Affiliation: | (1) School of Mathematics and Statistics, Lanzhou University, Lanzhou, 730000, P. R. China;(2) School of Mathematics and Statistics, Lanzhou University, Lanzhou, 730000, P. R. China;(3) Department of Mathematics, Lanzhou Teachers College, Lanzhou, 730070, P. R. China;(4) School of Mathematics and Statistics, Lanzhou University, Lanzhou, 730000, P. R. China |