1. School of Mathematics, Qilu Normal University, Jinan 250013, China
2. School of Mathematical Sciences, Qufu Normal University, Qufu 273165, China
3. School of Mathematics and Statistics, Ludong University, Yantai 264025, China
4. School of Mathematical and Statistical Sciences, University of Galway, Galway, Ireland
5. School of Mathematical Sciences, Chongqing Normal University, Chongqing 401331, China