1. Department of MathematicsUniversity of West BohemiaPlzen Czech Republic
2. Department of Computer ScienceFaculty of Mathematics, Physics and Informatics, Comenius UniversityBratislava Slovakia
3. Department of Computer ScienceComenius University, Faculty of Mathematics, Physics and InformaticsBratislava Slovakia
4. Department of MathematicsInstitute of Theoretical Computer Science, University of West BohemiaPlzen Czech Republic