Guo, Chengjun, Donal O‘Regan, and Ravi P. Agarwal. 2010. “Existence of Periodic Solutions for a Class of Second-Order Neutral Differential Equations With Multiple Deviating Arguments”. CUBO, A Mathematical Journal 12 (3):153–165. https://doi.org/10.4067/S0719-06462010000300010.