[1] Bharathiar Univ, Dept Math, Coimbatore 641046, Tamil Nadu, India
[2] UAE Univ, Coll Sci, Dept Math Sci, Al Ain, U Arab Emirates
[3] Vellore Inst Technol, Sch Adv Sci, Dept Math, Vellore 632014, Tamil Nadu, India
[4] Politecn Torino, Dipartimento Sci Matematiche, Corso Duca Abruzzi 24, I-10129 Turin, Italy