1. Department of Mathematics, Jadavpur University, Kolkata, 700032, West Bengal, India
2. Faculty of Mathematics, Physics and Informatics, Comenius University, Mlynská dolina, SK-842 48, Bratislava, Slovakia
3. Department of Real Functions Faculty of Mathematics, University of Łódź, Banacha 22, PL-90-238, Łódź, Poland