1. LMPA Joseph Liouville, FR CNRS 2956, Université du Littoral Côte d'Opale, B.P. 699, F-62229 Calais cedex, France
2. New Mexico State University Las Cruces, New Mexico, USA
3. Department of Mathematics, Statistics and Computer Science, University of Illinois at Chicago, 851 South Morgan Street, Chicago, IL, 60607-7045, USA