  The moduli space of algebraic foliations on P2 of a fixed degree and with a
center singularity has many irreducible components. We find a basis of the
Brieskorn module defined for a rational function and prove that set of
pull-back foliations forms an irreducible component of the moduli space. The
main tools are the Picard-Lefschetz theory of a rational function in two
variables, period integrals, and the Brieskorn module.
