1. Le Pont de l’Etang 8, 1323 Romainmôtier, Switzerland
2. Institute of Mathematics, Pedagogical University of Cracow, Podchora¸żych 2, PL-30-084 Kraków, Poland
3. Institute of Mathematics, Faculty of Mathematics and Computer Science, Jagiellonian University, Łojasiewicza 6, 30-348 Kraków, Poland