Bojilov, Assen
Borissov, Lyubomir
Borissov, Yuri
Funding for this research was provided by:
Science Foundation of Sofia University (80-10- 74/20.04.2017)
Foundation ”Georgi Tchilikov” of Institute of Mathematics and Informatics, Bulgarian Academy of Sciences
Article History
Received: 23 November 2017
Accepted: 5 October 2018
First Online: 25 October 2018