* The University of Chicago, Chicago, Ill, U.S.A.,

January/February, 1969;

* Forschungsinstitut für Mathematik, ETH-Zürich,

Switzerland, 1970-1972, Summer 1973 and academic year 1979/80;

* Université des Sciences et Techniques de Lille, Villeneuve

d'Ascq, France, April-September, 1975

* Der Ludwig Maximilians Universität, Munich, Germany,

April/May 1982, September/October 1983, and January to May 1985;

* Steklov Mathematics Institute, USSR Academy of Science,

Moscow, May 1984;

* Peking University, Beijing, China, December, 1985;

* Fudan Universityty, Shanghai, China, December, 1985;

* Università di Perugia (Scuola Matematica

Interuniversitaria), Perugia, Italy, July/August, 1987 and

August, 1989,

* Dalhousie University, Halifax, Canada, February/March, 1997.

* The University of Rochester, April/May 1997.