1. Department of Mathematics, University of Catania, Viale A. Doria 6, 95125 Catania, Italy.
2. Department of Mathematics, California Lutheran University, 60 W. Olsen Rd., MC 3750, Thousand Oaks, CA 91360 USA
3. Department of Mathematical Sciences, Central Connecticut State University, 1615 Stanley Street, New Britain, CT 06050 USA.