Periodic solutions to a kind of neutral functional differential equation in the critical case ✩

In this paper, we consider a kind of neutral functional differential equation as follows: x(t) − cx(t − τ) = g t,x t −μ(t ) + e(t ) in the critical case |c| = 1. By employing the topological degree theory and some analysis techniques, we obtain some new results on the existence of periodic solutions.  2004 Elsevier Inc. All rights reserved