Definition CV_radius (a : nat -> R) : Rbar :=
  Lub_Rbar (CV_disk a).

Lemma CV_radius_finite_DAlembert (a : nat -> R) (l : R) : (forall n:nat, a n <> 0) -> 0 < l -> is_lim_seq (fun n:nat => Rabs (a (S n) / a n)) l -> CV_radius a = Finite (/l).