1. Grad. Student, School of Engrg. and Appl. Sci., Northwestern Univ., Evanston, IL 60208.
2. Asst. Prof., Dept. of Mathematics, Univ. of Maryland, College Park, MD 20742-4015.
3. Prof. Dr., School of Engrg. and Appl. Sci., Northwestern Univ., Evanston, IL.