微分方程(dx)/(dt)=h(y)-F(x),(dy)/(dt)=-g(x)存在极限环的一个充分条件

For the equations dx/dt=h(y)-f(x), dy/dt=-g(x)there are some results about the existence of limiting cycle under the monotone condition provided on h(y). In this paper, the same result without the monotone assumption is proved.