Funding for this research was provided by:
University of Basilicata (RIL 2013, RIL 2015)
University of Salerno (grant no. 300393FRB17CIARL and grant "Fondo per il finanziamento iniziale dell'attività di ricerca")
Italian National Group of Mathematical Physics (Progetto Giovani 2018)
Ministerio de Economia y Competitividad of the Spanish Government (grant TEC2015-67462-C2-2-R)
Article History
Received: 26 October 2018
First Online: 27 March 2019