Visiting scientist at :

. The International Centre for Theoretical Physics , Trieste , Italy ,

December 1986 - December 1989 under Post-Doctoral Fellowship .

. University of Heidelberg , 6 - 13 December 1987 .

. Unversity of Bonn , 16-23 December 1988 .

. Unversity Paris VII , 3-16 July 1989 .

. Unversity of Bonn , 21 November - 5 December 1989 .

. The International Centre for Theoretical Physics , Trieste, Italy ,

February 1990 .

. Centro de Investigacion en Matematicas , Guanjuato, Mexico ,

1 June - 1 September 1990 .

. The International Centre for Theoretical Physics , Trieste, Italy ,

5 September - 25 October 1990 .

. The Mathematics Institut of Bonn University , Bonn , F.R. Germany ,

1 Mai 1992 - 30 August 1993 , under Humboldt Fellowship .

. The International Centre for Theoretical Physics , Trieste, Italy ,

1 September - 30 October 1993 .

. The International Centre for Theoretical Physics , Trieste, Italy , 1 August - 31 October 1994 .

. The ETH , Zurich , Switzerland , 1 November - 30 November 1994 .

. The Ohio University ,Athens, Ohio , USA , 26 March - 9 June 2001.

. Instituto de Matematicas U.N.A.M., Cuernavaca, Morelos, Mexico ,

6 Feruary - 7 March 2003.