Funding for this research was provided by:
National Natural Science Foundation of China (11001179)
State Scholarship Fund of China (201708440016)
NSF-DMS (1419028)
Article History
Received: 11 April 2018
Accepted: 8 October 2018
First Online: 22 October 2018
Compliance with ethical standards
:
: The authors declare that they have no conflict of interest.