* Partially supported by the Applied Mathematical Sciences subprogram of the Offlice of Energy Research, U.S. Department of Energy, under Contract W-31-109-Eng-38 while a Visit- ing Scientist at Argonne National Laboratory during the summer of 1987, and m part by the Board of Regents of the State of Louisiana under Grant LEQSF(86-89)-RD-A-11. + Partially supported by the Applied Mathematical Sciences subprogram of the Office of Energy Research, U.S. Department of Energy, under contract W-31-109-Eng-38. This author thanks the Department of Mathematics, University of Southwestern Louisiana, for an invitation to spend a semester there in the fall of 1986, during which the research reported in this paper was started.