1. Department of Mathematics , Yunnan University , Kunming , Yunnan 650091 , P. R. China ; and Department of Mathematics, Texas A&M University-Kingsville, TX 78363-8202, Kingsville, TX, USA
2. Department of Mathematics , Texas A&M University-Kingsville , TX 78363-8202 , Kingsville ; and Florida Institute of Technology, Melbourne, FL 32901 , USA
3. School of Mathematics, Statistics and Applied Mathematics , National University of Ireland , Galway , Ireland