(** ** Derivative of arctangent *)

(* Specifications: *)
Lemma derive_pt_atan : forall x,
derive_pt atan x (derivable_pt_atan x) = 1 / (1 + x²).
Lemma derivable_pt_lim_atan : forall x,
derivable_pt_lim atan x (/ (1 + x^2)).