“…For the center-focus determination and Hopf bifurcation on the center manifold, there have been some valid research approaches, such as the averaging theory considered in Llibre et al and Barreira et al, 18,19 the technique of inverse Jacobi multiplier studied in Buica et al, 20,21 the simplest normal form method given in Tian and Yu,22 and the formal first integral method given in Edneral et al 23 Here, we apply the formal series method introduced in Wang et al, 8 as an upgraded version of the method given in Liu and Li and Liu,24,25 which are also very valid to study the Hopf bifurcation, for two-dimensional system, some results are obtained, see eg, previous studies, [26][27][28][29] for three-dimensional system, see eg, previous studies. [30][31][32] The method in Wang et al 8 mainly consider the three-dimensional system (4) as follows:…”