Funding for this research was provided by:
National Natural Science Foundation of China (11231006, 11671273)
the Connotation Development Funds of Beijing University of Technology (006000514117029)
Division of Mathematical Sciences (1209153, 1614246)
National Natural Science Foundation of China (11471103)
Article History
Received: 13 February 2017
Revised: 21 August 2017
First Online: 4 September 2017