1. Laboratoire de Mathématiques, Institut de Sciences Appliquees and U.R.A. C.N.R.S. 1378, BP8, 76131 Mont-Saint-Aignan Cedex, France
2. Physics Department, University of Crete, 714 09 Heraklion, Greece
3. Mathematics Department, University of Crete, 714 09 Heraklion, Greece