Dr. Dachun Yang ProfessorSchool of Mathematical SciencesBeijing Normal UniversityBeijing 100875People"s Republic of China Address: Education Building 8, 414 West.Phone: 0086-10-58805472Fax: 0086-10-58805472 E-mail: dcyang@bnu.edu.cnhttp://math.bnu.edu.cn/~dcyang/