Library compcert.flocq.Core.Fcore_ulp
This file is part of the Flocq formalization of floating-point
arithmetic in Coq: http://flocq.gforge.inria.fr/
Copyright (C) 2010-2013 Sylvie Boldo
Copyright (C) 2010-2013 Guillaume Melquiond
This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 3 of the License, or (at your option) any later version.
This library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
COPYING file for more details.
Copyright (C) 2010-2013 Guillaume Melquiond
Unit in the Last Place: our definition using fexp and its properties, successor and predecessor
Require Import Fcore_Raux.
Require Import Fcore_defs.
Require Import Fcore_rnd.
Require Import Fcore_generic_fmt.
Require Import Fcore_float_prop.
Section Fcore_ulp.
Variable beta : radix.
Notation bpow e := (bpow beta e).
Variable fexp : Z → Z.
Require Import Fcore_defs.
Require Import Fcore_rnd.
Require Import Fcore_generic_fmt.
Require Import Fcore_float_prop.
Section Fcore_ulp.
Variable beta : radix.
Notation bpow e := (bpow beta e).
Variable fexp : Z → Z.
Definition and basic properties about the minimal exponent, when it exists
Lemma Z_le_dec_aux: ∀ x y : Z, (x ≤ y)%Z ∨ ¬ (x ≤ y)%Z.
Proof.
intros.
destruct (Z_le_dec x y).
now left.
now right.
Qed.
negligible_exp is either none (as in FLX) or Some n such that n <= fexp n.
Definition negligible_exp: option Z :=
match (LPO_Z _ (fun z ⇒ Z_le_dec_aux z (fexp z))) with
| inleft N ⇒ Some (proj1_sig N)
| inright _ ⇒ None
end.
Inductive negligible_exp_prop: option Z → Prop :=
| negligible_None: (∀ n, (fexp n < n)%Z) → negligible_exp_prop None
| negligible_Some: ∀ n, (n ≤ fexp n)%Z → negligible_exp_prop (Some n).
Lemma negligible_exp_spec: negligible_exp_prop negligible_exp.
Proof.
unfold negligible_exp; destruct LPO_Z as [(n,Hn)|Hn].
now apply negligible_Some.
apply negligible_None.
intros n; specialize (Hn n); omega.
Qed.
Lemma negligible_exp_spec': (negligible_exp = None ∧ ∀ n, (fexp n < n)%Z)
∨ ∃ n, (negligible_exp = Some n ∧ (n ≤ fexp n)%Z).
Proof.
unfold negligible_exp; destruct LPO_Z as [(n,Hn)|Hn].
right; simpl; ∃ n; now split.
left; split; trivial.
intros n; specialize (Hn n); omega.
Qed.
Context { valid_exp : Valid_exp fexp }.
Lemma fexp_negligible_exp_eq: ∀ n m, (n ≤ fexp n)%Z → (m ≤ fexp m)%Z → fexp n = fexp m.
Proof.
intros n m Hn Hm.
case (Zle_or_lt n m); intros H.
apply valid_exp; omega.
apply sym_eq, valid_exp; omega.
Qed.
match (LPO_Z _ (fun z ⇒ Z_le_dec_aux z (fexp z))) with
| inleft N ⇒ Some (proj1_sig N)
| inright _ ⇒ None
end.
Inductive negligible_exp_prop: option Z → Prop :=
| negligible_None: (∀ n, (fexp n < n)%Z) → negligible_exp_prop None
| negligible_Some: ∀ n, (n ≤ fexp n)%Z → negligible_exp_prop (Some n).
Lemma negligible_exp_spec: negligible_exp_prop negligible_exp.
Proof.
unfold negligible_exp; destruct LPO_Z as [(n,Hn)|Hn].
now apply negligible_Some.
apply negligible_None.
intros n; specialize (Hn n); omega.
Qed.
Lemma negligible_exp_spec': (negligible_exp = None ∧ ∀ n, (fexp n < n)%Z)
∨ ∃ n, (negligible_exp = Some n ∧ (n ≤ fexp n)%Z).
Proof.
unfold negligible_exp; destruct LPO_Z as [(n,Hn)|Hn].
right; simpl; ∃ n; now split.
left; split; trivial.
intros n; specialize (Hn n); omega.
Qed.
Context { valid_exp : Valid_exp fexp }.
Lemma fexp_negligible_exp_eq: ∀ n m, (n ≤ fexp n)%Z → (m ≤ fexp m)%Z → fexp n = fexp m.
Proof.
intros n m Hn Hm.
case (Zle_or_lt n m); intros H.
apply valid_exp; omega.
apply sym_eq, valid_exp; omega.
Qed.
Definition and basic properties about the ulp Now includes a nice ulp(0): ulp(0) is now 0 when there is no minimal
exponent, such as in FLX, and beta^(fexp n) when there is a n such
that n <= fexp n. For instance, the value of ulp(O) is then
beta^emin in FIX and FLT. The main lemma to use is ulp_neq_0 that
is equivalent to the previous "unfold ulp" provided the value is
not zero.
Definition ulp x := match Req_bool x 0 with
| true ⇒ match negligible_exp with
| Some n ⇒ bpow (fexp n)
| None ⇒ 0%R
end
| false ⇒ bpow (canonic_exp beta fexp x)
end.
Lemma ulp_neq_0 : ∀ x:R, (x ≠ 0)%R → ulp x = bpow (canonic_exp beta fexp x).
Proof.
intros x Hx.
unfold ulp; case (Req_bool_spec x); trivial.
intros H; now contradict H.
Qed.
Notation F := (generic_format beta fexp).
Theorem ulp_opp :
∀ x, ulp (- x) = ulp x.
Proof.
intros x.
unfold ulp.
case Req_bool_spec; intros H1.
rewrite Req_bool_true; trivial.
rewrite <- (Ropp_involutive x), H1; ring.
rewrite Req_bool_false.
now rewrite canonic_exp_opp.
intros H2; apply H1; rewrite H2; ring.
Qed.
Theorem ulp_abs :
∀ x, ulp (Rabs x) = ulp x.
Proof.
intros x.
unfold ulp; case (Req_bool_spec x 0); intros H1.
rewrite Req_bool_true; trivial.
now rewrite H1, Rabs_R0.
rewrite Req_bool_false.
now rewrite canonic_exp_abs.
now apply Rabs_no_R0.
Qed.
Theorem ulp_ge_0:
∀ x, (0 ≤ ulp x)%R.
Proof.
intros x; unfold ulp; case Req_bool_spec; intros.
case negligible_exp; intros.
apply bpow_ge_0.
apply Rle_refl.
apply bpow_ge_0.
Qed.
Theorem ulp_le_id:
∀ x,
(0 < x)%R →
F x →
(ulp x ≤ x)%R.
Proof.
intros x Zx Fx.
rewrite <- (Rmult_1_l (ulp x)).
pattern x at 2; rewrite Fx.
rewrite ulp_neq_0.
2: now apply Rgt_not_eq.
unfold F2R; simpl.
apply Rmult_le_compat_r.
apply bpow_ge_0.
apply (Z2R_le (Zsucc 0)).
apply Zlt_le_succ.
apply F2R_gt_0_reg with beta (canonic_exp beta fexp x).
now rewrite <- Fx.
Qed.
Theorem ulp_le_abs:
∀ x,
(x ≠ 0)%R →
F x →
(ulp x ≤ Rabs x)%R.
Proof.
intros x Zx Fx.
rewrite <- ulp_abs.
apply ulp_le_id.
now apply Rabs_pos_lt.
now apply generic_format_abs.
Qed.
Theorem round_UP_DN_ulp :
∀ x, ¬ F x →
round beta fexp Zceil x = (round beta fexp Zfloor x + ulp x)%R.
Proof.
intros x Fx.
rewrite ulp_neq_0.
unfold round. simpl.
unfold F2R. simpl.
rewrite Zceil_floor_neq.
rewrite Z2R_plus. simpl.
ring.
intros H.
apply Fx.
unfold generic_format, F2R. simpl.
rewrite <- H.
rewrite Ztrunc_Z2R.
rewrite H.
now rewrite scaled_mantissa_mult_bpow.
intros V; apply Fx.
rewrite V.
apply generic_format_0.
Qed.
Theorem ulp_bpow :
∀ e, ulp (bpow e) = bpow (fexp (e + 1)).
Proof.
intros e.
rewrite ulp_neq_0.
apply f_equal.
apply canonic_exp_fexp.
rewrite Rabs_pos_eq.
split.
ring_simplify (e + 1 - 1)%Z.
apply Rle_refl.
apply bpow_lt.
apply Zlt_succ.
apply bpow_ge_0.
apply Rgt_not_eq, Rlt_gt, bpow_gt_0.
Qed.
Lemma generic_format_ulp_0:
F (ulp 0).
Proof.
unfold ulp.
rewrite Req_bool_true; trivial.
case negligible_exp_spec.
intros _; apply generic_format_0.
intros n H1.
apply generic_format_bpow.
now apply valid_exp.
Qed.
Lemma generic_format_bpow_ge_ulp_0: ∀ e,
(ulp 0 ≤ bpow e)%R → F (bpow e).
Proof.
intros e; unfold ulp.
rewrite Req_bool_true; trivial.
case negligible_exp_spec.
intros H1 _.
apply generic_format_bpow.
specialize (H1 (e+1)%Z); omega.
intros n H1 H2.
apply generic_format_bpow.
case (Zle_or_lt (e+1) (fexp (e+1))); intros H4.
absurd (e+1 ≤ e)%Z.
omega.
apply Zle_trans with (1:=H4).
replace (fexp (e+1)) with (fexp n).
now apply le_bpow with beta.
now apply fexp_negligible_exp_eq.
omega.
Qed.
The three following properties are equivalent:
Exp_not_FTZ ; forall x, F (ulp x) ; forall x, ulp 0 <= ulp x
Lemma generic_format_ulp: Exp_not_FTZ fexp →
∀ x, F (ulp x).
Proof.
unfold Exp_not_FTZ; intros H x.
case (Req_dec x 0); intros Hx.
rewrite Hx; apply generic_format_ulp_0.
rewrite (ulp_neq_0 _ Hx).
apply generic_format_bpow; unfold canonic_exp.
apply H.
Qed.
Lemma not_FTZ_generic_format_ulp:
(∀ x, F (ulp x)) → Exp_not_FTZ fexp.
intros H e.
specialize (H (bpow (e-1))).
rewrite ulp_neq_0 in H.
2: apply Rgt_not_eq, bpow_gt_0.
unfold canonic_exp in H.
rewrite ln_beta_bpow in H.
apply generic_format_bpow_inv' in H...
now replace (e-1+1)%Z with e in H by ring.
Qed.
Lemma ulp_ge_ulp_0: Exp_not_FTZ fexp →
∀ x, (ulp 0 ≤ ulp x)%R.
Proof.
unfold Exp_not_FTZ; intros H x.
case (Req_dec x 0); intros Hx.
rewrite Hx; now right.
unfold ulp at 1.
rewrite Req_bool_true; trivial.
case negligible_exp_spec'.
intros (H1,H2); rewrite H1; apply ulp_ge_0.
intros (n,(H1,H2)); rewrite H1.
rewrite ulp_neq_0; trivial.
apply bpow_le; unfold canonic_exp.
generalize (ln_beta beta x); intros l.
case (Zle_or_lt l (fexp l)); intros Hl.
rewrite (fexp_negligible_exp_eq n l); trivial; apply Zle_refl.
case (Zle_or_lt (fexp n) (fexp l)); trivial; intros K.
absurd (fexp n ≤ fexp l)%Z.
omega.
apply Zle_trans with (2:= H _).
apply Zeq_le, sym_eq, valid_exp; trivial.
omega.
Qed.
Lemma not_FTZ_ulp_ge_ulp_0:
(∀ x, (ulp 0 ≤ ulp x)%R) → Exp_not_FTZ fexp.
Proof.
intros H e.
apply generic_format_bpow_inv' with beta.
apply generic_format_bpow_ge_ulp_0.
replace e with ((e-1)+1)%Z by ring.
rewrite <- ulp_bpow.
apply H.
Qed.
Theorem ulp_le_pos :
∀ { Hm : Monotone_exp fexp },
∀ x y: R,
(0 ≤ x)%R → (x ≤ y)%R →
(ulp x ≤ ulp y)%R.
Proof with auto with typeclass_instances.
intros Hm x y Hx Hxy.
destruct Hx as [Hx|Hx].
rewrite ulp_neq_0.
rewrite ulp_neq_0.
apply bpow_le.
apply Hm.
now apply ln_beta_le.
apply Rgt_not_eq, Rlt_gt.
now apply Rlt_le_trans with (1:=Hx).
now apply Rgt_not_eq.
rewrite <- Hx.
apply ulp_ge_ulp_0.
apply monotone_exp_not_FTZ...
Qed.
Theorem ulp_le :
∀ { Hm : Monotone_exp fexp },
∀ x y: R,
(Rabs x ≤ Rabs y)%R →
(ulp x ≤ ulp y)%R.
Proof.
intros Hm x y Hxy.
rewrite <- ulp_abs.
rewrite <- (ulp_abs y).
apply ulp_le_pos; trivial.
apply Rabs_pos.
