^a Department of Mechanical and Environmental Engineering, University of California, Santa Barbara, CA 93106, U.S.A.

^b Department of Mathematics, University of California, Santa Barbara, CA 93106, U.S.A.

^c Department of Mathematical Sciences, University of North Carolina at Wilmington, North Carolina 28403, U.S.A.

^d Department of Mechanical Engineering, University of Natal, Durban 4001, Republic of South Africa