1. School of Science , China University of Geosciences , Beijing 100083 , P. R. China ; and Department of Mathematics, University of Texas-Rio Grande Valley, Edinburg, TX 78539, USA
2. Department of Mathematics , Yunnan Normal University , Kunming , Yunnan 650500 , P. R. China ; and Department of Mathematics, University of Texas-Rio Grande Valley, Edinburg, TX 78539, USA
3. Department of Mathematics , University of Texas-Rio Grande Valley , Edinburg , TX 78539 , USA