Funding for this research was provided by:
National Natural Science Foundation of China (91130014, 11471312)
National Natural Science Foundation of China (91430216)
Division of Mathematical Sciences (DMS-1419040)
National Natural Science Foundation of China (11471031, NSAF U1530401)
Article History
Received: 26 November 2017
Revised: 12 April 2018
Accepted: 23 April 2018
First Online: 17 May 2018