1. Department of Probability Theory, Faculty of Mechanics and Mathematics, Moscow State University, Moscow, 119992, Russia.
2. Department of Applied Mathematics, National Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu 30050, Taiwan. This research was partially supported by a Republic of China NSC grant 92-2115-M-009-014.
3. Department of Mathematics, College at Geneseo, State University of New York, Geneseo, NY 14454, USA. This research was partially supported by NSF-DMS 0107179.
4. Department of Applied Mathematics, National Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu 30050, Taiwan. This research was partially supported by a Republic of China NSC grant 91-2005-M-009-008.