Affiliation: | (1) School of Mathematics, Cardiff University, Senghennydd Road, Cardiff, CF24 4YH, England;(2) Institute of Mathematics, Academy of Sciences of the Czech Republic, Žitná 25, 115 67 Prague 1, Czech Republic;(3) Department of Mathematics and Didactics, Pedagogical Faculty, Technical University of Liberec, Hálkova 6, 461 17 Liberec, Czech Republic |