1. School of Computer Science, Chengdu University, Chengdu 610106, P. R. China
2. Department of Mathematics, COMSATS University Islamabad, Attock 43600, Pakistan
3. Government Girls High School Behboodi, Hazro 43440, Pakistan
4. Department of Mathematics, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand