(************************************************************************)
(*         *   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)           *)
(*   \VV/  **************************************************************)
(*    //   *    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)         *)
(************************************************************************)
Local Open Scope R_scope.
Fixpoint Dichotomy_lb (x y:R) (P:R -> bool) (N:nat) {struct N} : R :=
match N with
| O => x
| S n =>
let down := Dichotomy_lb x y P n in
let up := Dichotomy_ub x y P n in
let z := (down + up) / 2 in if P z then down else z
end
with Dichotomy_ub (x y:R) (P:R -> bool) (N:nat) {struct N} : R :=
match N with
| O => y
| S n =>
let down := Dichotomy_lb x y P n in
let up := Dichotomy_ub x y P n in
let z := (down + up) / 2 in if P z then z else up
end.
Definition dicho_lb (x y:R) (P:R -> bool) (N:nat) : R := Dichotomy_lb x y P N.
Definition dicho_up (x y:R) (P:R -> bool) (N:nat) : R := Dichotomy_ub x y P N.
Definition pow_2_n (n:nat) := 2 ^ n.
Definition cond_positivity (x:R) : bool :=
match Rle_dec 0 x with
| left _ => true
| right _ => false
end.

(* A general purpose corollary. *)
Lemma cv_pow_half : forall a, Un_cv (fun n => a/2^n) 0.