Qed.
Definition and properties of pred and succ
Definition pred_pos x :=
if Req_bool x (bpow (ln_beta beta x - 1)) then
(x - bpow (fexp (ln_beta beta x - 1)))%R
else
(x - ulp x)%R.
Definition succ x :=
if (Rle_bool 0 x) then
(x+ulp x)%R
else
(- pred_pos (-x))%R.
Definition pred x := (- succ (-x))%R.
Theorem pred_eq_pos:
∀ x, (0 ≤ x)%R → (pred x = pred_pos x)%R.
Proof.
intros x Hx; unfold pred, succ.
case Rle_bool_spec; intros Hx'.
assert (K:(x = 0)%R).
apply Rle_antisym; try assumption.
apply Ropp_le_cancel.
now rewrite Ropp_0.
rewrite K; unfold pred_pos.
rewrite Req_bool_false.
2: apply Rlt_not_eq, bpow_gt_0.
rewrite Ropp_0; ring.
now rewrite 2!Ropp_involutive.
Qed.
Theorem succ_eq_pos:
∀ x, (0 ≤ x)%R → (succ x = x + ulp x)%R.
Proof.
intros x Hx; unfold succ.
now rewrite Rle_bool_true.
Qed.
Lemma pred_eq_opp_succ_opp: ∀ x, pred x = (- succ (-x))%R.
Proof.
reflexivity.
Qed.
Lemma succ_eq_opp_pred_opp: ∀ x, succ x = (- pred (-x))%R.
Proof.
intros x; unfold pred.
now rewrite 2!Ropp_involutive.
Qed.
Lemma succ_opp: ∀ x, (succ (-x) = - pred x)%R.
Proof.
intros x; rewrite succ_eq_opp_pred_opp.
now rewrite Ropp_involutive.
Qed.
Lemma pred_opp: ∀ x, (pred (-x) = - succ x)%R.
Proof.
intros x; rewrite pred_eq_opp_succ_opp.
now rewrite Ropp_involutive.
Qed.
pred and succ are in the format
Theorem id_m_ulp_ge_bpow :
∀ x e, F x →
x ≠ ulp x →
(bpow e < x)%R →
(bpow e ≤ x - ulp x)%R.
Proof.
intros x e Fx Hx' Hx.
assert (1 ≤ Ztrunc (scaled_mantissa beta fexp x))%Z.
assert (0 < Ztrunc (scaled_mantissa beta fexp x))%Z.
apply F2R_gt_0_reg with beta (canonic_exp beta fexp x).
rewrite <- Fx.
apply Rle_lt_trans with (2:=Hx).
apply bpow_ge_0.
omega.
case (Zle_lt_or_eq _ _ H); intros Hm.
pattern x at 1 ; rewrite Fx.
rewrite ulp_neq_0.
unfold F2R. simpl.
pattern (bpow (canonic_exp beta fexp x)) at 2 ; rewrite <- Rmult_1_l.
rewrite <- Rmult_minus_distr_r.
change 1%R with (Z2R 1).
rewrite <- Z2R_minus.
change (bpow e ≤ F2R (Float beta (Ztrunc (scaled_mantissa beta fexp x) - 1) (canonic_exp beta fexp x)))%R.
apply bpow_le_F2R_m1; trivial.
now rewrite <- Fx.
apply Rgt_not_eq, Rlt_gt.
apply Rlt_trans with (2:=Hx), bpow_gt_0.
contradict Hx'.
pattern x at 1; rewrite Fx.
rewrite <- Hm.
rewrite ulp_neq_0.
unfold F2R; simpl.
now rewrite Rmult_1_l.
apply Rgt_not_eq, Rlt_gt.
apply Rlt_trans with (2:=Hx), bpow_gt_0.
Qed.
Theorem id_p_ulp_le_bpow :
∀ x e, (0 < x)%R → F x →
(x < bpow e)%R →
(x + ulp x ≤ bpow e)%R.
Proof.
intros x e Zx Fx Hx.
pattern x at 1 ; rewrite Fx.
rewrite ulp_neq_0.
unfold F2R. simpl.
pattern (bpow (canonic_exp beta fexp x)) at 2 ; rewrite <- Rmult_1_l.
rewrite <- Rmult_plus_distr_r.
change 1%R with (Z2R 1).
rewrite <- Z2R_plus.
change (F2R (Float beta (Ztrunc (scaled_mantissa beta fexp x) + 1) (canonic_exp beta fexp x)) ≤ bpow e)%R.
apply F2R_p1_le_bpow.
apply F2R_gt_0_reg with beta (canonic_exp beta fexp x).
now rewrite <- Fx.
now rewrite <- Fx.
now apply Rgt_not_eq.
Qed.
Lemma generic_format_pred_aux1:
∀ x, (0 < x)%R → F x →
x ≠ bpow (ln_beta beta x - 1) →
F (x - ulp x).
Proof.
intros x Zx Fx Hx.
destruct (ln_beta beta x) as (ex, Ex).
simpl in Hx.
specialize (Ex (Rgt_not_eq _ _ Zx)).
assert (Ex' : (bpow (ex - 1) < x < bpow ex)%R).
rewrite Rabs_pos_eq in Ex.
destruct Ex as (H,H'); destruct H; split; trivial.
contradict Hx; easy.
now apply Rlt_le.
unfold generic_format, scaled_mantissa, canonic_exp.
rewrite ln_beta_unique with beta (x - ulp x)%R ex.
pattern x at 1 3 ; rewrite Fx.
rewrite ulp_neq_0.
unfold scaled_mantissa.
rewrite canonic_exp_fexp with (1 := Ex).
unfold F2R. simpl.
rewrite Rmult_minus_distr_r.
rewrite Rmult_assoc.
rewrite <- bpow_plus, Zplus_opp_r, Rmult_1_r.
change (bpow 0) with (Z2R 1).
rewrite <- Z2R_minus.
rewrite Ztrunc_Z2R.
rewrite Z2R_minus.
rewrite Rmult_minus_distr_r.
now rewrite Rmult_1_l.
now apply Rgt_not_eq.
rewrite Rabs_pos_eq.
split.
apply id_m_ulp_ge_bpow; trivial.
rewrite ulp_neq_0.
intro H.
assert (ex-1 < canonic_exp beta fexp x < ex)%Z.
split ; apply (lt_bpow beta) ; rewrite <- H ; easy.
clear -H0. omega.
now apply Rgt_not_eq.
apply Ex'.
apply Rle_lt_trans with (2 := proj2 Ex').
pattern x at 3 ; rewrite <- Rplus_0_r.
apply Rplus_le_compat_l.
rewrite <-Ropp_0.
apply Ropp_le_contravar.
apply ulp_ge_0.
apply Rle_0_minus.
pattern x at 2; rewrite Fx.
rewrite ulp_neq_0.
unfold F2R; simpl.
pattern (bpow (canonic_exp beta fexp x)) at 1; rewrite <- Rmult_1_l.
apply Rmult_le_compat_r.
apply bpow_ge_0.
replace 1%R with (Z2R 1) by reflexivity.
apply Z2R_le.
assert (0 < Ztrunc (scaled_mantissa beta fexp x))%Z.
apply F2R_gt_0_reg with beta (canonic_exp beta fexp x).
rewrite <- Fx.
apply Rle_lt_trans with (2:=proj1 Ex').
apply bpow_ge_0.
omega.
now apply Rgt_not_eq.
Qed.
Lemma generic_format_pred_aux2 :
∀ x, (0 < x)%R → F x →
let e := ln_beta_val beta x (ln_beta beta x) in
x = bpow (e - 1) →
F (x - bpow (fexp (e - 1))).
Proof.
intros x Zx Fx e Hx.
pose (f:=(x - bpow (fexp (e - 1)))%R).
fold f.
assert (He:(fexp (e-1) ≤ e-1)%Z).
apply generic_format_bpow_inv with beta; trivial.
rewrite <- Hx; assumption.
case (Zle_lt_or_eq _ _ He); clear He; intros He.
assert (f = F2R (Float beta (Zpower beta (e-1-(fexp (e-1))) -1) (fexp (e-1))))%R.
unfold f; rewrite Hx.
unfold F2R; simpl.
rewrite Z2R_minus, Z2R_Zpower.
rewrite Rmult_minus_distr_r, Rmult_1_l.
rewrite <- bpow_plus.
now replace (e - 1 - fexp (e - 1) + fexp (e - 1))%Z with (e-1)%Z by ring.
omega.
rewrite H.
apply generic_format_F2R.
intros _.
apply Zeq_le.
apply canonic_exp_fexp.
rewrite <- H.
unfold f; rewrite Hx.
rewrite Rabs_right.
split.
apply Rplus_le_reg_l with (bpow (fexp (e-1))).
ring_simplify.
apply Rle_trans with (bpow (e - 2) + bpow (e - 2))%R.
apply Rplus_le_compat ; apply bpow_le ; omega.
apply Rle_trans with (2×bpow (e - 2))%R;[right; ring|idtac].
apply Rle_trans with (bpow 1×bpow (e - 2))%R.
apply Rmult_le_compat_r.
apply bpow_ge_0.
replace 2%R with (Z2R 2) by reflexivity.
replace (bpow 1) with (Z2R beta).
apply Z2R_le.
apply <- Zle_is_le_bool.
now destruct beta.
simpl.
unfold Zpower_pos; simpl.
now rewrite Zmult_1_r.
rewrite <- bpow_plus.
replace (1+(e-2))%Z with (e-1)%Z by ring.
now right.
rewrite <- Rplus_0_r.
apply Rplus_lt_compat_l.
rewrite <- Ropp_0.
apply Ropp_lt_contravar.
apply bpow_gt_0.
apply Rle_ge; apply Rle_0_minus.
apply bpow_le.
omega.
replace f with 0%R.
apply generic_format_0.
unfold f.
rewrite Hx, He.
ring.
Qed.
Theorem generic_format_succ_aux1 :
∀ x, (0 < x)%R → F x →
F (x + ulp x).
Proof.
intros x Zx Fx.
destruct (ln_beta beta x) as (ex, Ex).
specialize (Ex (Rgt_not_eq _ _ Zx)).
assert (Ex' := Ex).
rewrite Rabs_pos_eq in Ex'.
destruct (id_p_ulp_le_bpow x ex) ; try easy.
unfold generic_format, scaled_mantissa, canonic_exp.
rewrite ln_beta_unique with beta (x + ulp x)%R ex.
pattern x at 1 3 ; rewrite Fx.
rewrite ulp_neq_0.
unfold scaled_mantissa.
rewrite canonic_exp_fexp with (1 := Ex).
unfold F2R. simpl.
rewrite Rmult_plus_distr_r.
rewrite Rmult_assoc.
rewrite <- bpow_plus, Zplus_opp_r, Rmult_1_r.
change (bpow 0) with (Z2R 1).
rewrite <- Z2R_plus.
rewrite Ztrunc_Z2R.
rewrite Z2R_plus.
rewrite Rmult_plus_distr_r.
now rewrite Rmult_1_l.
now apply Rgt_not_eq.
rewrite Rabs_pos_eq.
split.
apply Rle_trans with (1 := proj1 Ex').
pattern x at 1 ; rewrite <- Rplus_0_r.
apply Rplus_le_compat_l.
apply ulp_ge_0.
exact H.
apply Rplus_le_le_0_compat.
now apply Rlt_le.
apply ulp_ge_0.
rewrite H.
apply generic_format_bpow.
apply valid_exp.
destruct (Zle_or_lt ex (fexp ex)) ; trivial.
elim Rlt_not_le with (1 := Zx).
rewrite Fx.
replace (Ztrunc (scaled_mantissa beta fexp x)) with Z0.
rewrite F2R_0.
apply Rle_refl.
unfold scaled_mantissa.
rewrite canonic_exp_fexp with (1 := Ex).
destruct (mantissa_small_pos beta fexp x ex) ; trivial.
rewrite Ztrunc_floor.
apply sym_eq.
apply Zfloor_imp.
split.
now apply Rlt_le.
exact H2.
now apply Rlt_le.
now apply Rlt_le.
Qed.
Theorem generic_format_pred_pos :
∀ x, F x → (0 < x)%R →
F (pred_pos x).
Proof.
intros x Fx Zx.
unfold pred_pos; case Req_bool_spec; intros H.
now apply generic_format_pred_aux2.
now apply generic_format_pred_aux1.
Qed.
Theorem generic_format_succ :
∀ x, F x →
F (succ x).
Proof.
intros x Fx.
unfold succ; case Rle_bool_spec; intros Zx.
destruct Zx as [Zx|Zx].
now apply generic_format_succ_aux1.
rewrite <- Zx, Rplus_0_l.
apply generic_format_ulp_0.
apply generic_format_opp.
apply generic_format_pred_pos.
now apply generic_format_opp.
now apply Ropp_0_gt_lt_contravar.
Qed.
Theorem generic_format_pred :
∀ x, F x →
F (pred x).
Proof.
intros x Fx.
unfold pred.
apply generic_format_opp.
apply generic_format_succ.
now apply generic_format_opp.
Qed.
Theorem pred_pos_lt_id :
∀ x, (x ≠ 0)%R →
(pred_pos x < x)%R.
