These notes were written while the authors were members of the Forschungsinstitut fur Matheraatik, Eidg. Technische Hochschule, Zurich, during the summer of 1964. We wish to thank Professor B. Eckmann for having provided us with an opportunity to work together at the Forschungsinstitut.
Some of the results reported here were obtained while the first named author was partially supported by Princeton University, and the second named author by the Institute for Advanced Study and Cornell University.