(** * Iterated differential *)

(** ** Definition *)

Fixpoint Derive_n (f : R -> R) (n : nat) x :=
  match n with
    | O => f x
    | S n => Derive (Derive_n f n) x
  end.Definition ex_derive_n f n x :=
  match n with
  | O => True
  | S n => ex_derive (Derive_n f n) x
  end.Definition is_derive_n f n x l :=
  match n with
  | O => f x = l
  | S n => is_derive (Derive_n f n) x l
  end.

Lemma is_derive_Sn (f : R -> R) (n : nat) (x l : R) : locally x (ex_derive f) -> (is_derive_n f (S n) x l <-> is_derive_n (Derive f) n x l).