(** Convergence disk *)
Definition CV_disk (a : nat -> R) (r : R) :=
  ex_series (fun n => Rabs (a n * r^n)).

(* Specifications: *)
Lemma CV_disk_le (a : nat -> R) (r1 r2 : R) : Rabs r1 <= Rabs r2 -> CV_disk a r2 -> CV_disk a r1.
Lemma CV_disk_correct (a : nat -> R) (x : R) : CV_disk a x -> ex_pseries a x.
Lemma CV_disk_0 (a : nat -> R) : CV_disk a 0.