Visiting positions

University of Ghent (1978),
Universität Bonn (1979),
University of Helsinki (1980),
Steklov Institute, St. Petersburg (1980),
Lancaster University (1984),
University of California at Berkeley (academic year 1985/86),
Mathematical Sciences Research Institute, Berkeley (1988 and 1995),
University of Illinois, Chicago (1995),
Ohio State University (1997),
Banach Center, Warsaw (1997, 2000, 2007), 
Hayama, Japan (1998, 2000, 2004, 2007),
American Institute of Mathematics, Palo Alto (2000),
Mittag-Leffler Institute (2008),
Universite de Provence, Marseille (2008, 2009).