[1] Hohhot Univ Nationalities, Dept Math, Hohhot 010051, Peoples R China
[2] Univ British Columbia, Dept Math, Vancouver, BC V6T 1Z2, Canada
[3] Inner Mongolia Univ, Sch Math Sci, Hohhot 010021, Peoples R China
[4] Univ Witwatersrand, Sch Math, Ctr Differential Equat Continuum Mech & Applicat, ZA-2050 Johannesburg, South Africa