Consider the family of polynomial differential systems of degree 3, or simply cubic systems x ' = y , y ' = - x + a 1 x 2 + a 2 x y + a 3 y 2 + a 4 x 3 + a 5 x 2 y + a 6 x y 2 + a 7 y 3 , in the plane R 2 . An equilibrium point ( x 0 , y 0 ) of a planar ... ...
