| This paper deals with the existence of limit cycles of the following nonlinear differential system.(dx)/(dt)=p(y),(dy)/(dt)=-q(y)f(x)-g(x). We obtain two sufficient conditions for the existence of limit cycles surrounding multiple singular points of the system. The results generalize the corresponding results in [1,2,3], and correct the negligences appeared in [2,3,4,5]. |
