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

Lemma CV_disk_DAlembert_aux (a : nat -> R) (x k : R) : x <> 0 -> (forall n, a n <> 0) -> (is_lim_seq (fun n => Rabs (a (S n) / a n)) k <-> is_lim_seq (fun n => Rabs ((a (S n) * x^(S n)) / (a n * x ^ n))) (Rabs x * k)).