Affiliation: | 1 Institute of Mathematics, University of Debrecen, H-4010 Debrecen, P.O. Box 12, Hungary 2 Department of Mathematics, Technical University of Cluj-Napoca, Str. Gh. Bari?iu, nr. 25, Cluj-Napoca 3400, Romania 3 Hokkaido Tokai University, 5-1-1-1 Minamisawa, Minami-ku, Sapporo 005-8601, Japan |