Funding for this research was provided by:
Fundação para a Ciência e a Tecnologia (UID/MAT/00006/2013)
Deutsche Forschungsgemeinschaft (DR 271/6-2)
Article History
Received: 20 September 2017
Revised: 12 February 2018
Accepted: 22 February 2018
First Online: 6 March 2018