diff options
Diffstat (limited to 'theories/QArith/Qabs.v')
| -rw-r--r-- | theories/QArith/Qabs.v | 37 |
1 files changed, 36 insertions, 1 deletions
diff --git a/theories/QArith/Qabs.v b/theories/QArith/Qabs.v index 747c2c3c..557fabc8 100644 --- a/theories/QArith/Qabs.v +++ b/theories/QArith/Qabs.v @@ -1,6 +1,6 @@ (************************************************************************) (* v * The Coq Proof Assistant / The Coq Development Team *) -(* <O___,, * INRIA - CNRS - LIX - LRI - PPS - Copyright 1999-2011 *) +(* <O___,, * INRIA - CNRS - LIX - LRI - PPS - Copyright 1999-2010 *) (* \VV/ **************************************************************) (* // * This file is distributed under the terms of the *) (* * GNU Lesser General Public License Version 2.1 *) @@ -100,6 +100,13 @@ rewrite Zabs_Zmult. reflexivity. Qed. +Lemma Qabs_Qminus x y: Qabs (x - y) = Qabs (y - x). +Proof. + unfold Qminus, Qopp. simpl. + rewrite Pmult_comm, <- Zabs_Zopp. + do 2 f_equal. ring. +Qed. + Lemma Qle_Qabs : forall a, a <= Qabs a. Proof. intros a. @@ -122,3 +129,31 @@ apply Qabs_triangle. apply Qabs_wd. ring. Qed. + +Lemma Qabs_Qle_condition x y: Qabs x <= y <-> -y <= x <= y. +Proof. + split. + split. + rewrite <- (Qopp_opp x). + apply Qopp_le_compat. + apply Qle_trans with (Qabs (-x)). + apply Qle_Qabs. + now rewrite Qabs_opp. + apply Qle_trans with (Qabs x); auto using Qle_Qabs. + intros (H,H'). + apply Qabs_case; trivial. + intros. rewrite <- (Qopp_opp y). now apply Qopp_le_compat. +Qed. + +Lemma Qabs_diff_Qle_condition x y r: Qabs (x - y) <= r <-> x - r <= y <= x + r. +Proof. + intros. unfold Qminus. + rewrite Qabs_Qle_condition. + rewrite <- (Qplus_le_l (-r) (x+-y) (y+r)). + rewrite <- (Qplus_le_l (x+-y) r (y-r)). + setoid_replace (-r + (y + r)) with y by ring. + setoid_replace (r + (y - r)) with y by ring. + setoid_replace (x + - y + (y + r)) with (x + r) by ring. + setoid_replace (x + - y + (y - r)) with (x - r) by ring. + intuition. +Qed. |