1. Department of Applied Mathematics and Statistics, Comenius University, Mlynská dolina, Bratislava 84248, Slovakia
2. Graduate School of Mathematical Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8914, Japan
3. Department of Applied Mathematics and Informatics, Ryukoku University, Seta, Otsu 520-2194, Japan