Ardjouni, Abdelouaheb, and Ahcene Djoudi. 1. “Existence of Positive Periodic Solutions for Two Types of Second-Order Nonlinear Neutral Differential Equations With Variable Delay”. Proyecciones (Antofagasta, On Line) 32 (4):377-91. https://doi.org/10.4067/S0716-09172013000400006.