lean2/library/data/real/complete.lean

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/-
Copyright (c) 2015 Robert Y. Lewis. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Author: Robert Y. Lewis
The real numbers, constructed as equivalence classes of Cauchy sequences of rationals.
This construction follows Bishop and Bridges (1985).
At this point, we no longer proceed constructively: this file makes heavy use of decidability,
excluded middle, and Hilbert choice.
Here, we show that is complete.
-/
import data.real.basic data.real.order data.real.division data.rat data.nat data.pnat
open -[coercions] rat
local notation 0 := rat.of_num 0
local notation 1 := rat.of_num 1
open -[coercions] nat
open eq.ops pnat classical
local notation 2 := subtype.tag (nat.of_num 2) dec_trivial
local notation 3 := subtype.tag (nat.of_num 3) dec_trivial
namespace rat_seq
theorem rat_approx {s : seq} (H : regular s) :
∀ n : +, ∃ q : , ∃ N : +, ∀ m : +, m ≥ N → abs (s m - q) ≤ n⁻¹ :=
begin
intro n,
existsi (s (2 * n)),
existsi 2 * n,
intro m Hm,
apply rat.le.trans,
apply H,
rewrite -(add_halves n),
apply rat.add_le_add_right,
apply inv_ge_of_le Hm
end
theorem rat_approx_seq {s : seq} (H : regular s) :
∀ n : +, ∃ q : , s_le (s_abs (sadd s (sneg (const q)))) (const n⁻¹) :=
begin
intro m,
rewrite ↑s_le,
cases rat_approx H m with [q, Hq],
cases Hq with [N, HN],
existsi q,
apply nonneg_of_bdd_within,
repeat (apply reg_add_reg | apply reg_neg_reg | apply abs_reg_of_reg | apply const_reg
| assumption),
intro n,
existsi N,
intro p Hp,
rewrite ↑[sadd, sneg, s_abs, const],
apply rat.le.trans,
rotate 1,
apply rat.sub_le_sub_left,
apply HN,
apply pnat.le.trans,
apply Hp,
rewrite -*pnat.mul.assoc,
apply pnat.mul_le_mul_left,
rewrite [sub_self, -neg_zero],
apply neg_le_neg,
apply rat.le_of_lt,
apply inv_pos
end
theorem r_rat_approx (s : reg_seq) :
∀ n : +, ∃ q : , r_le (r_abs (radd s (rneg (r_const q)))) (r_const n⁻¹) :=
rat_approx_seq (reg_seq.is_reg s)
theorem const_bound {s : seq} (Hs : regular s) (n : +) :
s_le (s_abs (sadd s (sneg (const (s n))))) (const n⁻¹) :=
begin
rewrite ↑[s_le, nonneg, s_abs, sadd, sneg, const],
intro m,
apply iff.mp !rat.le_add_iff_neg_le_sub_left,
apply rat.le.trans,
apply Hs,
apply rat.add_le_add_right,
rewrite -*pnat.mul.assoc,
apply inv_ge_of_le,
apply pnat.mul_le_mul_left
end
theorem abs_const (a : ) : const (abs a) ≡ s_abs (const a) :=
by apply equiv.refl
theorem r_abs_const (a : ) : requiv (r_const (abs a) ) (r_abs (r_const a)) := abs_const a
theorem equiv_abs_of_ge_zero {s : seq} (Hs : regular s) (Hz : s_le zero s) : s_abs s ≡ s :=
begin
apply eq_of_bdd,
apply abs_reg_of_reg Hs,
apply Hs,
intro j,
rewrite ↑s_abs,
let Hz' := s_nonneg_of_ge_zero Hs Hz,
existsi 2 * j,
intro n Hn,
cases em (s n ≥ 0) with [Hpos, Hneg],
rewrite [rat.abs_of_nonneg Hpos, sub_self, abs_zero],
apply rat.le_of_lt,
apply inv_pos,
let Hneg' := lt_of_not_ge Hneg,
have Hsn : -s n - s n > 0, from add_pos (neg_pos_of_neg Hneg') (neg_pos_of_neg Hneg'),
rewrite [rat.abs_of_neg Hneg', rat.abs_of_pos Hsn],
apply rat.le.trans,
apply rat.add_le_add,
repeat (apply rat.neg_le_neg; apply Hz'),
rewrite *rat.neg_neg,
apply rat.le.trans,
apply rat.add_le_add,
repeat (apply inv_ge_of_le; apply Hn),
rewrite pnat.add_halves,
apply rat.le.refl
end
