1. Department of Mathematics, Basic Science Faculty, University of Bonab, P. O. Box 55513-95133, Bonab, Iran
2. Department of Mathematics, Clarkson University, Potsdam, NY 13699, USA
3. Department of Basic Sciences, University of Engineering and Technology, Peshawar 25000, Pakistan
4. Section of Mathematics, International Telematic University Uninettuno, Corso Vittorio Emanuele II, 39, Roma 00186, Italy