1. 1 Department of Engineering Mathematics, Faculty of Engineering , Cairo University , Orman, Giza 12221 , Egypt
2. 2 Department of Mathematics , University of Tennessee at Chattanooga , Chattanooga , , USA
3. 3 School of Control Science and Engineering , Shandong University , Jinan, Shandong 250061, P. R. China
4. 4 Department of Mathematics, Faculty of Arts and Sciences , Tokat Gaziosmanpasa University , , Tokat , Turkey