function popup(page,nom,hauteur_popup,largeur_popup)
        {
        var H = (screen.height - hauteur_popup) / 2;
        var L = (screen.width - largeur_popup) / 2;
        pop_up = window.open(page,nom,"status=yes,scrollbars=yes,resizable=yes,width=900,height=800,top="+H+",left="+L);
        pop_up.focus();
        }