1. PRINCETON UNIVERSITY PRINCETON NEW JERSEY
2. CNRS, LABORATOIRE G‐SCOP UNIVERSITY OF GRENOBLE FRANCE
3. LABORATOIRE G‐SCOP UNIVERSITY OF GRENOBLE FRANCE
4. WESLEYAN UNIVERSITY MIDDLETOWN CONNECTICUT
5. DEPARTMENT OF APPLIED MATHEMATICS AND COMPUTER SCIENCE TECHNICAL UNIVERSITY OF DENMARK LYNGBY DENMARK