Proof.
intros x Zx.
unfold pred_pos.
case Req_bool_spec; intros H.
rewrite <- Rplus_0_r.
apply Rplus_lt_compat_l.
rewrite <- Ropp_0.
apply Ropp_lt_contravar.
apply bpow_gt_0.
rewrite <- Rplus_0_r.
apply Rplus_lt_compat_l.
rewrite <- Ropp_0.
apply Ropp_lt_contravar.
rewrite ulp_neq_0; trivial.
apply bpow_gt_0.
Qed.
Theorem succ_gt_id :
∀ x, (x ≠ 0)%R →
(x < succ x)%R.
Proof.
intros x Zx; unfold succ.
case Rle_bool_spec; intros Hx.
pattern x at 1; rewrite <- (Rplus_0_r x).
apply Rplus_lt_compat_l.
rewrite ulp_neq_0; trivial.
apply bpow_gt_0.
pattern x at 1; rewrite <- (Ropp_involutive x).
apply Ropp_lt_contravar.
apply pred_pos_lt_id.
now auto with real.
Qed.
Theorem pred_lt_id :
∀ x, (x ≠ 0)%R →
(pred x < x)%R.
Proof.
intros x Zx; unfold pred.
pattern x at 2; rewrite <- (Ropp_involutive x).
apply Ropp_lt_contravar.
apply succ_gt_id.
now auto with real.
Qed.
Theorem succ_ge_id :
∀ x, (x ≤ succ x)%R.
Proof.
intros x; case (Req_dec x 0).
intros V; rewrite V.
unfold succ; rewrite Rle_bool_true;[idtac|now right].
rewrite Rplus_0_l; apply ulp_ge_0.
intros; left; now apply succ_gt_id.
Qed.
Theorem pred_le_id :
∀ x, (pred x ≤ x)%R.
Proof.
intros x; unfold pred.
pattern x at 2; rewrite <- (Ropp_involutive x).
apply Ropp_le_contravar.
apply succ_ge_id.
Qed.
Theorem pred_pos_ge_0 :
∀ x,
(0 < x)%R → F x → (0 ≤ pred_pos x)%R.
Proof.
intros x Zx Fx.
unfold pred_pos.
case Req_bool_spec; intros H.
apply Rle_0_minus.
rewrite H.
apply bpow_le.
destruct (ln_beta beta x) as (ex,Ex) ; simpl.
rewrite ln_beta_bpow.
ring_simplify (ex - 1 + 1 - 1)%Z.
apply generic_format_bpow_inv with beta; trivial.
simpl in H.
rewrite <- H; assumption.
apply Rle_0_minus.
now apply ulp_le_id.
Qed.
Theorem pred_ge_0 :
∀ x,
(0 < x)%R → F x → (0 ≤ pred x)%R.
Proof.
intros x Zx Fx.
rewrite pred_eq_pos.
now apply pred_pos_ge_0.
now left.
Qed.
Lemma pred_pos_plus_ulp_aux1 :
∀ x, (0 < x)%R → F x →
x ≠ bpow (ln_beta beta x - 1) →
((x - ulp x) + ulp (x-ulp x) = x)%R.
Proof.
intros x Zx Fx Hx.
replace (ulp (x - ulp x)) with (ulp x).
ring.
assert (H:(x ≠ 0)%R) by auto with real.
assert (H':(x ≠ bpow (canonic_exp beta fexp x))%R).
unfold canonic_exp; intros M.
case_eq (ln_beta beta x); intros ex Hex T.
assert (Lex:(ln_beta_val beta x (ln_beta beta x) = ex)%Z).
rewrite T; reflexivity.
rewrite Lex in ×.
clear T; simpl in *; specialize (Hex H).
rewrite Rabs_right in Hex.
2: apply Rle_ge; apply Rlt_le; easy.
assert (ex-1 < fexp ex < ex)%Z.
split ; apply (lt_bpow beta); rewrite <- M;[idtac|easy].
destruct (proj1 Hex);[trivial|idtac].
contradict Hx; auto with real.
omega.
rewrite 2!ulp_neq_0; try auto with real.
apply f_equal.
unfold canonic_exp; apply f_equal.
case_eq (ln_beta beta x); intros ex Hex T.
assert (Lex:(ln_beta_val beta x (ln_beta beta x) = ex)%Z).
rewrite T; reflexivity.
rewrite Lex in *; simpl in *; clear T.
specialize (Hex H).
apply sym_eq, ln_beta_unique.
rewrite Rabs_right.
rewrite Rabs_right in Hex.
2: apply Rle_ge; apply Rlt_le; easy.
split.
destruct Hex as ([H1|H1],H2).
apply Rle_trans with (x-ulp x)%R.
apply id_m_ulp_ge_bpow; trivial.
rewrite ulp_neq_0; trivial.
rewrite ulp_neq_0; trivial.
right; unfold canonic_exp; now rewrite Lex.
contradict Hx; auto with real.
apply Rle_lt_trans with (2:=proj2 Hex).
rewrite <- Rplus_0_r.
apply Rplus_le_compat_l.
rewrite <- Ropp_0.
apply Ropp_le_contravar.
apply bpow_ge_0.
apply Rle_ge.
apply Rle_0_minus.
rewrite Fx.
unfold F2R, canonic_exp; simpl.
rewrite Lex.
pattern (bpow (fexp ex)) at 1; rewrite <- Rmult_1_l.
apply Rmult_le_compat_r.
apply bpow_ge_0.
replace 1%R with (Z2R (Zsucc 0)) by reflexivity.
apply Z2R_le.
apply Zlt_le_succ.
apply F2R_gt_0_reg with beta (canonic_exp beta fexp x).
now rewrite <- Fx.
Qed.
Lemma pred_pos_plus_ulp_aux2 :
∀ x, (0 < x)%R → F x →
let e := ln_beta_val beta x (ln_beta beta x) in
x = bpow (e - 1) →
(x - bpow (fexp (e-1)) ≠ 0)%R →
((x - bpow (fexp (e-1))) + ulp (x - bpow (fexp (e-1))) = x)%R.
Proof.
intros x Zx Fx e Hxe Zp.
replace (ulp (x - bpow (fexp (e - 1)))) with (bpow (fexp (e - 1))).
ring.
assert (He:(fexp (e-1) ≤ e-1)%Z).
apply generic_format_bpow_inv with beta; trivial.
rewrite <- Hxe; assumption.
case (Zle_lt_or_eq _ _ He); clear He; intros He.
rewrite ulp_neq_0; trivial.
apply f_equal.
unfold canonic_exp; apply f_equal.
apply sym_eq.
apply ln_beta_unique.
rewrite Rabs_right.
split.
apply Rplus_le_reg_l with (bpow (fexp (e-1))).
ring_simplify.
apply Rle_trans with (bpow (e - 2) + bpow (e - 2))%R.
apply Rplus_le_compat; apply bpow_le; omega.
apply Rle_trans with (2×bpow (e - 2))%R;[right; ring|idtac].
apply Rle_trans with (bpow 1×bpow (e - 2))%R.
apply Rmult_le_compat_r.
apply bpow_ge_0.
replace 2%R with (Z2R 2) by reflexivity.
replace (bpow 1) with (Z2R beta).
apply Z2R_le.
apply <- Zle_is_le_bool.
now destruct beta.
simpl.
unfold Zpower_pos; simpl.
now rewrite Zmult_1_r.
rewrite <- bpow_plus.
replace (1+(e-2))%Z with (e-1)%Z by ring.
now right.
rewrite <- Rplus_0_r, Hxe.
apply Rplus_lt_compat_l.
rewrite <- Ropp_0.
apply Ropp_lt_contravar.
apply bpow_gt_0.
apply Rle_ge; apply Rle_0_minus.
rewrite Hxe.
apply bpow_le.
omega.
contradict Zp.
rewrite Hxe, He; ring.
Qed.
Lemma pred_pos_plus_ulp_aux3 :
∀ x, (0 < x)%R → F x →
let e := ln_beta_val beta x (ln_beta beta x) in
x = bpow (e - 1) →
(x - bpow (fexp (e-1)) = 0)%R →
(ulp 0 = x)%R.
Proof.
intros x Hx Fx e H1 H2.
assert (H3:(x = bpow (fexp (e - 1)))).
now apply Rminus_diag_uniq.
assert (H4: (fexp (e-1) = e-1)%Z).
apply bpow_inj with beta.
now rewrite <- H1.
unfold ulp; rewrite Req_bool_true; trivial.
case negligible_exp_spec.
intros K.
specialize (K (e-1)%Z).
contradict K; omega.
intros n Hn.
rewrite H3; apply f_equal.
case (Zle_or_lt n (e-1)); intros H6.
apply valid_exp; omega.
apply sym_eq, valid_exp; omega.
Qed.
The following one is false for x = 0 in FTZ
Theorem pred_pos_plus_ulp :
∀ x, (0 < x)%R → F x →
(pred_pos x + ulp (pred_pos x) = x)%R.
Proof.
intros x Zx Fx.
unfold pred_pos.
case Req_bool_spec; intros H.
case (Req_EM_T (x - bpow (fexp (ln_beta_val beta x (ln_beta beta x) -1))) 0); intros H1.
rewrite H1, Rplus_0_l.
now apply pred_pos_plus_ulp_aux3.
now apply pred_pos_plus_ulp_aux2.
now apply pred_pos_plus_ulp_aux1.
Qed.
Rounding x + small epsilon
Theorem ln_beta_plus_eps:
∀ x, (0 < x)%R → F x →
∀ eps, (0 ≤ eps < ulp x)%R →
ln_beta beta (x + eps) = ln_beta beta x :> Z.
Proof.
intros x Zx Fx eps Heps.
destruct (ln_beta beta x) as (ex, He).
simpl.
specialize (He (Rgt_not_eq _ _ Zx)).
apply ln_beta_unique.
rewrite Rabs_pos_eq.
rewrite Rabs_pos_eq in He.
split.
apply Rle_trans with (1 := proj1 He).
pattern x at 1 ; rewrite <- Rplus_0_r.
now apply Rplus_le_compat_l.
apply Rlt_le_trans with (x + ulp x)%R.
now apply Rplus_lt_compat_l.
pattern x at 1 ; rewrite Fx.
rewrite ulp_neq_0.
unfold F2R. simpl.
pattern (bpow (canonic_exp beta fexp x)) at 2 ; rewrite <- Rmult_1_l.
rewrite <- Rmult_plus_distr_r.
change 1%R with (Z2R 1).
rewrite <- Z2R_plus.
change (F2R (Float beta (Ztrunc (scaled_mantissa beta fexp x) + 1) (canonic_exp beta fexp x)) ≤ bpow ex)%R.
apply F2R_p1_le_bpow.
apply F2R_gt_0_reg with beta (canonic_exp beta fexp x).
now rewrite <- Fx.
now rewrite <- Fx.
now apply Rgt_not_eq.
now apply Rlt_le.
apply Rplus_le_le_0_compat.
now apply Rlt_le.
apply Heps.
Qed.
Theorem round_DN_plus_eps_pos:
∀ x, (0 ≤ x)%R → F x →
∀ eps, (0 ≤ eps < ulp x)%R →
round beta fexp Zfloor (x + eps) = x.
Proof.
intros x Zx Fx eps Heps.
destruct Zx as [Zx|Zx].
pattern x at 2 ; rewrite Fx.
unfold round.
unfold scaled_mantissa. simpl.
unfold canonic_exp at 1 2.
rewrite ln_beta_plus_eps ; trivial.
apply (f_equal (fun m ⇒ F2R (Float beta m _))).
rewrite Ztrunc_floor.
apply Zfloor_imp.
split.
apply (Rle_trans _ _ _ (Zfloor_lb _)).
apply Rmult_le_compat_r.
apply bpow_ge_0.
pattern x at 1 ; rewrite <- Rplus_0_r.
now apply Rplus_le_compat_l.
apply Rlt_le_trans with ((x + ulp x) × bpow (- canonic_exp beta fexp x))%R.
apply Rmult_lt_compat_r.
apply bpow_gt_0.
now apply Rplus_lt_compat_l.
rewrite Rmult_plus_distr_r.
rewrite Z2R_plus.
apply Rplus_le_compat.
pattern x at 1 3 ; rewrite Fx.
unfold F2R. simpl.
rewrite Rmult_assoc.
rewrite <- bpow_plus.
rewrite Zplus_opp_r.
rewrite Rmult_1_r.
rewrite Zfloor_Z2R.
apply Rle_refl.
rewrite ulp_neq_0.
