1. Department of Agricultural Machinery, Agrarian Industrial Faculty, University of Ruse, 8 Studentska Str., 7004 Ruse, Bulgaria
2. Department of Informational Modeling, Institute of Mathematics and Informatics, Bulgarian Academy of Sciences, 8 Acad. Georgi Bonchev Str., 1113 Sofia, Bulgaria
3. Department of Applied Mathematics and Statistics, Faculty of Natural Sciences and Education, University of Ruse, 8 Studentska Str., 7004 Ruse, Bulgaria