Guo, C., D. O‘Regan, and R. P. Agarwal. “Existence of Periodic Solutions for a Class of Second-Order Neutral Differential Equations With Multiple Deviating Arguments”. CUBO, A Mathematical Journal, vol. 12, no. 3, Oct. 2010, pp. 153–165, doi:10.4067/S0719-06462010000300010.