1. Department of Mathematical Sciences, University of North Carolina at Wilmington, Wilmington, NC 28403, U.S.A.
2. Department of Mathematics, University of California, Santa Barbara, CA 93106, U. S.A.
3. Department of Mechanical Engineering, University of Natal, Durban 4001, Republic of South Africa
4. Department of Mechanical and Environmental Engineering, University of California, Santa Barbara, CA 93106, U.S.A.