theorem equiv_neg_abs_of_le_zero {s : seq} (Hs : regular s) (Hz : s_le s zero) : s_abs s ≡ sneg s :=
begin
apply eq_of_bdd,
apply abs_reg_of_reg Hs,
apply reg_neg_reg Hs,
intro j,
rewrite [↑s_abs, ↑s_le at Hz],
have Hz' : nonneg (sneg s), begin
apply nonneg_of_nonneg_equiv,
rotate 3,
apply Hz,
rotate 2,
apply s_zero_add,
repeat (apply Hs | apply zero_is_reg | apply reg_neg_reg | apply reg_add_reg)
end,
existsi 2 * j,
intro n Hn,
cases em (s n ≥ 0) with [Hpos, Hneg],
have Hsn : s n + s n ≥ 0, from add_nonneg Hpos Hpos,
rewrite [rat.abs_of_nonneg Hpos, ↑sneg, rat.sub_neg_eq_add, rat.abs_of_nonneg Hsn],
rewrite [↑nonneg at Hz', ↑sneg at Hz'],
apply rat.le.trans,
apply rat.add_le_add,
repeat apply (rat.le_of_neg_le_neg !Hz'),
apply rat.le.trans,
apply rat.add_le_add,
repeat (apply inv_ge_of_le; apply Hn),
rewrite pnat.add_halves,
apply rat.le.refl,
let Hneg' := lt_of_not_ge Hneg,
rewrite [rat.abs_of_neg Hneg', ↑sneg, rat.sub_neg_eq_add, rat.neg_add_eq_sub, rat.sub_self,
abs_zero],
apply rat.le_of_lt,
apply inv_pos
end
theorem r_equiv_abs_of_ge_zero {s : reg_seq} (Hz : r_le r_zero s) : requiv (r_abs s) s :=
equiv_abs_of_ge_zero (reg_seq.is_reg s) Hz
theorem r_equiv_neg_abs_of_le_zero {s : reg_seq} (Hz : r_le s r_zero) : requiv (r_abs s) (-s) :=
equiv_neg_abs_of_le_zero (reg_seq.is_reg s) Hz
end rat_seq
namespace real
open [classes] rat_seq
private theorem rewrite_helper9 (a b c : ) : b - c = (b - a) - (c - a) :=
by rewrite[-sub_add_eq_sub_sub_swap,sub_add_cancel]
private theorem rewrite_helper10 (a b c d : ) : c - d = (c - a) + (a - b) + (b - d) :=
by rewrite[*add_sub,*sub_add_cancel]
noncomputable definition rep (x : ) : rat_seq.reg_seq := some (quot.exists_rep x)
definition re_abs (x : ) : :=
quot.lift_on x (λ a, quot.mk (rat_seq.r_abs a)) (take a b Hab, quot.sound (rat_seq.r_abs_well_defined Hab))
theorem r_abs_nonneg {x : } : zero ≤ x → re_abs x = x :=
quot.induction_on x (λ a Ha, quot.sound (rat_seq.r_equiv_abs_of_ge_zero Ha))
theorem r_abs_nonpos {x : } : x ≤ zero → re_abs x = -x :=
quot.induction_on x (λ a Ha, quot.sound (rat_seq.r_equiv_neg_abs_of_le_zero Ha))
private theorem abs_const' (a : ) : of_rat (rat.abs a) = re_abs (of_rat a) := quot.sound (rat_seq.r_abs_const a)
private theorem re_abs_is_abs : re_abs = real.abs := funext
(begin
intro x,
apply eq.symm,
cases em (zero ≤ x) with [Hor1, Hor2],
rewrite [abs_of_nonneg Hor1, r_abs_nonneg Hor1],
have Hor2' : x ≤ zero, from le_of_lt (lt_of_not_ge Hor2),
rewrite [abs_of_neg (lt_of_not_ge Hor2), r_abs_nonpos Hor2']
end)
theorem abs_const (a : ) : of_rat (rat.abs a) = abs (of_rat a) :=
by rewrite -re_abs_is_abs
private theorem rat_approx' (x : ) : ∀ n : +, ∃ q : , re_abs (x - of_rat q) ≤ of_rat n⁻¹ :=
quot.induction_on x (λ s n, rat_seq.r_rat_approx s n)
theorem rat_approx (x : ) : ∀ n : +, ∃ q : , abs (x - of_rat q) ≤ of_rat n⁻¹ :=
by rewrite -re_abs_is_abs; apply rat_approx'
noncomputable definition approx (x : ) (n : +) := some (rat_approx x n)
theorem approx_spec (x : ) (n : +) : abs (x - (of_rat (approx x n))) ≤ of_rat n⁻¹ :=
some_spec (rat_approx x n)
theorem approx_spec' (x : ) (n : +) : abs ((of_rat (approx x n)) - x) ≤ of_rat n⁻¹ :=
by rewrite abs_sub; apply approx_spec
notation `r_seq` := + →
noncomputable definition converges_to (X : r_seq) (a : ) (N : + → +) :=
∀ k : +, ∀ n : +, n ≥ N k → abs (X n - a) ≤ of_rat k⁻¹
