1. School of Mathematics and Statistics, Xi'an Jiaotong University, Xi'an, Shaanxi, People's Republic of China
2. School of Mathematics and Statistics & State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi, People's Republic of China
3. Department of Mathematics & Program in Applied Mathematical and Computational Sciences (AMCS), University of Iowa, Iowa City, IA, USA