The averaging principle and periodic solutions for nonlinear evolution equations at resonance

摘要

We study the existence of T-periodic solutions (T>0) for the first order differential equations being at resonance at infinity, where the right hand side is the perturbations of a sectorial operator. Our aim is to prove an index formula expressing the topological degree of the associated translation along trajectories operator on appropriately large ball, in terms of special geometrical assumptions imposed on the nonlinearity. We also prove that the geometrical assumptions are generalizations of well known Landesman-Lazer and strong resonance conditions. The obtained index formula is used to derive the criteria determining the existence of T-periodic solutions for the heat equation being at resonance at infinity.