In the present paper we investigate the number of periodic solutions of the following differential equation dy/dt=(A1(t)y+A2(t)y2+A3(t)y3)/(a0(t)+a1(t)y+a2(t)y2)(**) which was discussed in paper , and obtain the theorem by the method of cross-ratio of the solutions of (**) without the traditional condition assumption that the functions Ai(t),aj(t) (i=1,2,3; j=0,1,2) are differential. |