The center conditions and local bifurcation of critical periods for a Liénard system. - Qiujin Xu; Wentao Huang (Appl. Math. Comput., 2011)

Bifurcation of Limit cycles and Isochronous Centers for a quartic System. - Wentao Huang; Aiyong Chen; Qiujin Xu (Int. J. Bifurc. Chaos, 2013)