作者机构:[1]School of Mathematical and Informatical Sciences, Henan Polytechnic University, Jiaozuo 454003, China, [2]Department of Mathematics, Capital Normal University, Beijing 100048, China
相关基金:The first author is supported in part by National Natural Sciences Foundation of China (No. 10771052). The second author is supported in part by National Natural Sciences Foundation of China (No. 10871133) and a project of Beijing Education Bureau.