Professor Dieter Happel, chemnitz University, 1998, 2004.
Professor Helmut Lenzing, University of Paderborn, Germany, 1998.
Professor Hossin Zand, British Open University, 1998.
Professor Luise Unger, University of Bielefeld, Germany, 1999.
Professor Edward Green, Virginia State University, USA, 1999.
Professor bernhard Keller, University of Paris VII, 2003.
Professor Steffen Koenig, University of Leicester, UK, 2004
Professor Vincent Rivasseau, University of Paris, 2004.