def f(u_n):
    return 3*u_n+1
def seuil_recurrent(n_0,u_0,N):
    n=n_0
    U=u_0
    while U<N:
        n=n+1
        U=f(U)
    return n
def somme_recurrente(debut,fin,u_0):
    s=u_0
    u=u_0
    for i in range(debut,fin):
        u=f(u)
        s=s+u
    return s