2: now apply Rgt_not_eq.
rewrite <- bpow_plus.
rewrite Zplus_opp_r.
apply Rle_refl.
apply Rmult_le_pos.
now apply Rlt_le.
apply bpow_ge_0.
rewrite <- Zx, Rplus_0_l; rewrite <- Zx in Heps.
case (proj1 Heps); intros P.
unfold round, scaled_mantissa, canonic_exp.
revert Heps; unfold ulp.
rewrite Req_bool_true; trivial.
case negligible_exp_spec.
intros _ (H1,H2).
absurd (0 < 0)%R; auto with real.
now apply Rle_lt_trans with (1:=H1).
intros n Hn H.
assert (fexp (ln_beta beta eps) = fexp n).
apply valid_exp; try assumption.
assert(ln_beta beta eps-1 < fexp n)%Z;[idtac|omega].
apply lt_bpow with beta.
apply Rle_lt_trans with (2:=proj2 H).
destruct (ln_beta beta eps) as (e,He).
simpl; rewrite Rabs_pos_eq in He.
now apply He, Rgt_not_eq.
now left.
replace (Zfloor (eps × bpow (- fexp (ln_beta beta eps)))) with 0%Z.
unfold F2R; simpl; ring.
apply sym_eq, Zfloor_imp.
split.
apply Rmult_le_pos.
now left.
apply bpow_ge_0.
apply Rmult_lt_reg_r with (bpow (fexp n)).
apply bpow_gt_0.
rewrite Rmult_assoc, <- bpow_plus.
rewrite H0; ring_simplify (-fexp n + fexp n)%Z.
simpl; rewrite Rmult_1_l, Rmult_1_r.
apply H.
rewrite <- P, round_0; trivial.
apply valid_rnd_DN.
Qed.
Theorem round_UP_plus_eps_pos :
∀ x, (0 ≤ x)%R → F x →
∀ eps, (0 < eps ≤ ulp x)%R →
round beta fexp Zceil (x + eps) = (x + ulp x)%R.
Proof with auto with typeclass_instances.
intros x Zx Fx eps.
case Zx; intros Zx1.
intros (Heps1,[Heps2|Heps2]).
assert (Heps: (0 ≤ eps < ulp x)%R).
split.
now apply Rlt_le.
exact Heps2.
assert (Hd := round_DN_plus_eps_pos x Zx Fx eps Heps).
rewrite round_UP_DN_ulp.
rewrite Hd.
rewrite 2!ulp_neq_0.
unfold canonic_exp.
now rewrite ln_beta_plus_eps.
now apply Rgt_not_eq.
now apply Rgt_not_eq, Rplus_lt_0_compat.
intros Fs.
rewrite round_generic in Hd...
apply Rgt_not_eq with (2 := Hd).
pattern x at 2 ; rewrite <- Rplus_0_r.
now apply Rplus_lt_compat_l.
rewrite Heps2.
apply round_generic...
now apply generic_format_succ_aux1.
rewrite <- Zx1, 2!Rplus_0_l.
intros Heps.
case (proj2 Heps).
unfold round, scaled_mantissa, canonic_exp.
unfold ulp.
rewrite Req_bool_true; trivial.
case negligible_exp_spec.
intros H2.
intros J; absurd (0 < 0)%R; auto with real.
apply Rlt_trans with eps; try assumption; apply Heps.
intros n Hn H.
assert (fexp (ln_beta beta eps) = fexp n).
apply valid_exp; try assumption.
assert(ln_beta beta eps-1 < fexp n)%Z;[idtac|omega].
apply lt_bpow with beta.
apply Rle_lt_trans with (2:=H).
destruct (ln_beta beta eps) as (e,He).
simpl; rewrite Rabs_pos_eq in He.
now apply He, Rgt_not_eq.
now left.
replace (Zceil (eps × bpow (- fexp (ln_beta beta eps)))) with 1%Z.
unfold F2R; simpl; rewrite H0; ring.
apply sym_eq, Zceil_imp.
split.
simpl; apply Rmult_lt_0_compat.
apply Heps.
apply bpow_gt_0.
apply Rmult_le_reg_r with (bpow (fexp n)).
apply bpow_gt_0.
rewrite Rmult_assoc, <- bpow_plus.
rewrite H0; ring_simplify (-fexp n + fexp n)%Z.
simpl; rewrite Rmult_1_l, Rmult_1_r.
now left.
intros P; rewrite P.
apply round_generic...
apply generic_format_ulp_0.
Qed.
Theorem round_UP_pred_plus_eps_pos :
∀ x, (0 < x)%R → F x →
∀ eps, (0 < eps ≤ ulp (pred x) )%R →
round beta fexp Zceil (pred x + eps) = x.
Proof.
intros x Hx Fx eps Heps.
rewrite round_UP_plus_eps_pos; trivial.
rewrite pred_eq_pos.
apply pred_pos_plus_ulp; trivial.
now left.
now apply pred_ge_0.
apply generic_format_pred; trivial.
Qed.
Theorem round_DN_minus_eps_pos :
∀ x, (0 < x)%R → F x →
∀ eps, (0 < eps ≤ ulp (pred x))%R →
round beta fexp Zfloor (x - eps) = pred x.
Proof.
intros x Hpx Fx eps.
rewrite pred_eq_pos;[intros Heps|now left].
replace (x-eps)%R with (pred_pos x + (ulp (pred_pos x)-eps))%R.
2: pattern x at 3; rewrite <- (pred_pos_plus_ulp x); trivial.
2: ring.
rewrite round_DN_plus_eps_pos; trivial.
now apply pred_pos_ge_0.
now apply generic_format_pred_pos.
split.
apply Rle_0_minus.
now apply Heps.
rewrite <- Rplus_0_r.
apply Rplus_lt_compat_l.
rewrite <- Ropp_0.
apply Ropp_lt_contravar.
now apply Heps.
Qed.
Theorem round_DN_plus_eps:
∀ x, F x →
∀ eps, (0 ≤ eps < if (Rle_bool 0 x) then (ulp x)
else (ulp (pred (-x))))%R →
round beta fexp Zfloor (x + eps) = x.
Proof.
intros x Fx eps Heps.
case (Rle_or_lt 0 x); intros Zx.
apply round_DN_plus_eps_pos; try assumption.
split; try apply Heps.
rewrite Rle_bool_true in Heps; trivial.
now apply Heps.
rewrite Rle_bool_false in Heps; trivial.
rewrite <- (Ropp_involutive (x+eps)).
pattern x at 2; rewrite <- (Ropp_involutive x).
rewrite round_DN_opp.
apply f_equal.
replace (-(x+eps))%R with (pred (-x) + (ulp (pred (-x)) - eps))%R.
rewrite round_UP_pred_plus_eps_pos; try reflexivity.
now apply Ropp_0_gt_lt_contravar.
now apply generic_format_opp.
split.
apply Rplus_lt_reg_l with eps; ring_simplify.
apply Heps.
apply Rplus_le_reg_l with (eps-ulp (pred (- x)))%R; ring_simplify.
apply Heps.
unfold pred.
rewrite Ropp_involutive.
unfold succ; rewrite Rle_bool_false; try assumption.
rewrite Ropp_involutive; unfold Rminus.
rewrite <- Rplus_assoc, pred_pos_plus_ulp.
ring.
now apply Ropp_0_gt_lt_contravar.
now apply generic_format_opp.
Qed.
Theorem round_UP_plus_eps :
∀ x, F x →
∀ eps, (0 < eps ≤ if (Rle_bool 0 x) then (ulp x)
else (ulp (pred (-x))))%R →
round beta fexp Zceil (x + eps) = (succ x)%R.
Proof with auto with typeclass_instances.
intros x Fx eps Heps.
case (Rle_or_lt 0 x); intros Zx.
rewrite succ_eq_pos; try assumption.
rewrite Rle_bool_true in Heps; trivial.
apply round_UP_plus_eps_pos; assumption.
rewrite Rle_bool_false in Heps; trivial.
rewrite <- (Ropp_involutive (x+eps)).
rewrite <- (Ropp_involutive (succ x)).
rewrite round_UP_opp.
apply f_equal.
replace (-(x+eps))%R with (-succ x + (-eps + ulp (pred (-x))))%R.
apply round_DN_plus_eps_pos.
rewrite <- pred_opp.
apply pred_ge_0.
now apply Ropp_0_gt_lt_contravar.
now apply generic_format_opp.
now apply generic_format_opp, generic_format_succ.
split.
apply Rplus_le_reg_l with eps; ring_simplify.
apply Heps.
unfold pred; rewrite Ropp_involutive.
apply Rplus_lt_reg_l with (eps-ulp (- succ x))%R; ring_simplify.
apply Heps.
unfold succ; rewrite Rle_bool_false; try assumption.
apply trans_eq with (-x +-eps)%R;[idtac|ring].
pattern (-x)%R at 3; rewrite <- (pred_pos_plus_ulp (-x)).
rewrite pred_eq_pos.
ring.
left; now apply Ropp_0_gt_lt_contravar.
now apply Ropp_0_gt_lt_contravar.
now apply generic_format_opp.
Qed.
Lemma le_pred_pos_lt :
∀ x y,
F x → F y →
(0 ≤ x < y)%R →
(x ≤ pred_pos y)%R.
Proof with auto with typeclass_instances.
intros x y Fx Fy H.
case (proj1 H); intros V.
assert (Zy:(0 < y)%R).
apply Rle_lt_trans with (1:=proj1 H).
apply H.
assert (Zp: (0 < pred y)%R).
assert (Zp:(0 ≤ pred y)%R).
apply pred_ge_0 ; trivial.
destruct Zp; trivial.
generalize H0.
rewrite pred_eq_pos;[idtac|now left].
unfold pred_pos.
destruct (ln_beta beta y) as (ey,Hey); simpl.
case Req_bool_spec; intros Hy2.
intros Hy3.
assert (ey-1 = fexp (ey -1))%Z.
apply bpow_inj with beta.
rewrite <- Hy2, <- Rplus_0_l, Hy3.
ring.
assert (Zx: (x ≠ 0)%R).
now apply Rgt_not_eq.
destruct (ln_beta beta x) as (ex,Hex).
specialize (Hex Zx).
assert (ex ≤ ey)%Z.
apply bpow_lt_bpow with beta.
apply Rle_lt_trans with (1:=proj1 Hex).
apply Rlt_trans with (Rabs y).
rewrite 2!Rabs_right.
apply H.
now apply Rgt_ge.
now apply Rgt_ge.
apply Hey.
now apply Rgt_not_eq.
case (Zle_lt_or_eq _ _ H2); intros Hexy.
assert (fexp ex = fexp (ey-1))%Z.
apply valid_exp.
omega.
rewrite <- H1.
omega.
absurd (0 < Ztrunc (scaled_mantissa beta fexp x) < 1)%Z.
omega.
split.
apply F2R_gt_0_reg with beta (canonic_exp beta fexp x).
now rewrite <- Fx.
apply lt_Z2R.
apply Rmult_lt_reg_r with (bpow (canonic_exp beta fexp x)).
apply bpow_gt_0.
replace (Z2R (Ztrunc (scaled_mantissa beta fexp x)) ×
bpow (canonic_exp beta fexp x))%R with x.
rewrite Rmult_1_l.
unfold canonic_exp.
rewrite ln_beta_unique with beta x ex.
rewrite H3,<-H1, <- Hy2.
apply H.
exact Hex.
absurd (y ≤ x)%R.
now apply Rlt_not_le.
rewrite Rabs_right in Hex.
apply Rle_trans with (2:=proj1 Hex).
rewrite Hexy, Hy2.
now apply Rle_refl.
now apply Rgt_ge.
intros Hy3.
assert (y = bpow (fexp ey))%R.
apply Rminus_diag_uniq.
rewrite Hy3.
rewrite ulp_neq_0;[idtac|now apply Rgt_not_eq].
unfold canonic_exp.
rewrite (ln_beta_unique beta y ey); trivial.
apply Hey.
now apply Rgt_not_eq.
contradict Hy2.
rewrite H1.
apply f_equal.
apply Zplus_reg_l with 1%Z.
ring_simplify.
apply trans_eq with (ln_beta beta y).
apply sym_eq; apply ln_beta_unique.
rewrite H1, Rabs_right.
split.
apply bpow_le.
omega.
apply bpow_lt.
omega.
apply Rle_ge; apply bpow_ge_0.
apply ln_beta_unique.
apply Hey.
now apply Rgt_not_eq.
case (Rle_or_lt (ulp (pred_pos y)) (y-x)); intros H1.
apply Rplus_le_reg_r with (-x + ulp (pred_pos y))%R.
ring_simplify (x+(-x+ulp (pred_pos y)))%R.
apply Rle_trans with (1:=H1).
rewrite <- (pred_pos_plus_ulp y) at 1; trivial.
apply Req_le; ring.
replace x with (y-(y-x))%R by ring.
rewrite <- pred_eq_pos;[idtac|now left].
rewrite <- round_DN_minus_eps_pos with (eps:=(y-x)%R); try easy.
ring_simplify (y-(y-x))%R.
apply Req_le.
apply sym_eq.
apply round_generic...
split; trivial.
now apply Rlt_Rminus.
rewrite pred_eq_pos;[idtac|now left].
now apply Rlt_le.
rewrite <- V; apply pred_pos_ge_0; trivial.
apply Rle_lt_trans with (1:=proj1 H); apply H.
Qed.
Theorem succ_le_lt_aux:
∀ x y,
F x → F y →
(0 ≤ x)%R → (x < y)%R →
(succ x ≤ y)%R.
Proof with auto with typeclass_instances.
intros x y Hx Hy Zx H.
rewrite succ_eq_pos; trivial.
case (Rle_or_lt (ulp x) (y-x)); intros H1.
apply Rplus_le_reg_r with (-x)%R.
now ring_simplify (x+ulp x + -x)%R.
replace y with (x+(y-x))%R by ring.
absurd (x < y)%R.
