1. Mathematical Institute of the Serbian Academy of Science and Arts, Kneza Mihaila 36, 11000 Belgrade, Serbia
2. Department of Energy Engineering, University of Oradea, Universitatii 1, 410087 Oradea, Romania
3. Aurel Vlaicu University of Arad, 310330 Arad, Elena Dragoi 2, Romania
4. Agora University of Oradea, 410526 Oradea, P-ta Tineretului 8, Romania
5. Department of Economic Informatics & Statistics, University of Craiova, 200585, Craiova, Str. A.I. Cuza, nr. 13, Romania