[1] Vellore Inst Technol, Sch Adv Sci, Div Math, Chennai 600127, India
[2] Vellore Inst Technol, Sch Adv Sci, Div Math, Vellore 632014, India
[3] Salford Univ, Sch Sci Engn & Environm SEE, Multiphys Engn Sci Grp, Aeronaut Mech Engn, Manchester M5 4WT, Lancs, England
[4] Alipurduar Univ, Dept Math, Alipurduar 736121, W Bengal, India