Peter J. Olver, Guillermo Sapiro, Allen Tannenbaum.
Includes bibliographical references (p. 40-44).
Supported by the National Science Foundation. DMS-9204192 DMS-8811084 ECS-9122106 Supported by the Air Force Office of Scientific Research. AFOSR-90-0024 Supported by the Rothschild Foundation-Yad Hanadiv and by Image Evolutions, Ltd.