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

Lemma CV_disk_le (a : nat -> R) (r1 r2 : R) :
Rabs r1 <= Rabs r2 -> CV_disk a r2 -> CV_disk a r1.