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

Lemma CV_disk_DAlembert (a : nat -> R) (x:R) l : (forall n:nat, a n <> 0) -> is_lim_seq (fun n:nat => Rabs (a (S n) / a n)) (Finite l) -> (l = 0 \/ (l <> 0 /\ Rabs x < / l)) -> CV_disk a x.