noncomputable definition cauchy (X : r_seq) (M : + → +) :=
∀ k : +, ∀ m n : +, m ≥ M k → n ≥ M k → abs (X m - X n) ≤ of_rat k⁻¹
theorem cauchy_of_converges_to {X : r_seq} {a : } {N : + → +} (Hc : converges_to X a N) :
cauchy X (λ k, N (2 * k)) :=
begin
intro k m n Hm Hn,
rewrite (rewrite_helper9 a),
apply le.trans,
apply abs_add_le_abs_add_abs,
apply le.trans,
apply add_le_add,
apply Hc,
apply Hm,
krewrite abs_neg,
apply Hc,
apply Hn,
xrewrite of_rat_add,
apply of_rat_le_of_rat_of_le,
rewrite pnat.add_halves,
apply rat.le.refl
end
private definition Nb (M : + → +) := λ k, pnat.max (3 * k) (M (2 * k))
private theorem Nb_spec_right (M : + → +) (k : +) : M (2 * k) ≤ Nb M k := !max_right
private theorem Nb_spec_left (M : + → +) (k : +) : 3 * k ≤ Nb M k := !max_left
section lim_seq
parameter {X : r_seq}
parameter {M : + → +}
hypothesis Hc : cauchy X M
include Hc
noncomputable definition lim_seq : + → :=
λ k, approx (X (Nb M k)) (2 * k)
private theorem lim_seq_reg_helper {m n : +} (Hmn : M (2 * n) ≤M (2 * m)) :
abs (of_rat (lim_seq m) - X (Nb M m)) + abs (X (Nb M m) - X (Nb M n)) + abs
(X (Nb M n) - of_rat (lim_seq n)) ≤ of_rat (m⁻¹ + n⁻¹) :=
begin
apply le.trans,
apply add_le_add_three,
apply approx_spec',
rotate 1,
apply approx_spec,
rotate 1,
apply Hc,
rotate 1,
apply Nb_spec_right,
rotate 1,
apply pnat.le.trans,
apply Hmn,
apply Nb_spec_right,
rewrite [*of_rat_add, rat.add.assoc, pnat.add_halves],
apply of_rat_le_of_rat_of_le,
apply rat.add_le_add_right,
apply inv_ge_of_le,
apply pnat.mul_le_mul_left
end
theorem lim_seq_reg : rat_seq.regular lim_seq :=
begin
rewrite ↑rat_seq.regular,
intro m n,
apply le_of_rat_le_of_rat,
rewrite [abs_const, -of_rat_sub, (rewrite_helper10 (X (Nb M m)) (X (Nb M n)))],
apply real.le.trans,
apply abs_add_three,
cases em (M (2 * m) ≥ M (2 * n)) with [Hor1, Hor2],
apply lim_seq_reg_helper Hor1,
let Hor2' := pnat.le_of_lt (pnat.lt_of_not_le Hor2),
rewrite [real.abs_sub (X (Nb M n)), abs_sub (X (Nb M m)), abs_sub,
rat.add.comm, add_comm_three],
apply lim_seq_reg_helper Hor2'
end
theorem lim_seq_spec (k : +) :
rat_seq.s_le (rat_seq.s_abs (rat_seq.sadd lim_seq (rat_seq.sneg (rat_seq.const (lim_seq k))))) (rat_seq.const k⁻¹) :=
by apply rat_seq.const_bound; apply lim_seq_reg
private noncomputable definition r_lim_seq : rat_seq.reg_seq :=
rat_seq.reg_seq.mk lim_seq lim_seq_reg
private theorem r_lim_seq_spec (k : +) : rat_seq.r_le
(rat_seq.r_abs ((rat_seq.radd r_lim_seq (rat_seq.rneg (rat_seq.r_const ((rat_seq.reg_seq.sq r_lim_seq) k))))))
(rat_seq.r_const k⁻¹) :=
lim_seq_spec k
noncomputable definition lim : :=
quot.mk r_lim_seq
theorem re_lim_spec (k : +) : re_abs (lim - (of_rat (lim_seq k))) ≤ of_rat k⁻¹ :=
r_lim_seq_spec k
theorem lim_spec' (k : +) : abs (lim - (of_rat (lim_seq k))) ≤ of_rat k⁻¹ :=
by rewrite -re_abs_is_abs; apply re_lim_spec
theorem lim_spec (k : +) :
abs ((of_rat (lim_seq k)) - lim) ≤ of_rat k⁻¹ :=
by rewrite abs_sub; apply lim_spec'
theorem converges_of_cauchy : converges_to X lim (Nb M) :=
begin
intro k n Hn,
rewrite (rewrite_helper10 (X (Nb M n)) (of_rat (lim_seq n))),
apply le.trans,
apply abs_add_three,
apply le.trans,
apply add_le_add_three,
apply Hc,
apply pnat.le.trans,
rotate 1,
apply Hn,
rotate_right 1,
apply Nb_spec_right,
have HMk : M (2 * k) ≤ Nb M n, begin
apply pnat.le.trans,
apply Nb_spec_right,
