1. Dipartimento di Matematica Applicata "U.Dini", Università di Pisa, V. Bonanno 25/b, 56126 Pisa, Italy
2. Department of Mechanical Engineering, University of Pittsburgh, Pittsburgh, PA 15260, USA
3. Mathematics and Computer Science Division, Argonne National Laboratory, Argonne, IL 60439, USA
4. Department of Mathematics, University of Pittsburgh, Pittsburgh, PA 15260, USA