonal and blockdiagonal systems are given. Using these estimates and the theory [36, 42] of Kaashoek, Lunel and Latushkin a spectral gap mapping theorem for linearizations of (SH) in the “small” Banach space X is proven: An open spectral gap of the generator is mapped exponentially to an open spectral gap of the semigroup and vice versa. Hence, a phenomenon like in the counterexample [61] of Renardy cannot appear for linearizations of (SH). The results here differ to the work [48] of Lopes, Neves and Ribeiro in essential directions: First, the focus is on the “small” Banach space X (not L spaces), which is required for nonlinear problems like (SH). Second, degenerate and equal speed systems are considered needed for applications to laser dynamics. Existence of smooth center manifolds for (SH) is shown by applying the above results and general theory on persistence and smoothness of invariant manifolds, obtained by Bates, Lu and Zeng [7, 8], in the Banach space X. The results are applied to traveling wave models of semiconductor laser dynamics. For such models mode approximations (ODE systems which approximately describe the dynamics on center manifolds) are derived and justified, and generic bifurcations of modulated waves from rotating waves are shown. Global existence and smooth dependence of nonautonomous traveling wave models with more general solutions, which possess jumps, are considered, and mode approximations are derived for such nonautonomous models. In particular the theory applies to stability and bifurcation analysis for Turing models with correlated random walk [33, 31]. Moreover, the class (SH) includes neutral and retarded functional differential equations.