Affiliation: | (1) Department of Mathematics, University of the Aegean, Karlovassi, 832 00 Samos, Greece;(2) M. Sykiotis, Amalthias 18, 412 22 Larissa, Greece;(3) Present address: Department of Mathematics and Statistics, University of Cyprus, P.O. Box 20537, 1678 Nicosia, Cyprus |