作者机构:School of Mathematics, Shandong University, Jinan 250100, China.
相关基金:This paper was partially supported by the National Natural Science Foundation of China under Grant Nos. 61174141 and 61374025, Research Awards Young and Middle-Aged Scientists of Shandong Province under Grant Nos. BS2011SF009 and BS2011DX019, Excellent Youth Foundation of Shandong Province under Grant No. JQ201219.