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

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