Funding for this research was provided by:
Directorate for Mathematical and Physical Sciences (DMS-1418918)
Hong Kong RGC-GRF Grant (605513, 16302715)
Hong Kong RGC-CRF Grant (C6004-14G)
NSFC-RGC Joint Research Grant (N-HKUST620/15)
Article History
Received: 25 January 2017
Revised: 27 April 2017
Accepted: 2 May 2017
First Online: 16 May 2017