1. Software Engineering College, Zhengzhou University of Light Industry, Zhengzhou, Henan 450002, P. R. China
2. Institute of Mathematics, Physics and Mechanics, 1000Ljubljana, Slovenia; and Faculty of Applied Mathematics, AGH University of Science and Technology, 30-059 Kraków, Poland; and Department of Mathematics, University of Craiova, 200585 Craiova, Romania
3. Faculty of Education and Faculty of Mathematics and Physics, University of Ljubljana; and Institute of Mathematics, Physics and Mechanics, 1000Ljubljana, Slovenia
4. Department of Statistics, University of Georgia, Athens, GA 30602, USA