1. Department of Mathematics, Physics and Informatics, University of Forestry, 1756 Sofia, Bulgaria
2. Department of Informational Modeling, Institute of Mathematics and Informatics, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
3. Department of Applied Mathematics and Statistics, University of Ruse, 7017 Ruse, Bulgaria
4. Department of Parallel Algorithms, Institute of Information and Communication Technologies, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria