1. Simion Stoilow Institute of Mathematics of the Romanian Academy, Research unit nr. 5, P. O. Box 1-764, RO-014700 Bucharest, Romania
2. Department of Mathematics, College of Industrial Technology, Nihon University, 2-11-1 Shin-ei, Narashino, Chiba, Japan
3. Division of Pure and Applied Science, Faculty of Science and Technology, Gunma University, 1-5-1 Tenjin-cho, Kiryu, Gunma, Japan