Copyright © 2009 Shugui Kang and Sui Sun Cheng. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
This paper deals with the existence and uniqueness of periodic solutions for the first-order functional differential equation with periodic coefficients and delays. We choose the mixed monotone operator theory to approach
our problem because such methods, besides providing the usual existence results, may also sometimes provide uniqueness as well as additional numerical schemes for the computation of solutions.