2: apply H.
apply Rle_not_lt; apply Req_le.
rewrite <- round_DN_plus_eps_pos with (eps:=(y-x)%R); try easy.
ring_simplify (x+(y-x))%R.
apply sym_eq.
apply round_generic...
split; trivial.
apply Rlt_le; now apply Rlt_Rminus.
Qed.
Theorem succ_le_lt:
∀ x y,
F x → F y →
(x < y)%R →
(succ x ≤ y)%R.
Proof with auto with typeclass_instances.
intros x y Fx Fy H.
destruct (Rle_or_lt 0 x) as [Hx|Hx].
now apply succ_le_lt_aux.
unfold succ; rewrite Rle_bool_false; try assumption.
case (Rle_or_lt y 0); intros Hy.
rewrite <- (Ropp_involutive y).
apply Ropp_le_contravar.
apply le_pred_pos_lt.
now apply generic_format_opp.
now apply generic_format_opp.
split.
rewrite <- Ropp_0; now apply Ropp_le_contravar.
now apply Ropp_lt_contravar.
apply Rle_trans with (-0)%R.
apply Ropp_le_contravar.
apply pred_pos_ge_0.
rewrite <- Ropp_0; now apply Ropp_lt_contravar.
now apply generic_format_opp.
rewrite Ropp_0; now left.
Qed.
Theorem le_pred_lt :
∀ x y,
F x → F y →
(x < y)%R →
(x ≤ pred y)%R.
Proof.
intros x y Fx Fy Hxy.
rewrite <- (Ropp_involutive x).
unfold pred; apply Ropp_le_contravar.
apply succ_le_lt.
now apply generic_format_opp.
now apply generic_format_opp.
now apply Ropp_lt_contravar.
Qed.
Theorem lt_succ_le:
∀ x y,
F x → F y → (y ≠ 0)%R →
(x ≤ y)%R →
(x < succ y)%R.
Proof.
intros x y Fx Fy Zy Hxy.
case (Rle_or_lt (succ y) x); trivial; intros H.
absurd (succ y = y)%R.
apply Rgt_not_eq.
now apply succ_gt_id.
apply Rle_antisym.
now apply Rle_trans with x.
apply succ_ge_id.
Qed.
Theorem succ_pred_aux : ∀ x, F x → (0 < x)%R → succ (pred x)=x.
Proof.
intros x Fx Hx.
rewrite pred_eq_pos;[idtac|now left].
rewrite succ_eq_pos.
2: now apply pred_pos_ge_0.
now apply pred_pos_plus_ulp.
Qed.
Theorem pred_succ_aux_0 : (pred (succ 0)=0)%R.
Proof.
unfold succ; rewrite Rle_bool_true.
2: apply Rle_refl.
rewrite Rplus_0_l.
rewrite pred_eq_pos.
2: apply ulp_ge_0.
unfold ulp; rewrite Req_bool_true; trivial.
case negligible_exp_spec'.
intros (H1,H2); rewrite H1.
unfold pred_pos; rewrite Req_bool_false.
2: apply Rlt_not_eq, bpow_gt_0.
unfold ulp; rewrite Req_bool_true; trivial.
rewrite H1; ring.
intros (n,(H1,H2)); rewrite H1.
unfold pred_pos.
rewrite ln_beta_bpow.
replace (fexp n + 1 - 1)%Z with (fexp n) by ring.
rewrite Req_bool_true; trivial.
apply Rminus_diag_eq, f_equal.
apply sym_eq, valid_exp; omega.
Qed.
Theorem pred_succ_aux : ∀ x, F x → (0 < x)%R → pred (succ x)=x.
Proof.
intros x Fx Hx.
rewrite succ_eq_pos;[idtac|now left].
rewrite pred_eq_pos.
2: apply Rplus_le_le_0_compat;[now left| apply ulp_ge_0].
unfold pred_pos.
case Req_bool_spec; intros H1.
pose (l:=(ln_beta beta (x+ulp x))).
rewrite H1 at 1; fold l.
apply Rplus_eq_reg_r with (ulp x).
rewrite H1; fold l.
rewrite (ulp_neq_0 x) at 3.
2: now apply Rgt_not_eq.
unfold canonic_exp.
replace (fexp (ln_beta beta x)) with (fexp (l-1))%Z.
ring.
apply f_equal, sym_eq.
apply Zle_antisym.
assert (ln_beta beta x - 1 < l - 1)%Z;[idtac|omega].
apply lt_bpow with beta.
unfold l; rewrite <- H1.
apply Rle_lt_trans with x.
destruct (ln_beta beta x) as (e,He); simpl.
rewrite <- (Rabs_right x) at 1.
2: apply Rle_ge; now left.
now apply He, Rgt_not_eq.
apply Rplus_lt_reg_l with (-x)%R; ring_simplify.
rewrite ulp_neq_0.
apply bpow_gt_0.
now apply Rgt_not_eq.
apply le_bpow with beta.
unfold l; rewrite <- H1.
apply id_p_ulp_le_bpow; trivial.
rewrite <- (Rabs_right x) at 1.
2: apply Rle_ge; now left.
apply bpow_ln_beta_gt.
replace (ulp (x+ ulp x)) with (ulp x).
ring.
rewrite ulp_neq_0 at 1.
2: now apply Rgt_not_eq.
rewrite ulp_neq_0 at 1.
2: apply Rgt_not_eq, Rlt_gt.
2: apply Rlt_le_trans with (1:=Hx).
2: apply Rplus_le_reg_l with (-x)%R; ring_simplify.
2: apply ulp_ge_0.
apply f_equal; unfold canonic_exp; apply f_equal.
apply sym_eq, ln_beta_unique.
rewrite Rabs_right.
2: apply Rle_ge; left.
2: apply Rlt_le_trans with (1:=Hx).
2: apply Rplus_le_reg_l with (-x)%R; ring_simplify.
2: apply ulp_ge_0.
destruct (ln_beta beta x) as (e,He); simpl.
rewrite Rabs_right in He.
2: apply Rle_ge; now left.
split.
apply Rle_trans with x.
apply He, Rgt_not_eq; assumption.
apply Rplus_le_reg_l with (-x)%R; ring_simplify.
apply ulp_ge_0.
case (Rle_lt_or_eq_dec (x+ulp x) (bpow e)); trivial.
apply id_p_ulp_le_bpow; trivial.
apply He, Rgt_not_eq; assumption.
intros K; contradict H1.
rewrite K, ln_beta_bpow.
apply f_equal; ring.
Qed.
Theorem succ_pred : ∀ x, F x → succ (pred x)=x.
Proof.
intros x Fx.
case (Rle_or_lt 0 x); intros Hx.
destruct Hx as [Hx|Hx].
now apply succ_pred_aux.
rewrite <- Hx.
rewrite pred_eq_opp_succ_opp, succ_opp, Ropp_0.
rewrite pred_succ_aux_0; ring.
rewrite pred_eq_opp_succ_opp, succ_opp.
rewrite pred_succ_aux.
ring.
now apply generic_format_opp.
now apply Ropp_0_gt_lt_contravar.
Qed.
Theorem pred_succ : ∀ x, F x → pred (succ x)=x.
Proof.
intros x Fx.
case (Rle_or_lt 0 x); intros Hx.
destruct Hx as [Hx|Hx].
now apply pred_succ_aux.
rewrite <- Hx.
apply pred_succ_aux_0.
rewrite succ_eq_opp_pred_opp, pred_opp.
rewrite succ_pred_aux.
ring.
now apply generic_format_opp.
now apply Ropp_0_gt_lt_contravar.
Qed.
Theorem round_UP_pred_plus_eps :
∀ x, F x →
∀ eps, (0 < eps ≤ if (Rle_bool x 0) then (ulp x)
else (ulp (pred x)))%R →
round beta fexp Zceil (pred x + eps) = x.
Proof.
intros x Fx eps Heps.
rewrite round_UP_plus_eps.
now apply succ_pred.
now apply generic_format_pred.
unfold pred at 4.
rewrite Ropp_involutive, pred_succ.
rewrite ulp_opp.
generalize Heps; case (Rle_bool_spec x 0); intros H1 H2.
rewrite Rle_bool_false; trivial.
case H1; intros H1'.
apply Rlt_le_trans with (2:=H1).
apply pred_lt_id.
now apply Rlt_not_eq.
rewrite H1'; unfold pred, succ.
rewrite Ropp_0; rewrite Rle_bool_true;[idtac|now right].
rewrite Rplus_0_l.
rewrite <- Ropp_0; apply Ropp_lt_contravar.
apply Rlt_le_trans with (1:=proj1 H2).
apply Rle_trans with (1:=proj2 H2).
rewrite Ropp_0, H1'.
now right.
rewrite Rle_bool_true; trivial.
now apply pred_ge_0.
now apply generic_format_opp.
Qed.
Theorem round_DN_minus_eps:
∀ x, F x →
∀ eps, (0 < eps ≤ if (Rle_bool x 0) then (ulp x)
else (ulp (pred x)))%R →
round beta fexp Zfloor (x - eps) = pred x.
Proof.
intros x Fx eps Heps.
replace (x-eps)%R with (-(-x+eps))%R by ring.
rewrite round_DN_opp.
unfold pred; apply f_equal.
pattern (-x)%R at 1; rewrite <- (pred_succ (-x)).
apply round_UP_pred_plus_eps.
now apply generic_format_succ, generic_format_opp.
rewrite pred_succ.
rewrite ulp_opp.
generalize Heps; case (Rle_bool_spec x 0); intros H1 H2.
rewrite Rle_bool_false; trivial.
case H1; intros H1'.
apply Rlt_le_trans with (-x)%R.
now apply Ropp_0_gt_lt_contravar.
apply succ_ge_id.
rewrite H1', Ropp_0, succ_eq_pos;[idtac|now right].
rewrite Rplus_0_l.
apply Rlt_le_trans with (1:=proj1 H2).
rewrite H1' in H2; apply H2.
rewrite Rle_bool_true.
now rewrite succ_opp, ulp_opp.
rewrite succ_opp.
rewrite <- Ropp_0; apply Ropp_le_contravar.
now apply pred_ge_0.
now apply generic_format_opp.
now apply generic_format_opp.
Qed.
Error of a rounding, expressed in number of ulps false for x=0 in the FLX format
Theorem error_lt_ulp :
∀ rnd { Zrnd : Valid_rnd rnd } x,
(x ≠ 0)%R →
(Rabs (round beta fexp rnd x - x) < ulp x)%R.
Proof with auto with typeclass_instances.
intros rnd Zrnd x Zx.
destruct (generic_format_EM beta fexp x) as [Hx|Hx].
rewrite round_generic...
unfold Rminus.
rewrite Rplus_opp_r, Rabs_R0.
rewrite ulp_neq_0; trivial.
apply bpow_gt_0.
destruct (round_DN_or_UP beta fexp rnd x) as [H|H] ; rewrite H ; clear H.
rewrite Rabs_left1.
rewrite Ropp_minus_distr.
apply Rplus_lt_reg_l with (round beta fexp Zfloor x).
rewrite <- round_UP_DN_ulp with (1 := Hx).
ring_simplify.
assert (Hu: (x ≤ round beta fexp Zceil x)%R).
apply round_UP_pt...
destruct Hu as [Hu|Hu].
exact Hu.
elim Hx.
rewrite Hu.
apply generic_format_round...
apply Rle_minus.
apply round_DN_pt...
rewrite Rabs_pos_eq.
rewrite round_UP_DN_ulp with (1 := Hx).
apply Rplus_lt_reg_r with (x - ulp x)%R.
ring_simplify.
assert (Hd: (round beta fexp Zfloor x ≤ x)%R).
apply round_DN_pt...
destruct Hd as [Hd|Hd].
exact Hd.
elim Hx.
rewrite <- Hd.
apply generic_format_round...
apply Rle_0_minus.
apply round_UP_pt...
Qed.
Theorem error_le_ulp :
∀ rnd { Zrnd : Valid_rnd rnd } x,
(Rabs (round beta fexp rnd x - x) ≤ ulp x)%R.
Proof with auto with typeclass_instances.
intros rnd Zrnd x.
case (Req_dec x 0).
intros Zx; rewrite Zx, round_0...
unfold Rminus; rewrite Rplus_0_l, Ropp_0, Rabs_R0.
apply ulp_ge_0.
intros Zx; left.
now apply error_lt_ulp.
Qed.
Theorem error_le_half_ulp :
∀ choice x,
(Rabs (round beta fexp (Znearest choice) x - x) ≤ /2 × ulp x)%R.
