Funding for this research was provided by:
National Science Foundation (DMS-1620194)
National Natural Science Foundation of China (91630207, 11571115)
National Natural Science Foundation of China (11471194)
Natural Science Foundation of Shandong Province (ZR2011AM015)
Article History
Received: 17 March 2016
Revised: 24 March 2017
Accepted: 1 June 2017
First Online: 29 June 2017