(** ** Convergence criteria *)
(** D'Alembert criterion for power series *)

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.