1. Departament de Matemàtiques, Universitat Politècnica de Catalunya, Barcelona, Spain;
2. Department of Mathematics, Ben-Gurion University of the Negev, Beer-Sheva, Israel;
3. Departament d’Àlgebra i Geometria, Universitat de Barcelona, Barcelona, SpainDedicated to Professor Gennady Lyubeznik on the occasion of his 60th birthday.