A Morame, Universit  de Nantes, 44322 Nantes Cedex 3, France. 
email: morame@math.univ-nantes.fr 
 
F Truc, Institut Fourier, 38402 Saint Martin d'H res Cedex , France.  
email: Francoise.Truc@ujf-grenoble.fr 
 
Mathematical Subject Classification: 35P20