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(*         *   The Coq Proof Assistant / The Coq Development Team       *)
(*  v      *   INRIA, CNRS and contributors - Copyright 1999-2018       *)
(* <O___,, *       (see CREDITS file for the list of authors)           *)
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(*    //   *    This file is distributed under the terms of the         *)
(*         *     GNU Lesser General Public License Version 2.1          *)
(*         *     (see LICENSE file for the text of the license)         *)
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Local Open Scope R_scope.
(** Tools *)
Definition pos_half_prf : 0 < /2.
(* The following lemmas are general purpose lemmas about squares. They do not belong here *)
(** * Properties of tangent *)
(** Proof that tangent is a bijection *)
(* to be removed? *)
(* end hide *)
(* The following lemmas about PI should probably be in Rtrigo. *)
Definition frame_tan y : {x | 0 < x < PI/2 /\ Rabs y < tan x}.
Definition pre_atan (y : R) : {x : R | -PI/2 < x < PI/2 /\ tan x = y}.
Definition atan x := let (v, _) := pre_atan x in v.
(** atan's derivative value is the function 1 / (1+x²) *)
(** * Definition of the arctangent function as the sum of the arctan power series *)
(* Proof taken from Guillaume Melquiond's interval package for Coq *)
Definition Ratan_seq x :=  fun n => (x ^ (2 * n + 1) / INR (2 * n + 1))%R.

Lemma Ratan_seq_decreasing : forall x, (0 <= x <= 1)%R -> Un_decreasing (Ratan_seq x).