In this paper, in the case of not requiring the nonlinear terms to be non-negative the existence of nontrivial periodic solutions for the first order functional differential equations is considered by using the partial ordering theory. © 2004 Elsevier Ltd. All rights reserved.
Kang, S., & Zhang, G. (2005). Existence of nontrivial periodic solutions for first order functional differential equations. Applied Mathematics Letters, 18(1), 101–107. https://doi.org/10.1016/j.aml.2004.07.018