1. National Technical UniversityDepartment of MathematicsZografou Campus Athens 15780 Greece
2. Institute of MathematicsPhysics and Mechanics1000 Ljubljana Slovenia
3. Faculty of Applied MathematicsAGH University of Science and Technology30‐059 Kraków Poland
4. Institute of Mathematics“Simion Stoilow” of the Romanian AcademyP.O. Box 1‐764 014700 Bucharest Romania
5. Faculty of Education and Faculty of Mathematics and PhysicsUniversity of Ljubljana1000 Ljubljana Slovenia