1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
|
(************************************************************************)
(* v * The Coq Proof Assistant / The Coq Development Team *)
(* <O___,, * INRIA - CNRS - LIX - LRI - PPS - Copyright 1999-2016 *)
(* \VV/ **************************************************************)
(* // * This file is distributed under the terms of the *)
(* * GNU Lesser General Public License Version 2.1 *)
(************************************************************************)
(** This module proves some logical properties of the axiomatic of Reals.
- Decidability of arithmetical statements.
- Derivability of the Archimedean "axiom".
- Decidability of negated formulas.
*)
Require Import RIneq.
(** * Decidability of arithmetical statements *)
(** One can iterate this lemma and use classical logic to decide any
statement in the arithmetical hierarchy. *)
Section Arithmetical_dec.
Variable P : nat -> Prop.
Hypothesis HP : forall n, {P n} + {~P n}.
Lemma sig_forall_dec : {n | ~P n} + {forall n, P n}.
Proof.
assert (Hi: (forall n, 0 < INR n + 1)%R).
intros n.
apply Rle_lt_0_plus_1, pos_INR.
set (u n := (if HP n then 0 else / (INR n + 1))%R).
assert (Bu: forall n, (u n <= 1)%R).
intros n.
unfold u.
case HP ; intros _.
apply Rle_0_1.
rewrite <- S_INR, <- Rinv_1.
apply Rinv_le_contravar with (1 := Rlt_0_1).
apply (le_INR 1), le_n_S, le_0_n.
set (E y := exists n, y = u n).
destruct (completeness E) as [l [ub lub]].
exists R1.
intros y [n ->].
apply Bu.
exists (u O).
now exists O.
assert (Hnp: forall n, not (P n) -> ((/ (INR n + 1) <= l)%R)).
intros n Hp.
apply ub.
exists n.
unfold u.
now destruct (HP n).
destruct (Rle_lt_dec l 0) as [Hl|Hl].
right.
intros n.
destruct (HP n) as [H|H].
exact H.
exfalso.
apply Rle_not_lt with (1 := Hl).
apply Rlt_le_trans with (/ (INR n + 1))%R.
now apply Rinv_0_lt_compat.
now apply Hnp.
left.
set (N := Zabs_nat (up (/l) - 2)).
assert (H1l: (1 <= /l)%R).
rewrite <- Rinv_1.
apply Rinv_le_contravar with (1 := Hl).
apply lub.
now intros y [m ->].
assert (HN: (INR N + 1 = IZR (up (/ l)) - 1)%R).
unfold N.
rewrite INR_IZR_INZ.
rewrite inj_Zabs_nat.
replace (IZR (up (/ l)) - 1)%R with (IZR (up (/ l) - 2) + 1)%R.
apply (f_equal (fun v => IZR v + 1)%R).
apply Zabs_eq.
apply Zle_minus_le_0.
apply (Zlt_le_succ 1).
apply lt_IZR.
apply Rle_lt_trans with (1 := H1l).
apply archimed.
rewrite minus_IZR.
simpl.
ring.
assert (Hl': (/ (INR (S N) + 1) < l)%R).
rewrite <- (Rinv_involutive l) by now apply Rgt_not_eq.
apply Rinv_1_lt_contravar with (1 := H1l).
rewrite S_INR.
rewrite HN.
ring_simplify.
apply archimed.
exists N.
intros H.
apply Rle_not_lt with (2 := Hl').
apply lub.
intros y [n ->].
unfold u.
destruct (HP n) as [_|Hp].
apply Rlt_le.
now apply Rinv_0_lt_compat.
apply Rinv_le_contravar.
apply Hi.
apply Rplus_le_compat_r.
apply le_INR.
destruct (le_or_lt n N) as [Hn|Hn].
2: now apply lt_le_S.
exfalso.
destruct (le_lt_or_eq _ _ Hn) as [Hn'| ->].
2: now apply Hp.
apply Rlt_not_le with (2 := Hnp _ Hp).
rewrite <- (Rinv_involutive l) by now apply Rgt_not_eq.
apply Rinv_1_lt_contravar.
rewrite <- S_INR.
apply (le_INR 1), le_n_S, le_0_n.
apply Rlt_le_trans with (INR N + 1)%R.
apply Rplus_lt_compat_r.
now apply lt_INR.
rewrite HN.
apply Rplus_le_reg_r with (-/l + 1)%R.
ring_simplify.
apply archimed.
Qed.
End Arithmetical_dec.
(** * Derivability of the Archimedean axiom *)
(** This is a standard proof (it has been taken from PlanetMath). It is
formulated negatively so as to avoid the need for classical
logic. Using a proof of [{n | ~P n}+{forall n, P n}], we can in
principle also derive [up] and its specification. The proof above
cannot be used for that purpose, since it relies on the [archimed] axiom. *)
Theorem not_not_archimedean :
forall r : R, ~ (forall n : nat, (INR n <= r)%R).
Proof.
intros r H.
set (E := fun r => exists n : nat, r = INR n).
assert (exists x : R, E x) by
(exists 0%R; simpl; red; exists 0%nat; reflexivity).
assert (bound E) by (exists r; intros x (m,H2); rewrite H2; apply H).
destruct (completeness E) as (M,(H3,H4)); try assumption.
set (M' := (M + -1)%R).
assert (H2 : ~ is_upper_bound E M').
intro H5.
assert (M <= M')%R by (apply H4; exact H5).
apply (Rlt_not_le M M').
unfold M'.
pattern M at 2.
rewrite <- Rplus_0_l.
pattern (0 + M)%R.
rewrite Rplus_comm.
rewrite <- (Rplus_opp_r 1).
apply Rplus_lt_compat_l.
rewrite Rplus_comm.
apply Rlt_plus_1.
assumption.
apply H2.
intros N (n,H7).
rewrite H7.
unfold M'.
assert (H5 : (INR (S n) <= M)%R) by (apply H3; exists (S n); reflexivity).
rewrite S_INR in H5.
assert (H6 : (INR n + 1 + -1 <= M + -1)%R).
apply Rplus_le_compat_r.
assumption.
rewrite Rplus_assoc in H6.
rewrite Rplus_opp_r in H6.
rewrite (Rplus_comm (INR n) 0) in H6.
rewrite Rplus_0_l in H6.
assumption.
Qed.
(** * Decidability of negated formulas *)
Lemma sig_not_dec : forall P : Prop, {not (not P)} + {not P}.
Proof.
intros P.
set (E := fun x => x = R0 \/ (x = R1 /\ P)).
destruct (completeness E) as [x H].
exists R1.
intros x [->|[-> _]].
apply Rle_0_1.
apply Rle_refl.
exists R0.
now left.
destruct (Rle_lt_dec 1 x) as [H'|H'].
- left.
intros HP.
elim Rle_not_lt with (1 := H').
apply Rle_lt_trans with (2 := Rlt_0_1).
apply H.
intros y [->|[_ Hy]].
apply Rle_refl.
now elim HP.
- right.
intros HP.
apply Rlt_not_le with (1 := H').
apply H.
right.
now split.
Qed.
|
