1. Agricultural Machinery Department, Agrarian and Industrial Faculty, University of Ruse, 8 Studentska Str., 7017 Ruse, Bulgaria
2. Department of Mathematics, Faculty of Natural Sciences and Education, University of Ruse, 8 Studentska Str., 7017 Ruse, Bulgaria
3. Department of Applied Mathematics and Statistics, Faculty of Natural Sciences and Education, University of Ruse, 8 Studentska Str., 7017 Ruse, Bulgaria