  We give a simple proof of an isomorphism between the two
$\mathbb{C}[t]$-modules: the module of relative cohomologies $\Lambda^2/dH\land
\Lambda^1$ and the module of Abelian integrals corresponding to a regular at
infinity polynomial $H$ in two variables. Using this isomorphism, we prove
existence and deduce some properties of the corresponding Picard-Fuchs system.