Proof with auto with typeclass_instances.
intros choice x.
destruct (generic_format_EM beta fexp x) as [Hx|Hx].
rewrite round_generic...
unfold Rminus.
rewrite Rplus_opp_r, Rabs_R0.
apply Rmult_le_pos.
apply Rlt_le.
apply Rinv_0_lt_compat.
now apply (Z2R_lt 0 2).
apply ulp_ge_0.
set (d := round beta fexp Zfloor x).
destruct (round_N_pt beta fexp choice x) as (Hr1, Hr2).
destruct (Rle_or_lt (x - d) (d + ulp x - x)) as [H|H].
apply Rle_trans with (Rabs (d - x)).
apply Hr2.
apply (round_DN_pt beta fexp x).
rewrite Rabs_left1.
rewrite Ropp_minus_distr.
apply Rmult_le_reg_r with 2%R.
now apply (Z2R_lt 0 2).
apply Rplus_le_reg_r with (d - x)%R.
ring_simplify.
apply Rle_trans with (1 := H).
right. field.
apply Rle_minus.
apply (round_DN_pt beta fexp x).
assert (Hu: (d + ulp x)%R = round beta fexp Zceil x).
unfold d.
now rewrite <- round_UP_DN_ulp.
apply Rle_trans with (Rabs (d + ulp x - x)).
apply Hr2.
rewrite Hu.
apply (round_UP_pt beta fexp x).
rewrite Rabs_pos_eq.
apply Rmult_le_reg_r with 2%R.
now apply (Z2R_lt 0 2).
apply Rplus_le_reg_r with (- (d + ulp x - x))%R.
ring_simplify.
apply Rlt_le.
apply Rlt_le_trans with (1 := H).
right. field.
apply Rle_0_minus.
rewrite Hu.
apply (round_UP_pt beta fexp x).
Qed.
Theorem ulp_DN :
∀ x,
(0 < round beta fexp Zfloor x)%R →
ulp (round beta fexp Zfloor x) = ulp x.
Proof with auto with typeclass_instances.
intros x Hd.
rewrite 2!ulp_neq_0.
now rewrite canonic_exp_DN with (2 := Hd).
intros T; contradict Hd; rewrite T, round_0...
apply Rlt_irrefl.
now apply Rgt_not_eq.
Qed.
Theorem round_neq_0_negligible_exp:
negligible_exp=None → ∀ rnd { Zrnd : Valid_rnd rnd } x,
(x ≠ 0)%R → (round beta fexp rnd x ≠ 0)%R.
Proof with auto with typeclass_instances.
intros H rndn Hrnd x Hx K.
case negligible_exp_spec'.
intros (_,Hn).
destruct (ln_beta beta x) as (e,He).
absurd (fexp e < e)%Z.
apply Zle_not_lt.
apply exp_small_round_0 with beta rndn x...
apply (Hn e).
intros (n,(H1,_)).
rewrite H in H1; discriminate.
Qed.
∀ rnd { Zrnd : Valid_rnd rnd } x,
(x ≠ 0)%R →
(Rabs (round beta fexp rnd x - x) < ulp x)%R.
Proof with auto with typeclass_instances.
intros rnd Zrnd x Zx.
destruct (generic_format_EM beta fexp x) as [Hx|Hx].
rewrite round_generic...
unfold Rminus.
rewrite Rplus_opp_r, Rabs_R0.
rewrite ulp_neq_0; trivial.
apply bpow_gt_0.
destruct (round_DN_or_UP beta fexp rnd x) as [H|H] ; rewrite H ; clear H.
rewrite Rabs_left1.
rewrite Ropp_minus_distr.
apply Rplus_lt_reg_l with (round beta fexp Zfloor x).
rewrite <- round_UP_DN_ulp with (1 := Hx).
ring_simplify.
assert (Hu: (x ≤ round beta fexp Zceil x)%R).
apply round_UP_pt...
destruct Hu as [Hu|Hu].
exact Hu.
elim Hx.
rewrite Hu.
apply generic_format_round...
apply Rle_minus.
apply round_DN_pt...
rewrite Rabs_pos_eq.
rewrite round_UP_DN_ulp with (1 := Hx).
apply Rplus_lt_reg_r with (x - ulp x)%R.
ring_simplify.
assert (Hd: (round beta fexp Zfloor x ≤ x)%R).
apply round_DN_pt...
destruct Hd as [Hd|Hd].
exact Hd.
elim Hx.
rewrite <- Hd.
apply generic_format_round...
apply Rle_0_minus.
apply round_UP_pt...
Qed.
Theorem error_le_ulp :
∀ rnd { Zrnd : Valid_rnd rnd } x,
(Rabs (round beta fexp rnd x - x) ≤ ulp x)%R.
Proof with auto with typeclass_instances.
intros rnd Zrnd x.
case (Req_dec x 0).
intros Zx; rewrite Zx, round_0...
unfold Rminus; rewrite Rplus_0_l, Ropp_0, Rabs_R0.
apply ulp_ge_0.
intros Zx; left.
now apply error_lt_ulp.
Qed.
Theorem error_le_half_ulp :
∀ choice x,
(Rabs (round beta fexp (Znearest choice) x - x) ≤ /2 × ulp x)%R.
Proof with auto with typeclass_instances.
intros choice x.
destruct (generic_format_EM beta fexp x) as [Hx|Hx].
rewrite round_generic...
unfold Rminus.
rewrite Rplus_opp_r, Rabs_R0.
apply Rmult_le_pos.
apply Rlt_le.
apply Rinv_0_lt_compat.
now apply (Z2R_lt 0 2).
apply ulp_ge_0.
set (d := round beta fexp Zfloor x).
destruct (round_N_pt beta fexp choice x) as (Hr1, Hr2).
destruct (Rle_or_lt (x - d) (d + ulp x - x)) as [H|H].
apply Rle_trans with (Rabs (d - x)).
apply Hr2.
apply (round_DN_pt beta fexp x).
rewrite Rabs_left1.
rewrite Ropp_minus_distr.
apply Rmult_le_reg_r with 2%R.
now apply (Z2R_lt 0 2).
apply Rplus_le_reg_r with (d - x)%R.
ring_simplify.
apply Rle_trans with (1 := H).
right. field.
apply Rle_minus.
apply (round_DN_pt beta fexp x).
assert (Hu: (d + ulp x)%R = round beta fexp Zceil x).
unfold d.
now rewrite <- round_UP_DN_ulp.
apply Rle_trans with (Rabs (d + ulp x - x)).
apply Hr2.
rewrite Hu.
apply (round_UP_pt beta fexp x).
rewrite Rabs_pos_eq.
apply Rmult_le_reg_r with 2%R.
now apply (Z2R_lt 0 2).
apply Rplus_le_reg_r with (- (d + ulp x - x))%R.
ring_simplify.
apply Rlt_le.
apply Rlt_le_trans with (1 := H).
right. field.
apply Rle_0_minus.
rewrite Hu.
apply (round_UP_pt beta fexp x).
Qed.
Theorem ulp_DN :
∀ x,
(0 < round beta fexp Zfloor x)%R →
ulp (round beta fexp Zfloor x) = ulp x.
Proof with auto with typeclass_instances.
intros x Hd.
rewrite 2!ulp_neq_0.
now rewrite canonic_exp_DN with (2 := Hd).
intros T; contradict Hd; rewrite T, round_0...
apply Rlt_irrefl.
now apply Rgt_not_eq.
Qed.
Theorem round_neq_0_negligible_exp:
negligible_exp=None → ∀ rnd { Zrnd : Valid_rnd rnd } x,
(x ≠ 0)%R → (round beta fexp rnd x ≠ 0)%R.
Proof with auto with typeclass_instances.
intros H rndn Hrnd x Hx K.
case negligible_exp_spec'.
intros (_,Hn).
destruct (ln_beta beta x) as (e,He).
absurd (fexp e < e)%Z.
apply Zle_not_lt.
apply exp_small_round_0 with beta rndn x...
apply (Hn e).
intros (n,(H1,_)).
rewrite H in H1; discriminate.
Qed.
allows rnd x to be 0
Theorem error_lt_ulp_round :
∀ { Hm : Monotone_exp fexp } rnd { Zrnd : Valid_rnd rnd } x,
( x ≠ 0)%R →
(Rabs (round beta fexp rnd x - x) < ulp (round beta fexp rnd x))%R.
Proof with auto with typeclass_instances.
intros Hm.
cut (∀ rnd : R → Z, Valid_rnd rnd → ∀ x : R, (0 < x)%R →
(Rabs (round beta fexp rnd x - x) < ulp (round beta fexp rnd x))%R).
intros M rnd Hrnd x Zx.
case (Rle_or_lt 0 x).
intros H; destruct H.
now apply M.
contradict H; now apply sym_not_eq.
intros H.
rewrite <- (Ropp_involutive x).
rewrite round_opp, ulp_opp.
replace (- round beta fexp (Zrnd_opp rnd) (- x) - - - x)%R with
(-(round beta fexp (Zrnd_opp rnd) (- x) - (-x)))%R by ring.
rewrite Rabs_Ropp.
apply M.
now apply valid_rnd_opp.
now apply Ropp_0_gt_lt_contravar.
intros rnd Hrnd x Hx.
case (Rle_lt_or_eq_dec 0 (round beta fexp Zfloor x)).
apply round_ge_generic...
apply generic_format_0.
now left.
intros Hx2.
apply Rlt_le_trans with (ulp x).
apply error_lt_ulp...
now apply Rgt_not_eq.
rewrite <- ulp_DN; trivial.
apply ulp_le_pos.
now left.
case (round_DN_or_UP beta fexp rnd x); intros V; rewrite V.
apply Rle_refl.
apply Rle_trans with x.
apply round_DN_pt...
apply round_UP_pt...
intros Hx2.
case (round_DN_or_UP beta fexp rnd x); intros V; rewrite V; clear V.
rewrite <- Hx2.
unfold Rminus; rewrite Rplus_0_l, Rabs_Ropp.
unfold ulp; rewrite Req_bool_true; trivial.
case negligible_exp_spec.
intros K; contradict Hx2.
apply Rlt_not_eq.
apply F2R_gt_0_compat; simpl.
apply Zlt_le_trans with 1%Z.
apply Pos2Z.is_pos.
apply Zfloor_lub.
simpl; unfold scaled_mantissa, canonic_exp.
destruct (ln_beta beta x) as (e,He); simpl.
apply Rle_trans with (bpow (e-1) × bpow (- fexp e))%R.
rewrite <- bpow_plus.
replace 1%R with (bpow 0) by reflexivity.
apply bpow_le.
specialize (K e); omega.
apply Rmult_le_compat_r.
apply bpow_ge_0.
rewrite <- (Rabs_pos_eq x).
now apply He, Rgt_not_eq.
now left.
intros n Hn.
rewrite Rabs_pos_eq;[idtac|now left].
case (Rle_or_lt (bpow (fexp n)) x); trivial.
intros K; contradict Hx2.
apply Rlt_not_eq.
apply Rlt_le_trans with (bpow (fexp n)).
apply bpow_gt_0.
apply round_ge_generic...
apply generic_format_bpow.
now apply valid_exp.
apply Rlt_le_trans with (ulp x).
apply error_lt_ulp...
now apply Rgt_not_eq.
apply ulp_le_pos.
now left.
apply round_UP_pt...
Qed.
∀ { Hm : Monotone_exp fexp } rnd { Zrnd : Valid_rnd rnd } x,
( x ≠ 0)%R →
(Rabs (round beta fexp rnd x - x) < ulp (round beta fexp rnd x))%R.
Proof with auto with typeclass_instances.
intros Hm.
cut (∀ rnd : R → Z, Valid_rnd rnd → ∀ x : R, (0 < x)%R →
(Rabs (round beta fexp rnd x - x) < ulp (round beta fexp rnd x))%R).
intros M rnd Hrnd x Zx.
case (Rle_or_lt 0 x).
intros H; destruct H.
now apply M.
contradict H; now apply sym_not_eq.
intros H.
rewrite <- (Ropp_involutive x).
rewrite round_opp, ulp_opp.
replace (- round beta fexp (Zrnd_opp rnd) (- x) - - - x)%R with
(-(round beta fexp (Zrnd_opp rnd) (- x) - (-x)))%R by ring.
rewrite Rabs_Ropp.
apply M.
now apply valid_rnd_opp.
now apply Ropp_0_gt_lt_contravar.
intros rnd Hrnd x Hx.
case (Rle_lt_or_eq_dec 0 (round beta fexp Zfloor x)).
apply round_ge_generic...
apply generic_format_0.
now left.
intros Hx2.
apply Rlt_le_trans with (ulp x).
apply error_lt_ulp...
now apply Rgt_not_eq.
rewrite <- ulp_DN; trivial.
apply ulp_le_pos.
now left.
case (round_DN_or_UP beta fexp rnd x); intros V; rewrite V.
apply Rle_refl.
apply Rle_trans with x.
apply round_DN_pt...
apply round_UP_pt...
intros Hx2.
case (round_DN_or_UP beta fexp rnd x); intros V; rewrite V; clear V.
rewrite <- Hx2.
unfold Rminus; rewrite Rplus_0_l, Rabs_Ropp.
unfold ulp; rewrite Req_bool_true; trivial.
case negligible_exp_spec.
intros K; contradict Hx2.
apply Rlt_not_eq.
apply F2R_gt_0_compat; simpl.
apply Zlt_le_trans with 1%Z.
apply Pos2Z.is_pos.
apply Zfloor_lub.
simpl; unfold scaled_mantissa, canonic_exp.
destruct (ln_beta beta x) as (e,He); simpl.
apply Rle_trans with (bpow (e-1) × bpow (- fexp e))%R.
rewrite <- bpow_plus.
replace 1%R with (bpow 0) by reflexivity.
apply bpow_le.
specialize (K e); omega.
apply Rmult_le_compat_r.
apply bpow_ge_0.
rewrite <- (Rabs_pos_eq x).
now apply He, Rgt_not_eq.
now left.
intros n Hn.
rewrite Rabs_pos_eq;[idtac|now left].
case (Rle_or_lt (bpow (fexp n)) x); trivial.
intros K; contradict Hx2.
apply Rlt_not_eq.
apply Rlt_le_trans with (bpow (fexp n)).
apply bpow_gt_0.
apply round_ge_generic...
apply generic_format_bpow.
now apply valid_exp.
apply Rlt_le_trans with (ulp x).
apply error_lt_ulp...
now apply Rgt_not_eq.
apply ulp_le_pos.
now left.
apply round_UP_pt...