apply pnat.le.trans,
apply Hn,
apply pnat.le.trans,
apply mul_le_mul_left 3,
apply Nb_spec_left
end,
apply HMk,
rewrite ↑lim_seq,
apply approx_spec,
apply lim_spec,
rewrite 2 of_rat_add,
apply of_rat_le_of_rat_of_le,
apply rat.le.trans,
apply rat.add_le_add_three,
apply rat.le.refl,
apply inv_ge_of_le,
apply pnat_mul_le_mul_left',
apply pnat.le.trans,
rotate 1,
apply Hn,
rotate_right 1,
apply Nb_spec_left,
apply inv_ge_of_le,
apply pnat.le.trans,
rotate 1,
apply Hn,
rotate_right 1,
apply Nb_spec_left,
rewrite [-*pnat.mul.assoc, pnat.p_add_fractions],
apply rat.le.refl
end
end lim_seq
-------------------------------------------
-- int embedding theorems
-- archimedean properties, integer floor and ceiling
section ints
open int
theorem archimedean_upper (x : ) : ∃ z : , x ≤ of_rat (of_int z) :=
begin
apply quot.induction_on x,
intro s,
cases rat_seq.bdd_of_regular (rat_seq.reg_seq.is_reg s) with [b, Hb],
existsi ubound b,
have H : rat_seq.s_le (rat_seq.reg_seq.sq s) (rat_seq.const (rat.of_nat (ubound b))), begin
apply rat_seq.s_le_of_le_pointwise (rat_seq.reg_seq.is_reg s),
apply rat_seq.const_reg,
intro n,
apply rat.le.trans,
apply Hb,
apply ubound_ge
end,
apply H
end
theorem archimedean_upper_strict (x : ) : ∃ z : , x < of_rat (of_int z) :=
begin
cases archimedean_upper x with [z, Hz],
existsi z + 1,
apply lt_of_le_of_lt,
apply Hz,
apply of_rat_lt_of_rat_of_lt,
apply iff.mpr !of_int_lt_of_int,
apply int.lt_add_of_pos_right,
apply dec_trivial
end
theorem archimedean_lower (x : ) : ∃ z : , x ≥ of_rat (of_int z) :=
begin
cases archimedean_upper (-x) with [z, Hz],
existsi -z,
rewrite [of_int_neg, of_rat_neg],
apply iff.mp !neg_le_iff_neg_le Hz
end
theorem archimedean_lower_strict (x : ) : ∃ z : , x > of_rat (of_int z) :=
begin
cases archimedean_upper_strict (-x) with [z, Hz],
existsi -z,
rewrite [of_int_neg, of_rat_neg],
apply iff.mp !neg_lt_iff_neg_lt Hz
end
definition ex_floor (x : ) :=
(@ex_largest_of_bdd (λ z, x ≥ of_rat (of_int z)) _
(begin
existsi some (archimedean_upper_strict x),
let Har := some_spec (archimedean_upper_strict x),
intros z Hz,
apply not_le_of_gt,
apply lt_of_lt_of_le,
apply Har,
have H : of_rat (of_int (some (archimedean_upper_strict x))) ≤ of_rat (of_int z), begin
apply of_rat_le_of_rat_of_le,
apply iff.mpr !of_int_le_of_int,
apply Hz
end,
exact H
end)
(by existsi some (archimedean_lower x); apply some_spec (archimedean_lower x)))
noncomputable definition floor (x : ) : :=
some (ex_floor x)
noncomputable definition ceil (x : ) : := - floor (-x)
theorem floor_spec (x : ) : of_rat (of_int (floor x)) ≤ x :=
and.left (some_spec (ex_floor x))
theorem floor_largest {x : } {z : } (Hz : z > floor x) : x < of_rat (of_int z) :=
begin
apply lt_of_not_ge,
cases some_spec (ex_floor x),
apply a_1 _ Hz
end
theorem ceil_spec (x : ) : of_rat (of_int (ceil x)) ≥ x :=
begin
rewrite [↑ceil, of_int_neg, of_rat_neg],
apply iff.mp !le_neg_iff_le_neg,
apply floor_spec
end
theorem ceil_smallest {x : } {z : } (Hz : z < ceil x) : x > of_rat (of_int z) :=
begin
rewrite ↑ceil at Hz,
let Hz' := floor_largest (iff.mp !int.lt_neg_iff_lt_neg Hz),
rewrite [of_int_neg at Hz', of_rat_neg at Hz'],
apply lt_of_neg_lt_neg Hz'
end
theorem floor_succ (x : ) : (floor x) + 1 = floor (x + 1) :=
begin
apply by_contradiction,
intro H,
cases int.lt_or_gt_of_ne H with [Hlt, Hgt],
