Small Noise Asymptotic Expansions for Stochastic pde's.i : The Case of a Dissipative Polynomially Bounded non Linearity

We study a reaction-diusion evolution equation perturbed by a Gaussian noise. Here the leading operator is the innitesimal generator of a C0-semigroup of strictly negative type, the nonlinear term has at most polynomial growth and is such that the whole system is dissipative. The corresponding It^o stochastic equation describes a process on a Hilbert space with dissipative nonlinear drift and a Gaussian noise. Under smoothness assumptions on the non-linearity, asymptotics to all orders in a small parameter in front of the noise are given, with uniform estimates on the remainders. Applications to nonlinear SPDEs with a linear term in the drift given by a Laplacian in a bounded domain are included. As a particular example we consider the small noise asymptotic expansions for the stochastic FitzHugh-Nagumo equations of neurobiology around deterministic solutions. 2010 Mathematics Subject Classication. Primary 35K57 , 35R60, 35C20 ; Secondary 92B20