School of Mathematics and Computer Science, Northwest University for Nationalities,Gansu 730030, P.R.China;
Department of Mathematics, Gansu Normal University for Nationalities,Gansu 747000, P.R.China;
School of Mathematics and Computer Science, Northwest University for Nationalities,Gansu 730030, P.R.China;
School of Mathematics and Computer Science, Northwest University for Nationalities,Gansu 730030, P.R.China;