Godbillon-Vey sequence and Francoise algorithm

Abstract

We consider foliations given by deformations $dF+\epsilon \omega$ of exact forms $dF$ in ${\mathbb{C}}^2$ in a neighborhood of a family of cycles $\gamma (t)\subset F^{-1}(t)$. In 1996 Francoise gave an algorithm for calculating the first nonzero term of the displacement function $\Delta$ along $\gamma$ of such deformations. This algorithm recalls the well-known Godbillon-Vey sequences discovered in 1971 for investigation integrability of a form $\omega$. In this paper, we establish the correspondence between the two approaches and translate some results by Casale relating types of integrability for finite Godbillon-Vey sequences to the Francoise algorithm settings.