let Hl := floor_largest (iff.mp !int.add_lt_iff_lt_sub_right Hlt),
rewrite [of_int_sub at Hl, -of_rat_sub at Hl],
apply not_le_of_gt (iff.mpr !add_lt_iff_lt_sub_right Hl) !floor_spec,
let Hl := floor_largest Hgt,
rewrite [of_int_add at Hl, -of_rat_add at Hl],
apply not_le_of_gt (lt_of_add_lt_add_right Hl) !floor_spec
end
theorem floor_succ_lt (x : ) : floor (x - 1) < floor x :=
begin
apply @int.lt_of_add_lt_add_right _ 1,
rewrite [floor_succ (x - 1), sub_add_cancel],
apply int.lt_add_of_pos_right dec_trivial
end
theorem ceil_succ (x : ) : ceil x < ceil (x + 1) :=
begin
rewrite [↑ceil, neg_add],
apply int.neg_lt_neg,
apply floor_succ_lt
end
end ints
--------------------------------------------------
-- supremum property
-- this development roughly follows the proof of completeness done in Isabelle.
-- It does not depend on the previous proof of Cauchy completeness. Much of the same
-- machinery can be used to show that Cauchy completeness implies the supremum property.
section supremum
open prod nat
local postfix `~` := nat_of_pnat
-- The top part of this section could be refactored. What is the appropriate place to define
-- bounds, supremum, etc? In algebra/ordered_field? They potentially apply to more than just .
local notation 2 := (1 : ) + 1
parameter X : → Prop
-- this definition belongs somewhere else. Where?
definition rpt {A : Type} (op : A → A) : → A → A
| rpt 0 := λ a, a
| rpt (succ k) := λ a, op (rpt k a)
definition ub (x : ) := ∀ y : , X y → y ≤ x
definition sup (x : ) := ub x ∧ ∀ y : , ub y → x ≤ y
definition lb (x : ) := ∀ y : , X y → x ≤ y
definition inf (x : ) := lb x ∧ ∀ y : , lb y → y ≤ x
parameter elt :
hypothesis inh : X elt
parameter bound :
hypothesis bdd : ub bound
include inh bdd
-- this should exist somewhere, no? I can't find it
theorem not_forall_of_exists_not {A : Type} {P : A → Prop} (H : ∃ a : A, ¬ P a) :
¬ ∀ a : A, P a :=
begin
intro Hall,
cases H with [a, Ha],
apply Ha (Hall a)
end
definition avg (a b : ) := a / 2 + b / 2
noncomputable definition bisect (ab : × ) :=
if ub (avg (pr1 ab) (pr2 ab)) then
(pr1 ab, (avg (pr1 ab) (pr2 ab)))
else
(avg (pr1 ab) (pr2 ab), pr2 ab)
noncomputable definition under : := of_int (floor (elt - 1))
theorem under_spec1 : of_rat under < elt :=
have H : of_rat under < of_rat (of_int (floor elt)), begin
apply of_rat_lt_of_rat_of_lt,
apply iff.mpr !of_int_lt_of_int,
apply floor_succ_lt
end,
lt_of_lt_of_le H !floor_spec
theorem under_spec : ¬ ub under :=
begin
rewrite ↑ub,
apply not_forall_of_exists_not,
existsi elt,
apply iff.mpr not_implies_iff_and_not,
apply and.intro,
apply inh,
apply not_le_of_gt under_spec1
end
noncomputable definition over : := of_int (ceil (bound + 1)) -- b
theorem over_spec1 : bound < of_rat over :=
have H : of_rat (of_int (ceil bound)) < of_rat over, begin
apply of_rat_lt_of_rat_of_lt,
apply iff.mpr !of_int_lt_of_int,
apply ceil_succ
end,
lt_of_le_of_lt !ceil_spec H
theorem over_spec : ub over :=
begin
rewrite ↑ub,
intro y Hy,
apply le_of_lt,
apply lt_of_le_of_lt,
apply bdd,
apply Hy,
apply over_spec1
end
noncomputable definition under_seq := λ n : , pr1 (rpt bisect n (under, over)) -- A
noncomputable definition over_seq := λ n : , pr2 (rpt bisect n (under, over)) -- B
noncomputable definition avg_seq := λ n : , avg (over_seq n) (under_seq n) -- C
