1. Department of Mathematics and Computer Science, Faculty of Education, University of Ljubljana, Kardeljeva pl. 16, 1000 Ljubljana, Slovenia
2. Department of Mathematics, IAM, University of Primorska, Muzejski trg 2, 6000 Koper, Slovenia
3. Department of Mathematics, FAMNIT, Univerza na Primorskem, Glagoljaška 8, 6000 Koper, Slovenia