1. Faculty of Mathematics and Informatics , West University of Timişoara, Timişoara 300223, Romania, darius.galis@e-uvt.ro
2. Faculty of Mathematics and Informatics , West University of Timişoara, Timişoara 300223, Romania, ciprian.pungila@e-uvt.ro
3. Faculty of Mathematics and Informatics , West University of Timişoara, Timişoara 300223, Romania; ICAM–Environmental Advanced Research Institute, West University of Timisoara, Timişoara 300223, Romania, viorel.negru@e-uvt.ro