Dear Guys,

This is my contribution for the proccedings of Copromaph3. It is not quite finished : 
- The text must be slightly shortened (maybe I should withdraw the proof of Prop 4.2)
- The bibliography must be reduced to the cited.

I would be glaad to have your remark/comments, but the time turns short as the texte is due 
to the 15th of Jan. (deadline already extended).

Best wishes fom
Gérard

PS : The format is given by the Latex2e package with the 
world Scientific Publishing style files (These style files may be downloaded 
from the 
http://www.wspc.com/others/style_files/proceedings/proceedings_style_files.shtml
. The trim size 9.00" x 6.00". Latex2e set of files  is the appropriate one), 
tell me if you have difficulties.