1. School of Mathematics and Statistics, HNP-LAMA, Central South University, Changsha, Hunan 410083, People’s Republic of China.
2. Faculty of Applied Mathematics, AGH University of Science and Technology, Kraków, Poland, and Department of Mathematics, University of Craiova, 200585 Craiova, Romania, and Brno University of Technology, Faculty of Electrical Engineering and Communication, Technická, Brno 61600, Czech Republic, and School of Mathematics, Zhejiang Normal University, Jinhua, Zhejiang 321004, China, and Simion Stoilow Institute of Mathematics of the Romanian Academy, 010702 Bucharest, Romania.
3. Corresponding author. School of Mathematics and Statistics, HNP-LAMA, Central South University, Changsha, Hunan 410083, People’s Republic of China, and Department of Mathematics, University of Craiova, 200585 Craiova, Romania.