Funding for this research was provided by:
Vietnam National Foundation for Science and Technology Development (101.04-2015.26)
Vietnam National Foundation for Science and Technology Development (101.04-2015.26)
Vietnam Institute for Advanced Study in Mathematics
Article History
Received: 29 December 2016
Revised: 10 August 2017
Accepted: 21 October 2017
First Online: 2 February 2018