1. School of Mathematics, Xi'an University of Finance and Economics, Xi'an, China
2. Kwangwoon Global Education Center, Kwangwoon University, Seoul 139-701, Republic of Korea
3. Department of Mathematics, Kwangwoon University, Seoul 139-701, Republic of Korea
4. School of Science, Xi'an Technological University, Xi'an 710021, Shaanxi, China
5. Department of Mathematics, Sogang University, Seoul 121-742, Republic of Korea