Qed.
allows both x and rnd x to be 0
Theorem error_le_half_ulp_round :
∀ { Hm : Monotone_exp fexp },
∀ choice x,
(Rabs (round beta fexp (Znearest choice) x - x) ≤ /2 × ulp (round beta fexp (Znearest choice) x))%R.
Proof with auto with typeclass_instances.
intros Hm choice x.
case (Req_dec (round beta fexp (Znearest choice) x) 0); intros Hfx.
case (Req_dec x 0); intros Hx.
apply Rle_trans with (1:=error_le_half_ulp _ _).
rewrite Hx, round_0...
right; ring.
generalize (error_le_half_ulp choice x).
rewrite Hfx.
unfold Rminus; rewrite Rplus_0_l, Rabs_Ropp.
intros N.
unfold ulp; rewrite Req_bool_true; trivial.
case negligible_exp_spec'.
intros (H1,H2).
contradict Hfx.
apply round_neq_0_negligible_exp...
intros (n,(H1,Hn)); rewrite H1.
apply Rle_trans with (1:=N).
right; apply f_equal.
rewrite ulp_neq_0; trivial.
apply f_equal.
unfold canonic_exp.
apply valid_exp; trivial.
assert (ln_beta beta x -1 < fexp n)%Z;[idtac|omega].
apply lt_bpow with beta.
destruct (ln_beta beta x) as (e,He).
simpl.
apply Rle_lt_trans with (Rabs x).
now apply He.
apply Rle_lt_trans with (Rabs (round beta fexp (Znearest choice) x - x)).
right; rewrite Hfx; unfold Rminus; rewrite Rplus_0_l.
apply sym_eq, Rabs_Ropp.
apply Rlt_le_trans with (ulp 0).
rewrite <- Hfx.
apply error_lt_ulp_round...
unfold ulp; rewrite Req_bool_true, H1; trivial.
now right.
case (round_DN_or_UP beta fexp (Znearest choice) x); intros Hx.
case (Rle_or_lt 0 (round beta fexp Zfloor x)).
intros H; destruct H.
rewrite Hx at 2.
rewrite ulp_DN; trivial.
apply error_le_half_ulp.
rewrite Hx in Hfx; contradict Hfx; auto with real.
intros H.
apply Rle_trans with (1:=error_le_half_ulp _ _).
apply Rmult_le_compat_l.
auto with real.
apply ulp_le.
rewrite Hx; rewrite (Rabs_left1 x), Rabs_left; try assumption.
apply Ropp_le_contravar.
apply (round_DN_pt beta fexp x).
case (Rle_or_lt x 0); trivial.
intros H1; contradict H.
apply Rle_not_lt.
apply round_ge_generic...
apply generic_format_0.
now left.
case (Rle_or_lt 0 (round beta fexp Zceil x)).
intros H; destruct H.
apply Rle_trans with (1:=error_le_half_ulp _ _).
apply Rmult_le_compat_l.
auto with real.
apply ulp_le_pos; trivial.
case (Rle_or_lt 0 x); trivial.
intros H1; contradict H.
apply Rle_not_lt.
apply round_le_generic...
apply generic_format_0.
now left.
rewrite Hx; apply (round_UP_pt beta fexp x).
rewrite Hx in Hfx; contradict Hfx; auto with real.
intros H.
rewrite Hx at 2; rewrite <- (ulp_opp (round beta fexp Zceil x)).
rewrite <- round_DN_opp.
rewrite ulp_DN; trivial.
pattern x at 1 2; rewrite <- Ropp_involutive.
rewrite round_N_opp.
unfold Rminus.
rewrite <- Ropp_plus_distr, Rabs_Ropp.
apply error_le_half_ulp.
rewrite round_DN_opp; apply Ropp_0_gt_lt_contravar; apply Rlt_gt; assumption.
Qed.
Theorem pred_le: ∀ x y,
F x → F y → (x ≤ y)%R → (pred x ≤ pred y)%R.
Proof.
intros x y Fx Fy Hxy.
assert (V:( ((x = 0) ∧ (y = 0)) ∨ (x ≠0 ∨ x < y))%R).
case (Req_dec x 0); intros Zx.
case Hxy; intros Zy.
now right; right.
left; split; trivial; now rewrite <- Zy.
now right; left.
destruct V as [(V1,V2)|V].
rewrite V1,V2; now right.
apply le_pred_lt; try assumption.
apply generic_format_pred; try assumption.
case V; intros V1.
apply Rlt_le_trans with (2:=Hxy).
now apply pred_lt_id.
apply Rle_lt_trans with (2:=V1).
now apply pred_le_id.
Qed.
Theorem succ_le: ∀ x y,
F x → F y → (x ≤ y)%R → (succ x ≤ succ y)%R.
Proof.
intros x y Fx Fy Hxy.
rewrite 2!succ_eq_opp_pred_opp.
apply Ropp_le_contravar, pred_le; try apply generic_format_opp; try assumption.
now apply Ropp_le_contravar.
Qed.
Theorem pred_le_inv: ∀ x y, F x → F y
→ (pred x ≤ pred y)%R → (x ≤ y)%R.
Proof.
intros x y Fx Fy Hxy.
rewrite <- (succ_pred x), <- (succ_pred y); try assumption.
apply succ_le; trivial; now apply generic_format_pred.
Qed.
Theorem succ_le_inv: ∀ x y, F x → F y
→ (succ x ≤ succ y)%R → (x ≤ y)%R.
Proof.
intros x y Fx Fy Hxy.
rewrite <- (pred_succ x), <- (pred_succ y); try assumption.
apply pred_le; trivial; now apply generic_format_succ.
Qed.
Theorem le_round_DN_lt_UP :
∀ x y, F y →
(y < round beta fexp Zceil x → y ≤ round beta fexp Zfloor x)%R.
Proof with auto with typeclass_instances.
intros x y Fy Hlt.
apply round_DN_pt...
apply Rnot_lt_le.
contradict Hlt.
apply RIneq.Rle_not_lt.
apply round_UP_pt...
now apply Rlt_le.
Qed.
Theorem round_UP_le_gt_DN :
∀ x y, F y →
(round beta fexp Zfloor x < y → round beta fexp Zceil x ≤ y)%R.
Proof with auto with typeclass_instances.
intros x y Fy Hlt.
apply round_UP_pt...
apply Rnot_lt_le.
contradict Hlt.
apply RIneq.Rle_not_lt.
apply round_DN_pt...
now apply Rlt_le.
Qed.
Theorem pred_UP_le_DN :
∀ x, (pred (round beta fexp Zceil x) ≤ round beta fexp Zfloor x)%R.
Proof with auto with typeclass_instances.
intros x.
destruct (generic_format_EM beta fexp x) as [Fx|Fx].
rewrite !round_generic...
apply pred_le_id.
case (Req_dec (round beta fexp Zceil x) 0); intros Zx.
rewrite Zx; unfold pred; rewrite Ropp_0.
unfold succ; rewrite Rle_bool_true;[idtac|now right].
rewrite Rplus_0_l; unfold ulp; rewrite Req_bool_true; trivial.
case negligible_exp_spec'.
intros (H1,H2).
contradict Zx; apply round_neq_0_negligible_exp...
intros L; apply Fx; rewrite L; apply generic_format_0.
intros (n,(H1,Hn)); rewrite H1.
case (Rle_or_lt (- bpow (fexp n)) (round beta fexp Zfloor x)); trivial; intros K.
absurd (round beta fexp Zceil x ≤ - bpow (fexp n))%R.
apply Rlt_not_le.
rewrite Zx, <- Ropp_0.
apply Ropp_lt_contravar, bpow_gt_0.
apply round_UP_le_gt_DN; try assumption.
apply generic_format_opp, generic_format_bpow.
now apply valid_exp.
assert (let u := round beta fexp Zceil x in pred u < u)%R as Hup.
now apply pred_lt_id.
apply le_round_DN_lt_UP...
apply generic_format_pred...
now apply round_UP_pt.
Qed.
Theorem pred_UP_eq_DN :
∀ x, ¬ F x →
(pred (round beta fexp Zceil x) = round beta fexp Zfloor x)%R.
Proof with auto with typeclass_instances.
intros x Fx.
apply Rle_antisym.
now apply pred_UP_le_DN.
apply le_pred_lt; try apply generic_format_round...
pose proof round_DN_UP_lt _ _ _ Fx as HE.
now apply Rlt_trans with (1 := proj1 HE) (2 := proj2 HE).
Qed.
Theorem succ_DN_eq_UP :
∀ x, ¬ F x →
(succ (round beta fexp Zfloor x) = round beta fexp Zceil x)%R.
Proof with auto with typeclass_instances.
intros x Fx.
rewrite <- pred_UP_eq_DN; trivial.
rewrite succ_pred; trivial.
apply generic_format_round...
Qed.
Theorem round_DN_eq_betw: ∀ x d, F d
→ (d ≤ x < succ d)%R
→ round beta fexp Zfloor x = d.
Proof with auto with typeclass_instances.
intros x d Fd (Hxd1,Hxd2).
generalize (round_DN_pt beta fexp x); intros (T1,(T2,T3)).
apply sym_eq, Rle_antisym.
now apply T3.
destruct (generic_format_EM beta fexp x) as [Fx|NFx].
rewrite round_generic...
apply succ_le_inv; try assumption.
apply succ_le_lt; try assumption.
apply generic_format_succ...
apply succ_le_inv; try assumption.
rewrite succ_DN_eq_UP; trivial.
apply round_UP_pt...
apply generic_format_succ...
now left.
Qed.
Theorem round_UP_eq_betw: ∀ x u, F u
→ (pred u < x ≤ u)%R
→ round beta fexp Zceil x = u.
Proof with auto with typeclass_instances.
intros x u Fu Hux.
rewrite <- (Ropp_involutive (round beta fexp Zceil x)).
rewrite <- round_DN_opp.
rewrite <- (Ropp_involutive u).
apply f_equal.
apply round_DN_eq_betw; try assumption.
now apply generic_format_opp.
split;[now apply Ropp_le_contravar|idtac].
rewrite succ_opp.
now apply Ropp_lt_contravar.
Qed.
∀ { Hm : Monotone_exp fexp },
∀ choice x,
(Rabs (round beta fexp (Znearest choice) x - x) ≤ /2 × ulp (round beta fexp (Znearest choice) x))%R.
Proof with auto with typeclass_instances.
intros Hm choice x.
case (Req_dec (round beta fexp (Znearest choice) x) 0); intros Hfx.
case (Req_dec x 0); intros Hx.
apply Rle_trans with (1:=error_le_half_ulp _ _).
rewrite Hx, round_0...
right; ring.
generalize (error_le_half_ulp choice x).
rewrite Hfx.
unfold Rminus; rewrite Rplus_0_l, Rabs_Ropp.
intros N.
unfold ulp; rewrite Req_bool_true; trivial.
case negligible_exp_spec'.
intros (H1,H2).
contradict Hfx.
apply round_neq_0_negligible_exp...
intros (n,(H1,Hn)); rewrite H1.
apply Rle_trans with (1:=N).
right; apply f_equal.
rewrite ulp_neq_0; trivial.
apply f_equal.
unfold canonic_exp.
apply valid_exp; trivial.
assert (ln_beta beta x -1 < fexp n)%Z;[idtac|omega].
apply lt_bpow with beta.
destruct (ln_beta beta x) as (e,He).
simpl.
apply Rle_lt_trans with (Rabs x).
now apply He.
apply Rle_lt_trans with (Rabs (round beta fexp (Znearest choice) x - x)).
right; rewrite Hfx; unfold Rminus; rewrite Rplus_0_l.
apply sym_eq, Rabs_Ropp.
apply Rlt_le_trans with (ulp 0).
rewrite <- Hfx.
apply error_lt_ulp_round...
unfold ulp; rewrite Req_bool_true, H1; trivial.
now right.
case (round_DN_or_UP beta fexp (Znearest choice) x); intros Hx.
case (Rle_or_lt 0 (round beta fexp Zfloor x)).
intros H; destruct H.
rewrite Hx at 2.
rewrite ulp_DN; trivial.
apply error_le_half_ulp.
rewrite Hx in Hfx; contradict Hfx; auto with real.
intros H.
apply Rle_trans with (1:=error_le_half_ulp _ _).
apply Rmult_le_compat_l.
auto with real.
apply ulp_le.
rewrite Hx; rewrite (Rabs_left1 x), Rabs_left; try assumption.
apply Ropp_le_contravar.
apply (round_DN_pt beta fexp x).
case (Rle_or_lt x 0); trivial.
intros H1; contradict H.
apply Rle_not_lt.
apply round_ge_generic...
apply generic_format_0.
now left.
case (Rle_or_lt 0 (round beta fexp Zceil x)).
intros H; destruct H.
apply Rle_trans with (1:=error_le_half_ulp _ _).
apply Rmult_le_compat_l.
auto with real.
apply ulp_le_pos; trivial.
case (Rle_or_lt 0 x); trivial.
intros H1; contradict H.
apply Rle_not_lt.
apply round_le_generic...
apply generic_format_0.
now left.
rewrite Hx; apply (round_UP_pt beta fexp x).
rewrite Hx in Hfx; contradict Hfx; auto with real.
intros H.
rewrite Hx at 2; rewrite <- (ulp_opp (round beta fexp Zceil x)).
rewrite <- round_DN_opp.
rewrite ulp_DN; trivial.
pattern x at 1 2; rewrite <- Ropp_involutive.
rewrite round_N_opp.
unfold Rminus.
rewrite <- Ropp_plus_distr, Rabs_Ropp.
apply error_le_half_ulp.
rewrite round_DN_opp; apply Ropp_0_gt_lt_contravar; apply Rlt_gt; assumption.
