Funding for this research was provided by:
National Science Foundation (RNMS grant No. 1107444, DMS 1514826)
National Science Foundation (RNMS11-07444, DMS–1522184)
National Science Foundation (DMS–1107291)
National Natural Science Foundation of China (91330203)
Article History
Received: 20 June 2016
Revised: 31 December 2016
Accepted: 1 January 2017
First Online: 6 January 2017