JOAN C. ARTÉS AND JAUME LLIBRE
Abstract
Let $H$ be a cubic polynomial in two variables over $\Bbb R$.
Then $H$ defines a quadratic Hamiltonian vector field
$(\partial H/\partial y,-\partial H/\partial x)$. The purpose of this
paper is to prove that there are exactly 28 non--equivalent topologic phase
portraits of quadratic Hamiltonian vector fields.