Ⅲa＝0（－1＜n＜0，0＜l＜1）类二次系统存在极限环的充要条件

A SUFFICIENT AND NECESSARY CONDITION OF THE EXISTENCE OF LIMIT CYCLE IN THE

本文利用旋转向量场理论得到了系统ｘ＝－ｙ＋δｘ＋ｌｘ２＋ｍｘｙ＋ｎｙ２，ｙ＝ｘ（１＋ｙ），｛（－１＜ｎ＜０，０＜ｌ＜１）存在极限环的充要条件．

In this paper, we obtain a sufficient and necessary condition of the existence of limit cycle in the system =-y+δx+lx 2+mxy+ny 2, =x(1+y), (-1<n<0,0<l<1) by use of the rotated vector field theory.