1. a Dept. of Mathematics , University of Oslo , P. O. Box 1053, Blindern, Oslo, N-0316, Norway
2. b Dept. of Mathematics , University of Oslo , P. O. Box 1053, Blindern, Oslo, N-0316, Norway
3. c Domaine de Voluceau-Rocquencourt , INRIA , B.P. 105, Le Chesnay Cedex, F 78153, France