 April 1, 2009 ---- April 30, 2009, Visiting professor, Wayne State University, USA  August 15, 2008 --- October 14, 2008, Visiting professor, National Cental University, TaiWan.  July 15, 2007 --- October 15, 2007, Visiting professor, The Abdus Salam International Centre for Theoretical Physics (at Trieste in Italy).  November 1, 2006---Janary 1, 2007, Visiting professor, Beijing International Center for Mathematical Research