School of Mathematics, Shandong University, Jinan Shandong, P. R. China.|Department of Mathematics and Statistics, South Dakota State University, Brookings, SD, USA.|School of Mathematics, Shandong University, Jinan Shandong, P. R. China.|Department of Mathematics and Statistics, South Dakota State University, Brookings, SD, USA.|Department of Agronomy, Horticulture and Plant Science, South Dakota State University, Brookings, SD, USA.