This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 3.0 License.
Chang-Jun Li
School of Mathematical Sciences, Ocean University of China, Lane 238, Songling Road, Laoshan District, Qingdao City, Shandong Province, 266100, People’s Republic of China.
Xiang Gao
School of Mathematical Sciences, Ocean University of China, Lane 238, Songling Road, Laoshan District, Qingdao City, Shandong Province, 266100, People’s Republic of China.