• April - May, 1988 : Friedrich-Schiller University, Jena, Germany

• September-December, 1990 : Belarusian State University, Minsk, Belarus

• October-November, 1991 : Hue College of Education, Hue, Vietnam

• February-March, 1992 : Quynhon College of Education, Quynhon, Vietnam

• January 1993-September 1994 : Alexander von Humboldt Postdoctoral Fellow, Free University of Berlin, Berlin, Germany

• November, 1993 : Friedrich–Schiller University, Jena, Germany

• August–September, 1995 : University of Tokyo and Fukuoka University, Tokyo-Fukuoka, Japan

• January-February, 1996 : Free University of Berlin, Germany

• July-August, 1997 : Wolfram Research, Champaign, USA

• January, 1999 : Gunma University, Kiryu, Japan

• June-July, 1999 : Fukuoka University, Fukuoka, Japan

• August-September, 2002 : City University of Hong Kong, Hong Kong

• December, 2004 : Kuwait University, Kuwait

• April, 2006 : University Hassan II, Casablanca, Morocco

• May, 2006 : University of Porto, Porto, Portugal