1. a Faculty of Mathematical Sciences , University of Tabriz , Tabriz 51666-14766 , Iran
2. b School of Mathematics , University of the Witwatersrand , Johannesburg , Wits 2050 , South Africa
3. c Department of Mathematics and Computer Science, Faculty of Science and Mathematics , University of Niš , Višegradska 33, Niš 18000 , Serbia
4. d Applied Mathematics Research Center, Center for Research and Education in Optical Sciences and Applications, Department of Mathematical Sciences , Delaware State University , Dover , DE 19901-2277 , USA