1. Department of Mathematics, Faculty of Nuclear Sciences and Physical EngineeringCzech Technical University in Prague Trojanova 13, Prague, 12000 Czech Republic
2. Department of Mathematics, School of Science and TechnologyMeiji University 1‐1‐1 Higashi‐Mita, Tama‐ku, Kawasaki‐shi, Kanagawa 214‐8571 Japan
3. Department of Applied Mathematics and Statistics, Faculty of Mathematics Physics and InformaticsComenius University in Bratislava Bratislava Slovakia