1. Department of Information Technology, Mathematics and Physics, Petroleum-Gas University of Ploieşti, Bulevardul Bucureşti No. 39, 100680 Ploieşti, Romania
2. Faculty of Mathematics and Computer Science, Research Center in Geometry, Topology and Algebra, University of Bucharest, Street Academiei No. 14, Sector 1, 70109 Bucharest, Romania
3. Department of Mathematical Modelling, Economic Analysis and Statistics, Petroleum-Gas University of Ploieşti, Bulevardul Bucureşti No. 39, 100680 Ploieşti, Romania