August 1993 - September 1993:
Visiting researcher at the Department of Electronics and Electric Engineering of the University of Pretoria, South Africa.
April 25 - April 29, 2005:
Visiting researcher in the frame of an ERASMUS exchange program at the Department of Algebra (Prof. Jiri Tuma) of the Charles University, Prag, Czech Republic.
January 16 - April 30, 2006:
Visiting researcher in the Analytica project at the Department of Computer Science (Prof. Edmund M. Clarke & Prof. Klaus Sutner) at Carnegie Mellon University, Pittsburgh, USA.