[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 276-0028, Japan;
[3]Departement de Mathématique, Université de Strasbourg, Strasbourg 67084, France;