**Author details**

Virgilius-Aurelian Minuță1\* and Eugen Petac2

\*Address all correspondence to: aurelmva@gmail.com

1 Babeș-Bolyai University, Cluj-Napoca, Department of Mathematics and Computer Science, Romania

2 'Ovidius' University of Constanța, Constanța, Department of Mathematics and Computer Science, Romania
