1. LAREMA, UMR 6093, UNIV Angers, CNRS, SFR Math-STIC, France
2. Institut de Recherche en Mathématique et Physique, UCLouvain, Chemin du Cyclotron 2, B-1348 Louvain-La-Neuve, Belgium
3. Department of Mathematics, Colorado State University, 1874 Campus Delivery, Fort Collins, CO 80523 and Department of Mathematics and Statistics, Concordia University, Montréal, QC