Affiliation: | Institutt for matematiske fag, Norges teknisk-naturvitenskapelige universitet, N-7491 Trondheim, Norway ; Department of Mathematics, University of Leicester, University Road, Leicester LE1 7RH, England ; Institutt for matematiske fag, Norges teknisk-naturvitenskapelige universitet, N-7491 Trondheim, Norway ; Department of Mathematics, Northeastern University, 360 Huntington Avenue, Boston, Massachusetts 02115 |