Visited the following centers as visiting researcher:

• 04-05.1990, 02-04.1991, 01-06.1992, 10-12.1993  -  IMAG, Université Fourier de Grenoble, Grenoble, France
• 06.1990, 05.1991, 03-04.2011  - Institut für Diskrete Mathematik, Universität Bonn, Germany
•  10.1992, 11-12.1996  - Kyoto University, Japan
•  01-06.1993  - Stanford University, Palo Alto, USA
•  07.1993  - DIMACS, Rutgers University, New Brunswick, USA
•  02-04.1995, 09-10.1996 - Université Paris-11, Orsay, France
•  10-11.1997, 03-04.1998, 02-05.1999  - SFB 343, Bielefeld University, Bielefeld, Germany
• 11-12.1999  - Fields Institute for Research in Math. Sci., Toronto, Canada
• 10-11.2006  - CWI, Amsterdam, the Netherlands
• 10.2007-03.2008 - Université Paris-6, Paris, France