1. Department of Mathematics, College of Science, University of Ha'il, Ha'il 81451, Saudi Arabia, e.ahmad@uoh.edu.sa
2. Department of Mathematics and Computer Science, Faculty of Science, Port Said University, Port Said 42521, Egypt, ekram_008eg@yahoo.com
3. Department of Mathematics, Faculty of Computer Science and Engineering, "1 Decembrie 1918" University of Alba Iulia, Street Nicolae Iorga 11-13, R-510009 Alba Iulia, Romania, nbreaz@uab.ro, nicoletabreaz@yahoo.com
4. Department of Mathematics, Faculty of Science, Damietta University, New Damietta 34517, Egypt, r_elashwah@yahoo.com