Institution: | 1. Department of Mathematics, Yibin University, Yibin 644007, Sichuan Province, P. R. China 2. Department of Mathematics, Science and Research Branch, Islamic Azad University, Tehran 14778, Iran 3. Department of Mathematics and Computer Sciences, Tarbiat Moallem University of Sabzevar, P. O. Box 397, Sabzevar, Iran |