Funding for this research was provided by:
National Science Foundation (DMS-1418936, DMS-1107291)
NSFC (11771035, 91430216)
NSFC (U1530401)
Article History
Received: 24 March 2017
Revised: 25 October 2017
Accepted: 8 November 2017
First Online: 25 November 2017