On the decay problem for the Zakharov and Klein-Gordon Zakharov systems in one dimension

Abstract: We are interested in the long time asymptotic behavoir of solutions to the scalar Zakharov system [ i u_{t} + \Delta u = nu,] [n_{tt} - \Delta n= \Delta |u|^2] and the Klein-Gordon Zakharov system [ u_{tt} - \Delta u + u = - nu,] [ n_{tt} - \Delta n= \Delta |u|^2] in one dimension of space. For these two systems, we give two results proving decay of solutions for initial data in the energy space. The first result deals with decay over compact intervals asuming smallness and parity conditions ($u$ odd). The second result proves decay in far field regions along curves for solutions whose growth can be dominated by an increasing $C^1$ function. No smallness condition is needed to prove this last result for the Zakharov system. We argue relying on the use of suitable virial identities appropiate for the equations and follow the technics of Kowalczyk-Martel-Mu~noz and Mu~noz-Ponce-Saut.