1. Department of Applied Mathematics and Statistics, Comenius University, 84248 Bratislava, Slovakia. E-mails: fila@fmph.uniba.sk, jlankeit@math.upb.de
2. Graduate School of Mathematical Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8914, Japan. E-mail: ishige@ms.u-tokyo.ac.jp
3. Department of Applied Mathematics and Informatics, Ryukoku University, Seta Otsu 520-2194, Japan. E-mail: kawakami@math.ryukoku.ac.jp