1. Institute of Applied Mathematics, University of Bonn, D-53115 Bonn; SFB 256 (Bonn); BiBoS SFB237 (Essen-Bochum-Düsseldorf); CERFIM (Locarno), Locarno
2. Department of Mathematics, University of California, San Diego, La Jolla, CA 92093-0112, USA
3. Department of Mathematics, Louisiana State University, Baton Rouge, LA 70803-4918, USA