Most of the material in this book has been presented in lectures at Brown University, either in courses taught in the Division of Applied Mathematics or in the author's Re- search Seminar in Pattern Theory. I would like to thank the several members of the Division of Applied Mathematics that have participated in the discussions and in particular w. Freiberger, S. Geman, C.-R. Hwang, D. McClure and P. Thrift. I would also like to thank F. John, J...