Qed.
Theorem pred_le: ∀ x y,
F x → F y → (x ≤ y)%R → (pred x ≤ pred y)%R.
Proof.
intros x y Fx Fy Hxy.
assert (V:( ((x = 0) ∧ (y = 0)) ∨ (x ≠0 ∨ x < y))%R).
case (Req_dec x 0); intros Zx.
case Hxy; intros Zy.
now right; right.
left; split; trivial; now rewrite <- Zy.
now right; left.
destruct V as [(V1,V2)|V].
rewrite V1,V2; now right.
apply le_pred_lt; try assumption.
apply generic_format_pred; try assumption.
case V; intros V1.
apply Rlt_le_trans with (2:=Hxy).
now apply pred_lt_id.
apply Rle_lt_trans with (2:=V1).
now apply pred_le_id.
Qed.
Theorem succ_le: ∀ x y,
F x → F y → (x ≤ y)%R → (succ x ≤ succ y)%R.
Proof.
intros x y Fx Fy Hxy.
rewrite 2!succ_eq_opp_pred_opp.
apply Ropp_le_contravar, pred_le; try apply generic_format_opp; try assumption.
now apply Ropp_le_contravar.
Qed.
Theorem pred_le_inv: ∀ x y, F x → F y
→ (pred x ≤ pred y)%R → (x ≤ y)%R.
Proof.
intros x y Fx Fy Hxy.
rewrite <- (succ_pred x), <- (succ_pred y); try assumption.
apply succ_le; trivial; now apply generic_format_pred.
Qed.
Theorem succ_le_inv: ∀ x y, F x → F y
→ (succ x ≤ succ y)%R → (x ≤ y)%R.
Proof.
intros x y Fx Fy Hxy.
rewrite <- (pred_succ x), <- (pred_succ y); try assumption.
apply pred_le; trivial; now apply generic_format_succ.
Qed.
Theorem le_round_DN_lt_UP :
∀ x y, F y →
(y < round beta fexp Zceil x → y ≤ round beta fexp Zfloor x)%R.
Proof with auto with typeclass_instances.
intros x y Fy Hlt.
apply round_DN_pt...
apply Rnot_lt_le.
contradict Hlt.
apply RIneq.Rle_not_lt.
apply round_UP_pt...
now apply Rlt_le.
Qed.
Theorem round_UP_le_gt_DN :
∀ x y, F y →
(round beta fexp Zfloor x < y → round beta fexp Zceil x ≤ y)%R.
Proof with auto with typeclass_instances.
intros x y Fy Hlt.
apply round_UP_pt...
apply Rnot_lt_le.
contradict Hlt.
apply RIneq.Rle_not_lt.
apply round_DN_pt...
now apply Rlt_le.
Qed.
Theorem pred_UP_le_DN :
∀ x, (pred (round beta fexp Zceil x) ≤ round beta fexp Zfloor x)%R.
Proof with auto with typeclass_instances.
intros x.
destruct (generic_format_EM beta fexp x) as [Fx|Fx].
rewrite !round_generic...
apply pred_le_id.
case (Req_dec (round beta fexp Zceil x) 0); intros Zx.
rewrite Zx; unfold pred; rewrite Ropp_0.
unfold succ; rewrite Rle_bool_true;[idtac|now right].
rewrite Rplus_0_l; unfold ulp; rewrite Req_bool_true; trivial.
case negligible_exp_spec'.
intros (H1,H2).
contradict Zx; apply round_neq_0_negligible_exp...
intros L; apply Fx; rewrite L; apply generic_format_0.
intros (n,(H1,Hn)); rewrite H1.
case (Rle_or_lt (- bpow (fexp n)) (round beta fexp Zfloor x)); trivial; intros K.
absurd (round beta fexp Zceil x ≤ - bpow (fexp n))%R.
apply Rlt_not_le.
rewrite Zx, <- Ropp_0.
apply Ropp_lt_contravar, bpow_gt_0.
apply round_UP_le_gt_DN; try assumption.
apply generic_format_opp, generic_format_bpow.
now apply valid_exp.
assert (let u := round beta fexp Zceil x in pred u < u)%R as Hup.
now apply pred_lt_id.
apply le_round_DN_lt_UP...
apply generic_format_pred...
now apply round_UP_pt.
Qed.
Theorem pred_UP_eq_DN :
∀ x, ¬ F x →
(pred (round beta fexp Zceil x) = round beta fexp Zfloor x)%R.
Proof with auto with typeclass_instances.
intros x Fx.
apply Rle_antisym.
now apply pred_UP_le_DN.
apply le_pred_lt; try apply generic_format_round...
pose proof round_DN_UP_lt _ _ _ Fx as HE.
now apply Rlt_trans with (1 := proj1 HE) (2 := proj2 HE).
Qed.
Theorem succ_DN_eq_UP :
∀ x, ¬ F x →
(succ (round beta fexp Zfloor x) = round beta fexp Zceil x)%R.
Proof with auto with typeclass_instances.
intros x Fx.
rewrite <- pred_UP_eq_DN; trivial.
rewrite succ_pred; trivial.
apply generic_format_round...
Qed.
Theorem round_DN_eq_betw: ∀ x d, F d
→ (d ≤ x < succ d)%R
→ round beta fexp Zfloor x = d.
Proof with auto with typeclass_instances.
intros x d Fd (Hxd1,Hxd2).
generalize (round_DN_pt beta fexp x); intros (T1,(T2,T3)).
apply sym_eq, Rle_antisym.
now apply T3.
destruct (generic_format_EM beta fexp x) as [Fx|NFx].
rewrite round_generic...
apply succ_le_inv; try assumption.
apply succ_le_lt; try assumption.
apply generic_format_succ...
apply succ_le_inv; try assumption.
rewrite succ_DN_eq_UP; trivial.
apply round_UP_pt...
apply generic_format_succ...
now left.
Qed.
Theorem round_UP_eq_betw: ∀ x u, F u
→ (pred u < x ≤ u)%R
→ round beta fexp Zceil x = u.
Proof with auto with typeclass_instances.
intros x u Fu Hux.
rewrite <- (Ropp_involutive (round beta fexp Zceil x)).
rewrite <- round_DN_opp.
rewrite <- (Ropp_involutive u).
apply f_equal.
apply round_DN_eq_betw; try assumption.
now apply generic_format_opp.
split;[now apply Ropp_le_contravar|idtac].
rewrite succ_opp.
now apply Ropp_lt_contravar.
Qed.
Properties of rounding to nearest and ulp
Theorem round_N_le_midp: ∀ choice u v,
F u → (v < (u + succ u)/2)%R
→ (round beta fexp (Znearest choice) v ≤ u)%R.
Proof with auto with typeclass_instances.
intros choice u v Fu H.
assert (V: ((succ u = 0 ∧ u = 0) ∨ u < succ u)%R).
specialize (succ_ge_id u); intros P; destruct P as [P|P].
now right.
case (Req_dec u 0); intros Zu.
left; split; trivial.
now rewrite <- P.
right; now apply succ_gt_id.
destruct V as [(V1,V2)|V].
rewrite V2; apply round_le_generic...
apply generic_format_0.
left; apply Rlt_le_trans with (1:=H).
rewrite V1,V2; right; field.
assert (T: (u < (u + succ u) / 2 < succ u)%R).
split.
apply Rmult_lt_reg_l with 2%R.
now auto with real.
apply Rplus_lt_reg_l with (-u)%R.
apply Rle_lt_trans with u;[right; ring|idtac].
apply Rlt_le_trans with (1:=V).
right; field.
apply Rmult_lt_reg_l with 2%R.
now auto with real.
apply Rplus_lt_reg_l with (-succ u)%R.
apply Rle_lt_trans with u;[right; field|idtac].
apply Rlt_le_trans with (1:=V).
right; ring.
destruct T as (T1,T2).
apply Rnd_N_pt_monotone with F v ((u + succ u) / 2)%R...
apply round_N_pt...
apply Rnd_DN_pt_N with (succ u)%R.
pattern u at 3; replace u with (round beta fexp Zfloor ((u + succ u) / 2)).
apply round_DN_pt...
apply round_DN_eq_betw; trivial.
split; try left; assumption.
pattern (succ u) at 2; replace (succ u) with (round beta fexp Zceil ((u + succ u) / 2)).
apply round_UP_pt...
apply round_UP_eq_betw; trivial.
apply generic_format_succ...
rewrite pred_succ; trivial.
split; try left; assumption.
right; field.
Qed.
Theorem round_N_ge_midp: ∀ choice u v,
F u → ((u + pred u)/2 < v)%R
→ (u ≤ round beta fexp (Znearest choice) v)%R.
Proof with auto with typeclass_instances.
intros choice u v Fu H.
rewrite <- (Ropp_involutive v).
rewrite round_N_opp.
rewrite <- (Ropp_involutive u).
apply Ropp_le_contravar.
apply round_N_le_midp.
now apply generic_format_opp.
apply Ropp_lt_cancel.
rewrite Ropp_involutive.
apply Rle_lt_trans with (2:=H).
unfold pred.
right; field.
Qed.
Lemma round_N_eq_DN: ∀ choice x,
let d:=round beta fexp Zfloor x in
let u:=round beta fexp Zceil x in
(x<(d+u)/2)%R →
round beta fexp (Znearest choice) x = d.
Proof with auto with typeclass_instances.
intros choice x d u H.
apply Rle_antisym.
destruct (generic_format_EM beta fexp x) as [Fx|Fx].
rewrite round_generic...
apply round_DN_pt; trivial; now right.
apply round_N_le_midp.
apply round_DN_pt...
apply Rlt_le_trans with (1:=H).
right; apply f_equal2; trivial; apply f_equal.
now apply sym_eq, succ_DN_eq_UP.
apply round_ge_generic; try apply round_DN_pt...
Qed.
Lemma round_N_eq_DN_pt: ∀ choice x d u,
Rnd_DN_pt F x d → Rnd_UP_pt F x u →
(x<(d+u)/2)%R →
round beta fexp (Znearest choice) x = d.
Proof with auto with typeclass_instances.
intros choice x d u Hd Hu H.
assert (H0:(d = round beta fexp Zfloor x)%R).
apply Rnd_DN_pt_unicity with (1:=Hd).
apply round_DN_pt...
rewrite H0.
apply round_N_eq_DN.
rewrite <- H0.
rewrite Rnd_UP_pt_unicity with F x (round beta fexp Zceil x) u; try assumption.
apply round_UP_pt...
Qed.
Lemma round_N_eq_UP: ∀ choice x,
let d:=round beta fexp Zfloor x in
let u:=round beta fexp Zceil x in
((d+u)/2 < x)%R →
round beta fexp (Znearest choice) x = u.
Proof with auto with typeclass_instances.
intros choice x d u H.
apply Rle_antisym.
apply round_le_generic; try apply round_UP_pt...
destruct (generic_format_EM beta fexp x) as [Fx|Fx].
rewrite round_generic...
apply round_UP_pt; trivial; now right.
apply round_N_ge_midp.
apply round_UP_pt...
apply Rle_lt_trans with (2:=H).
right; apply f_equal2; trivial; rewrite Rplus_comm; apply f_equal2; trivial.
now apply pred_UP_eq_DN.
Qed.
Lemma round_N_eq_UP_pt: ∀ choice x d u,
Rnd_DN_pt F x d → Rnd_UP_pt F x u →
((d+u)/2 < x)%R →
round beta fexp (Znearest choice) x = u.
Proof with auto with typeclass_instances.
intros choice x d u Hd Hu H.
assert (H0:(u = round beta fexp Zceil x)%R).
apply Rnd_UP_pt_unicity with (1:=Hu).
apply round_UP_pt...
rewrite H0.
apply round_N_eq_UP.
rewrite <- H0.
rewrite Rnd_DN_pt_unicity with F x (round beta fexp Zfloor x) d; try assumption.
apply round_DN_pt...
Qed.
End Fcore_ulp.