theorem avg_symm (n : ) : avg_seq n = avg (under_seq n) (over_seq n) :=
by rewrite [↑avg_seq, ↑avg, rat.add.comm]
theorem over_0 : over_seq 0 = over := rfl
theorem under_0 : under_seq 0 = under := rfl
theorem succ_helper (n : ) :
avg (pr1 (rpt bisect n (under, over))) (pr2 (rpt bisect n (under, over))) = avg_seq n :=
by rewrite avg_symm
theorem under_succ (n : ) : under_seq (succ n) =
(if ub (avg_seq n) then under_seq n else avg_seq n) :=
begin
cases em (ub (avg_seq n)) with [Hub, Hub],
rewrite [if_pos Hub],
have H : pr1 (bisect (rpt bisect n (under, over))) = under_seq n, by
rewrite [↑under_seq, ↑bisect at {2}, -succ_helper at Hub, if_pos Hub],
apply H,
rewrite [if_neg Hub],
have H : pr1 (bisect (rpt bisect n (under, over))) = avg_seq n, by
rewrite [↑bisect at {2}, -succ_helper at Hub, if_neg Hub, avg_symm],
apply H
end
theorem over_succ (n : ) : over_seq (succ n) =
(if ub (avg_seq n) then avg_seq n else over_seq n) :=
begin
cases em (ub (avg_seq n)) with [Hub, Hub],
rewrite [if_pos Hub],
have H : pr2 (bisect (rpt bisect n (under, over))) = avg_seq n, by
rewrite [↑bisect at {2}, -succ_helper at Hub, if_pos Hub, avg_symm],
apply H,
rewrite [if_neg Hub],
have H : pr2 (bisect (rpt bisect n (under, over))) = over_seq n, by
rewrite [↑over_seq, ↑bisect at {2}, -succ_helper at Hub, if_neg Hub],
apply H
end
theorem width (n : ) : over_seq n - under_seq n = (over - under) / (rat.pow 2 n) :=
nat.induction_on n
(by xrewrite [over_0, under_0, rat.pow_zero, rat.div_one])
(begin
intro a Ha,
rewrite [over_succ, under_succ],
let Hou := calc
(over_seq a) / 2 - (under_seq a) / 2 = ((over - under) / rat.pow 2 a) / 2 :
by rewrite [rat.div_sub_div_same, Ha]
... = (over - under) / (rat.pow 2 a * 2) : rat.div_div_eq_div_mul
... = (over - under) / rat.pow 2 (a + 1) : by rewrite rat.pow_add,
cases em (ub (avg_seq a)),
rewrite [*if_pos a_1, -add_one, -Hou, ↑avg_seq, ↑avg, rat.add.assoc, rat.div_two_sub_self],
rewrite [*if_neg a_1, -add_one, -Hou, ↑avg_seq, ↑avg, rat.sub_add_eq_sub_sub,
rat.sub_self_div_two]
end)
theorem binary_nat_bound (a : ) : of_nat a ≤ (rat.pow 2 a) :=
nat.induction_on a (rat.zero_le_one)
(take n, assume Hn,
calc
of_nat (succ n) = (of_nat n) + 1 : of_nat_add
... ≤ rat.pow 2 n + 1 : rat.add_le_add_right Hn
... ≤ rat.pow 2 n + rat.pow 2 n :
rat.add_le_add_left (rat.pow_ge_one_of_ge_one rat.two_ge_one _)
... = rat.pow 2 (succ n) : rat.pow_two_add)
theorem binary_bound (a : ) : ∃ n : , a ≤ rat.pow 2 n :=
exists.intro (ubound a) (calc
a ≤ of_nat (ubound a) : ubound_ge
... ≤ rat.pow 2 (ubound a) : binary_nat_bound)
theorem rat_power_two_le (k : +) : rat_of_pnat k ≤ rat.pow 2 k~ :=
!binary_nat_bound
theorem width_narrows : ∃ n : , over_seq n - under_seq n ≤ 1 :=
begin
cases binary_bound (over - under) with [a, Ha],
existsi a,
rewrite (width a),
apply rat.div_le_of_le_mul,
apply rat.pow_pos dec_trivial,
rewrite rat.mul_one,
apply Ha
end
noncomputable definition over' := over_seq (some width_narrows)
noncomputable definition under' := under_seq (some width_narrows)
noncomputable definition over_seq' := λ n, over_seq (n + some width_narrows)
noncomputable definition under_seq' := λ n, under_seq (n + some width_narrows)
theorem over_seq'0 : over_seq' 0 = over' :=
by rewrite [↑over_seq', nat.zero_add]
theorem under_seq'0 : under_seq' 0 = under' :=
