1. Department of Mathematical and Computer Sciences, Physical and Earth Sciences, University of Messina, Viale F. Stagno d’Alcontres 31, 98166 Messina, Italy, E-mail: imbesim@unime.it
2. Department of Mathematics, Faculty of Mathematics, University of Sistan and Baluchestan, Zahedan, Iran, E-mail: lashkari@hamoon.usb.ac.ir
3. Department of Mathematics, Faculty of Mathematics, University of Sistan and Baluchestan, Zahedan, Iran, E-mail: z.ahmadiz@yahoo.com