1. Institute for Applied Mathematics, and Hausdorff Center for Mathematics, University of Bonn, Endenicher Allee 60, D-53115 Bonn, Germany. E-mail: michelangeli@iam.uni-bonn.de
2. Department of Mathematics, University of Rome La Sapienza, Piazzale Aldo Moro 5, 00185 Rome, Italy. E-mail: nicola.santamaria@uniroma1.it