by rewrite [↑under_seq', nat.zero_add]
theorem under_over' : over' - under' ≤ 1 := some_spec width_narrows
theorem width' (n : ) : over_seq' n - under_seq' n ≤ 1 / rat.pow 2 n :=
nat.induction_on n
(begin
xrewrite [over_seq'0, under_seq'0, rat.pow_zero, rat.div_one],
apply under_over'
end)
(begin
intros a Ha,
rewrite [↑over_seq' at *, ↑under_seq' at *, *succ_add at *, width at *,
-add_one, -(add_one a), rat.pow_add, rat.pow_add _ a 1, *rat.pow_one],
apply rat.div_mul_le_div_mul_of_div_le_div_pos' Ha dec_trivial
end)
theorem PA (n : ) : ¬ ub (under_seq n) :=
nat.induction_on n
(by rewrite under_0; apply under_spec)
(begin
intro a Ha,
rewrite under_succ,
cases em (ub (avg_seq a)),
rewrite (if_pos a_1),
assumption,
rewrite (if_neg a_1),
assumption
end)
theorem PB (n : ) : ub (over_seq n) :=
nat.induction_on n
(by rewrite over_0; apply over_spec)
(begin
intro a Ha,
rewrite over_succ,
cases em (ub (avg_seq a)),
rewrite (if_pos a_1),
assumption,
rewrite (if_neg a_1),
assumption
end)
theorem under_lt_over : under < over :=
begin
cases exists_not_of_not_forall under_spec with [x, Hx],
cases iff.mp not_implies_iff_and_not Hx with [HXx, Hxu],
apply lt_of_rat_lt_of_rat,
apply lt_of_lt_of_le,
apply lt_of_not_ge Hxu,
apply over_spec _ HXx
end
theorem under_seq_lt_over_seq : ∀ m n : , under_seq m < over_seq n :=
begin
intros,
cases exists_not_of_not_forall (PA m) with [x, Hx],
cases iff.mp not_implies_iff_and_not Hx with [HXx, Hxu],
apply lt_of_rat_lt_of_rat,
apply lt_of_lt_of_le,
apply lt_of_not_ge Hxu,
apply PB,
apply HXx
end
theorem under_seq_lt_over_seq_single : ∀ n : , under_seq n < over_seq n :=
by intros; apply under_seq_lt_over_seq
theorem under_seq'_lt_over_seq' : ∀ m n : , under_seq' m < over_seq' n :=
by intros; apply under_seq_lt_over_seq
theorem under_seq'_lt_over_seq'_single : ∀ n : , under_seq' n < over_seq' n :=
by intros; apply under_seq_lt_over_seq
theorem under_seq_mono_helper (i k : ) : under_seq i ≤ under_seq (i + k) :=
(nat.induction_on k
(by rewrite nat.add_zero; apply rat.le.refl)
(begin
intros a Ha,
rewrite [add_succ, under_succ],
cases em (ub (avg_seq (i + a))) with [Havg, Havg],
rewrite (if_pos Havg),
apply Ha,
rewrite [if_neg Havg, ↑avg_seq, ↑avg],
apply rat.le.trans,
apply Ha,
rewrite -rat.add_halves at {1},
apply rat.add_le_add_right,
apply rat.div_le_div_of_le_of_pos,
apply rat.le_of_lt,
apply under_seq_lt_over_seq,
apply dec_trivial
end))
theorem under_seq_mono (i j : ) (H : i ≤ j) : under_seq i ≤ under_seq j :=
begin
cases le.elim H with [k, Hk'],
rewrite -Hk',
apply under_seq_mono_helper
end
theorem over_seq_mono_helper (i k : ) : over_seq (i + k) ≤ over_seq i :=
nat.induction_on k
(by rewrite nat.add_zero; apply rat.le.refl)
(begin
intros a Ha,
rewrite [add_succ, over_succ],
cases em (ub (avg_seq (i + a))) with [Havg, Havg],
rewrite [if_pos Havg, ↑avg_seq, ↑avg],
apply rat.le.trans,
rotate 1,
apply Ha,
rotate 1,
rewrite -{over_seq (i + a)}rat.add_halves at {2},
apply rat.add_le_add_left,
apply rat.div_le_div_of_le_of_pos,
apply rat.le_of_lt,
apply under_seq_lt_over_seq,
apply dec_trivial,
rewrite [if_neg Havg],
apply Ha
end)
theorem over_seq_mono (i j : ) (H : i ≤ j) : over_seq j ≤ over_seq i :=
begin
cases le.elim H with [k, Hk'],
rewrite -Hk',
apply over_seq_mono_helper
end
theorem rat_power_two_inv_ge (k : +) : 1 / rat.pow 2 k~ ≤ k⁻¹ :=
