1. School of Mathematics, Hangzhou Normal University , Hangzhou 311121 , China
2. Department of Mathematics, Bidhan Chandra College , Asansol 713304 , India
3. Department of Mathematics, Mrinalini Datta Mahavidyapith , Kolkata 700 051 , India
4. Department of Mathematics, College of Science, King Khalid University , 61421 Abha , Saudi Arabia