The center conditions for a Liénard System. - Leonid A. Cherkas; Valery G. Romanovski (Comput. Math. Appl., 2006)

Limit Cycles and Centers in a cubic Planar System. - Leonid A. Cherkas; Valery G. Romanovski; Yepeng Xing (Int. J. Bifurc. Chaos, 2010)