Institution: | 1. Institute of Mathematics and Informatics, Bulgarian Academy of Sciences, Acad. G. Bonchev 8, 1113Sofia, Bulgaria.;2. Faculty of Information Sciences, State University of Library Studies and Information Technologies, Shipchenski prohod 69A, 1574Sofia, Bulgaria.;3. Faculty of Mathematics and Informatics, Adam Mickiewicz University, Umultowska 87, 61-001Poznań, Poland. |