Théorie de bifurcation et de stabilité pour une équation de Schrödinger avec une non-linéarité compacte

The nonlinear Schrödinger equation i∂tw + Δw +V(x)|w|p-1w = 0   w = w(t,x) : Ι × RN → C, N ≥ 2,     (1) is studied, with p > 1, V : RN \ {0} → R and Ι ⊂ R an interval. The coefficient V is subject to various hypotheses. In particular, it is always assumed that V (x) → 0 as |x| → ∞. Situations where V is unbounded at the origin are considered. A special attention is paid to the radial case. Seeking solutions of (1) as standing waves w(t,x) = eiλtu(x) leads naturally to the semilinear elliptic equation Δu - λu + V(x)|u|p-1u = 0   u : RN → R, N ≥ 2.     (2) The main goals of the thesis are to establish existence and bifurcation results for (2), to discuss the orbital stability of the standing waves of (1) corresponding to the solutions found in (A). First, in Chapter 1, in the case where V is radial, a variational approach shows the existence of ground states for (2). A non-degeneracy property of these solutions is proved, which plays a crucial role in the continuation arguments of Chapter 2. The first part of Chapter 2 establishes local existence and bifurcation results for (2), without any symmetry assumption on V . Under certain hypotheses on the power p and the coefficient V , two branches of solutions are obtained, in a neighborhood of λ = 0 and in a neighborhood of λ = +∞. The branches are of class Cr if V ∈ Cr(RN \ {0},R), for r = 0, 1. These independent results are proved by requiring respectively that lim|x|→∞ V (x)|x|b = B > 0 with b ∈ (0, 2) and that limx→0 V (x)|x|a = A > 0 with a ∈ (0, 2). The asymptotic behaviour along the branches is discussed in detail and depends on the value of p. The second part of Chapter 2 proves the existence of a global branch of solutions of (2), in the case where V is radial. Under appropriate hypotheses, in particular if a ∈ (0, b], the global branch "sticks together" the two local branches obtained in the first part. Chapter 3 is concerned with the orbital stability of the standing waves of (1) corresponding to the solutions of (2) found in the first part of Chapter 2. It is explained in detail how to apply the general theory of orbital stability to (1). Local stability/instability results are proved, in a neighborhood of λ = 0 and in a neighborhood of λ = +∞.