rat.div_le_div_of_le !rat_of_pnat_is_pos !rat_power_two_le
open rat_seq
theorem regular_lemma_helper {s : seq} {m n : +} (Hm : m ≤ n)
(H : ∀ n i : +, i ≥ n → under_seq' n~ ≤ s i ∧ s i ≤ over_seq' n~) :
rat.abs (s m - s n) ≤ m⁻¹ + n⁻¹ :=
begin
cases H m n Hm with [T1under, T1over],
cases H m m (!pnat.le.refl) with [T2under, T2over],
apply rat.le.trans,
apply rat.dist_bdd_within_interval,
apply under_seq'_lt_over_seq'_single,
rotate 1,
repeat assumption,
apply rat.le.trans,
apply width',
apply rat.le.trans,
apply rat_power_two_inv_ge,
apply rat.le_add_of_nonneg_right,
apply rat.le_of_lt (!inv_pos)
end
theorem regular_lemma (s : seq) (H : ∀ n i : +, i ≥ n → under_seq' n~ ≤ s i ∧ s i ≤ over_seq' n~) :
regular s :=
begin
rewrite ↑regular,
intros,
cases em (m ≤ n) with [Hm, Hn],
apply regular_lemma_helper Hm H,
let T := regular_lemma_helper (pnat.le_of_lt (pnat.lt_of_not_le Hn)) H,
rewrite [rat.abs_sub at T, {n⁻¹ + _}rat.add.comm at T],
exact T
end
noncomputable definition p_under_seq : seq := λ n : +, under_seq' n~
noncomputable definition p_over_seq : seq := λ n : +, over_seq' n~
theorem under_seq_regular : regular p_under_seq :=
begin
apply regular_lemma,
intros n i Hni,
apply and.intro,
apply under_seq_mono,
apply nat.add_le_add_right Hni,
apply rat.le_of_lt,
apply under_seq_lt_over_seq
end
theorem over_seq_regular : regular p_over_seq :=
begin
apply regular_lemma,
intros n i Hni,
apply and.intro,
apply rat.le_of_lt,
apply under_seq_lt_over_seq,
apply over_seq_mono,
apply nat.add_le_add_right Hni
end
noncomputable definition sup_over : := quot.mk (reg_seq.mk p_over_seq over_seq_regular)
noncomputable definition sup_under : := quot.mk (reg_seq.mk p_under_seq under_seq_regular)
theorem over_bound : ub sup_over :=
begin
rewrite ↑ub,
intros y Hy,
apply le_of_le_reprs,
intro n,
apply PB,
apply Hy
end
theorem under_lowest_bound : ∀ y : , ub y → sup_under ≤ y :=
begin
intros y Hy,
apply le_of_reprs_le,
intro n,
cases exists_not_of_not_forall (PA _) with [x, Hx],
cases iff.mp not_implies_iff_and_not Hx with [HXx, Hxn],
apply le.trans,
apply le_of_lt,
apply lt_of_not_ge Hxn,
apply Hy,
apply HXx
end
theorem under_over_equiv : p_under_seq ≡ p_over_seq :=
begin
intros,
apply rat.le.trans,
have H : p_under_seq n < p_over_seq n, from !under_seq_lt_over_seq,
rewrite [rat.abs_of_neg (iff.mpr !rat.sub_neg_iff_lt H), rat.neg_sub],
apply width',
apply rat.le.trans,
apply rat_power_two_inv_ge,
apply rat.le_add_of_nonneg_left,
apply rat.le_of_lt !inv_pos
end
theorem under_over_eq : sup_under = sup_over := quot.sound under_over_equiv
theorem ex_sup_of_inh_of_bdd : ∃ x : , sup x :=
exists.intro sup_over (and.intro over_bound (under_over_eq ▸ under_lowest_bound))
end supremum
definition bounding_set (X : → Prop) (x : ) : Prop := ∀ y : , X y → x ≤ y
theorem ex_inf_of_inh_of_bdd (X : → Prop) (elt : ) (inh : X elt) (bound : )
(bdd : lb X bound) : ∃ x : , inf X x :=
begin
have Hinh : bounding_set X bound, begin
intros y Hy,
apply bdd,
apply Hy
end,
have Hub : ub (bounding_set X) elt, begin
intros y Hy,
apply Hy,
apply inh
end,
cases ex_sup_of_inh_of_bdd _ _ Hinh _ Hub with [supr, Hsupr],
existsi supr,
cases Hsupr with [Hubs1, Hubs2],
apply and.intro,
intros,
apply Hubs2,
intros z Hz,
apply Hz,
apply a,
intros y Hlby,
apply Hubs1,
intros z Hz,
apply Hlby,
apply Hz
end
end real