Affiliation: | (1) Department of Mathematics, University of Umea, S-90187 Umea, Sweden;(2) Department of Mathematics, Mid Sweden University, S-85 170 Sundsvall, Sweden;(3) Institute of Mathematics, Jagiellonian University, Reymonta 4, 30-059 Kraków, Poland;(4) Institut de Mathématiques, Université Paul Sabatier, 118 Route de Narbonne, 31062 Toulouse cedex, France |