1. UNIHAVRE, LMAH, FR-CNRS-3335, ISCN, Normandie University, 76600 Le Havre, France
2. The Hudson School of Mathematics, New York, NY 10001, USA
3. Department of Mathematics, Sidho-Kanho-Birsha University, Purulia 723104, India
4. Department of Mathematical Sciences, University of Essex, Wivenhoe Park, Colchester CO4 3SQ, UK
5. Department of Mathematics and Computing, Indian Institute of Technology (Indian School of Mines), Dhanbad 826004, India