Laboratory of Nonlinear Analysis;
Department of Mathematics;
Central China Normal University;
Wuhan 430079;
P. R. China;
Department of Mathematical and Statistical Sciences;
University of Alberta;
Alberta T6G 2G1;
Canada;
Wuhan Institute of Physics and Mathematics;
The Chinese Academy of Sciences;
Wuhan 430071;
P. R. China;
Graduate School of the Chinese Academy of Sciences;