INSTABILITY OF THE STANDING WAVES FOR A BENNEY-ROSKES/ZAKHAROV-RUBENCHIK SYSTEM AND BLOW-UP FOR THE ZAKHAROV EQUATIONS

In this paper we establish the nonlinear orbital instability of ground state standing waves for a Benney-Roskes/Zakharov-Rubenchik system that models the interaction of low amplitude high frequency waves, acustic type waves in N = 2 and N = 3 spatial directions. For N = 2, we follow M. Weinstein's approach used in the case of the Schrodinger equation, by establishing a virial identity that relates the second variation of a momentum type functional with the energy (Hamiltonian) on a class of solutions for the Benney-Roskes/Zakharov-Rubenchik system. From this identity, it is possible to show that solutions for the Benney-Roskes/Zakharov-Rubenchik system blow up in finite time, in the case that the energy (Hamiltonian) of the initial data is negative, indicating a possible blow-up result for non radial solutions to the Zakharov equations. For N = 3, we establish the instability by using a scaling argument and the existence of invariant regions under the flow due to a concavity argument.