Comment on: "An observation on the periodic solutions to nonlinear physical models by means of the auxiliary equation with a sixth-degree nonlinear term" [Commun Nonlinear Sci Numer Simulat 2013;18: 2177-2187]. - Hong-Zhun Liu; Xiao-Quan Sun; Li-Jiang Chen (Commun. Nonlinear Sci. Numer. Simul., 2014)