Funding for this research was provided by:
NSF of China (11731006, 11822111)
NSF of China (11688101, 91630203)
NSF of China (11571351, 11731006)
NSF grant (DMS-1416742)
Article History
Received: 19 September 2018
Revised: 20 February 2019
Accepted: 28 February 2019
First Online: 7 March 2019