Funding for this research was provided by:
National Natural Science Foundation of China (NSFC 11471031, NSFC 91430216)
National Natural Science Foundation of China (NASF U1530401)
National Science Foundation (DMS-1419040)
Article History
Received: 16 March 2018
Revised: 16 July 2018
Accepted: 6 August 2018
First Online: 5 September 2018