TWO PROBLEMS OF PLANAR QUADRATIC SYSTEMS

The main results of this paper are as follows: (ⅰ) The important formulas, given by Bautin, of three focal quantities for the specific form of quadratle system (E2) have been generalized to the general form of (E2). (ⅱ) By using the method in [13], a kind of (E2) possessing at least four limit cycles is given. Theorem 2 herein contains the results in [11--13] on (1,3)-distribution of limit cycles of (E2).