Affiliation: | aDepartment of Mathematics, University of Oslo, P.O. Box 1053 Blindern, NO-0316 Oslo, Norway;bInstitut de Mathématiques de Jussieu, Université Paris 7 Denis Dedirot, France;cFaculty of Engineering, Oslo University College, P.O. Box 4 St. Olavs plass, NO-0130 Oslo, Norway |