Funding for this research was provided by:
National Natural Science Foundation of China (11401117)
National Natural Science Foundation of China (11401060)
National Natural Science Foundation of China (11401217)
National Natural Science Foundation of China (11771168)
National Science Foundation (DMS-1412454)
Article History
Received: 20 December 2017
Revised: 24 March 2018
First Online: 28 April 2018