This book consists of updated and considerably expanded lecture notes. I am grateful to F. John, J. P. LaSalle, L. Sirovich and G. Whitham for accepting this preliminary manu­ script for the series Applied Mathematical Sciences published by Springer-Verlag. Many colleagues and students have helped with their comments and suggestions. Among them are D. E. McClure, I. Frolow, J. Silverstein, R. A. Vitale, and I would like to thank especially W. Freiberger for all his encouragement and help in many different ways. The work has been supported by the Division of Mathematica and Computer Sciences of the National Science Foundation, and I appreciate the positive interest shown by John Pasta, Kent curtis, Bruce Barnes and other program directors, over the years. I am indebted to E. Fonseca for her untiring and careful preparation of the manuscript, to E. Addison for her skillful help with the many diagrams, and to K. H. Avery, D. P. Libutti, K. MacDougall and S. V. Spinacci for the final typing. I gratefully acknowledge permission to reproduce figures, as mentioned in the text, to: American Elsevier Publishing Company, Inc., A. J. Larkin, and M. Gardner. I dedicate this work to my wife, Emma-Stina. June, 1975 Ulf Grenander, Providence TABLE OF CONTENTS INTRODUCTION 1 CHAPTER 1. THE GENERATORS 6 l. Generators and Their Properties 6 2. Abstract Generators 10 3. Concrete Generators 13 4.