1. Department of Mathematics, Shanghai Normal University, Shanghai 200234, People’s Republic of China.
2. Corresponding author. School of Mathematics and Information Science, Henan Normal University, Xinxiang, People’s Republic of China, and Laboratoire de Mathématiques et Applications, Université de Poitiers, SP2MI, Boulevard Marie et Pierre Curie, Téléport 2, F-86962, Chasseneuil Futuroscope Cedex, France.