1. Faculty of Mechanical Engineering, University of Niš, A. Medvedeva 14, 18000 Niš, Serbia. melanija.mitrovic@masfak.ni.ac.rs
2. Int. Chair in Mathematical Physics and Applications (ICMPA-UNESCO Chair), University of Abomey-Calavi, 072 B.P. 50 Cotonou, Republic of Benin, Africa. norbert.hounkonnou@cipma.uac.bj (with copy to hounkonnou@yahoo.fr)
3. Dunarea de Jos University of Galati, Romania. marian.baroni@ugal.ro