1. Department of Mathematics, University of Architecture, Civil Engineering and Geodesy, 1 Hr. Smirnenski Blvd., 1046 Sofia, Bulgaria
2. Faculty of Economics and Business Administration, Sofia University “St. Kliment Ohridski”, 15 Tsar Osvoboditel Blvd., 1504 Sofia, Bulgaria
3. Department of Mathematics and Informatics, Faculty of Economics, Agricultural University-Plovdiv, 12 Mendeleev Blvd., 4000 Plovdiv, Bulgaria