1Many thanks to Henri Cohen who started writing this program.
2After this program is available, many people pointed out to
3me (mathematical as well as programming) bugs :
4B. Poonen, E. Schaefer, C. Stahlke, M. Stoll, F. Villegas.
5So thanks to all of them. Thanks also go to Ph. Depouilly
6who help me to compile the program.