He was visiting professor at the University of Strathclyde, Glasgow, U.K., in 1996, and later Visiting Professor at the Mittag-Leffler Institute, Stockholm, Sweden, 2003.
Han var gæsteforsker ved University of Strathclyde, Glasgow, U.K., i 1996, og senere gæsteprofessor ved Mittag-Leffler Instituttet, Stockholm, Sverige, 2003.