lean2/library/algebra/order.lean

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/-
Copyright (c) 2014 Jeremy Avigad. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Module: algebra.order
Author: Jeremy Avigad
Various types of orders. We develop weak orders "≤" and strict orders "<" separately. We also
consider structures with both, where the two are related by
x < y ↔ (x ≤ y ∧ x ≠ y) (order_pair)
x ≤ y ↔ (x < y x = y) (strong_order_pair)
These might not hold constructively in some applications, but we can define additional structures
with both < and ≤ as needed.
-/
import logic.eq logic.connectives
open eq eq.ops
namespace algebra
variable {A : Type}
/- overloaded symbols -/
structure has_le [class] (A : Type) :=
(le : A → A → Prop)
structure has_lt [class] (A : Type) :=
(lt : A → A → Prop)
infixl `<=` := has_le.le
infixl `≤` := has_le.le
infixl `<` := has_lt.lt
definition has_le.ge [reducible] {A : Type} [s : has_le A] (a b : A) := b ≤ a
notation a ≥ b := has_le.ge a b
notation a >= b := has_le.ge a b
definition has_lt.gt [reducible] {A : Type} [s : has_lt A] (a b : A) := b < a
notation a > b := has_lt.gt a b
/- weak orders -/
structure weak_order [class] (A : Type) extends has_le A :=
(le_refl : ∀a, le a a)
(le_trans : ∀a b c, le a b → le b c → le a c)
(le_antisymm : ∀a b, le a b → le b a → a = b)
section
variable [s : weak_order A]
include s
theorem le.refl (a : A) : a ≤ a := !weak_order.le_refl
theorem le.trans {a b c : A} : a ≤ b → b ≤ c → a ≤ c := !weak_order.le_trans
calc_trans le.trans
theorem ge.trans {a b c : A} (H1 : a ≥ b) (H2: b ≥ c) : a ≥ c := le.trans H2 H1
calc_trans ge.trans
theorem le.antisymm {a b : A} : a ≤ b → b ≤ a → a = b := !weak_order.le_antisymm
end
structure linear_weak_order [class] (A : Type) extends weak_order A :=
(le_total : ∀a b, le a b le b a)
theorem le.total [s : linear_weak_order A] (a b : A) : a ≤ b b ≤ a :=
!linear_weak_order.le_total
/- strict orders -/
structure strict_order [class] (A : Type) extends has_lt A :=
(lt_irrefl : ∀a, ¬ lt a a)
(lt_trans : ∀a b c, lt a b → lt b c → lt a c)
section
variable [s : strict_order A]
include s
theorem lt.irrefl (a : A) : ¬ a < a := !strict_order.lt_irrefl
theorem lt.trans {a b c : A} : a < b → b < c → a < c := !strict_order.lt_trans
calc_trans lt.trans
theorem gt.trans {a b c : A} (H1 : a > b) (H2: b > c) : a > c := lt.trans H2 H1
calc_trans gt.trans
theorem ne_of_lt {a b : A} (lt_ab : a < b) : a ≠ b :=
assume eq_ab : a = b,
show false, from lt.irrefl b (eq_ab ▸ lt_ab)
theorem ne_of_gt {a b : A} (gt_ab : a > b) : a ≠ b :=
ne.symm (ne_of_lt gt_ab)
theorem lt.asymm {a b : A} (H : a < b) : ¬ b < a :=
assume H1 : b < a, lt.irrefl _ (lt.trans H H1)
end
/- well-founded orders -/
-- TODO: do these duplicate what Leo has done? if so, eliminate
structure wf_strict_order [class] (A : Type) extends strict_order A :=
(wf_rec : ∀P : A → Type, (∀x, (∀y, lt y x → P y) → P x) → ∀x, P x)
definition wf.rec_on {A : Type} [s : wf_strict_order A] {P : A → Type}
(x : A) (H : ∀x, (∀y, wf_strict_order.lt y x → P y) → P x) : P x :=
wf_strict_order.wf_rec P H x
theorem wf.ind_on.{u v} {A : Type.{u}} [s : wf_strict_order.{u 0} A] {P : A → Prop}
(x : A) (H : ∀x, (∀y, wf_strict_order.lt y x → P y) → P x) : P x :=
wf.rec_on x H
/- structures with a weak and a strict order -/
structure order_pair [class] (A : Type) extends weak_order A, has_lt A :=
(lt_iff_le_and_ne : ∀a b, lt a b ↔ (le a b ∧ a ≠ b))
section
variable [s : order_pair A]
variables {a b c : A}
include s
theorem lt_iff_le_and_ne : a < b ↔ (a ≤ b ∧ a ≠ b) :=
!order_pair.lt_iff_le_and_ne
theorem le_of_lt (H : a < b) : a ≤ b :=
and.elim_left (iff.mp lt_iff_le_and_ne H)
theorem lt_of_le_of_ne (H1 : a ≤ b) (H2 : a ≠ b) : a < b :=
iff.mp (iff.symm lt_iff_le_and_ne) (and.intro H1 H2)
private theorem lt_irrefl (s' : order_pair A) (a : A) : ¬ a < a :=
assume H : a < a,
have H1 : a ≠ a, from and.elim_right (iff.mp !lt_iff_le_and_ne H),
H1 rfl
private theorem lt_trans (s' : order_pair A) (a b c: A) (lt_ab : a < b) (lt_bc : b < c) : a < c :=
have le_ab : a ≤ b, from le_of_lt lt_ab,
have le_bc : b ≤ c, from le_of_lt lt_bc,
have le_ac : a ≤ c, from le.trans le_ab le_bc,
have ne_ac : a ≠ c, from
assume eq_ac : a = c,
have le_ba : b ≤ a, from eq_ac⁻¹ ▸ le_bc,
have eq_ab : a = b, from le.antisymm le_ab le_ba,
have ne_ab : a ≠ b, from and.elim_right (iff.mp lt_iff_le_and_ne lt_ab),
ne_ab eq_ab,
show a < c, from lt_of_le_of_ne le_ac ne_ac
definition order_pair.to_strict_order [instance] [coercion] [reducible] : strict_order A :=
⦃ strict_order, s, lt_irrefl := lt_irrefl s, lt_trans := lt_trans s ⦄
theorem lt_of_lt_of_le : a < b → b ≤ c → a < c :=
assume lt_ab : a < b,
assume le_bc : b ≤ c,
have le_ac : a ≤ c, from le.trans (le_of_lt lt_ab) le_bc,
have ne_ac : a ≠ c, from
assume eq_ac : a = c,
have le_ba : b ≤ a, from eq_ac⁻¹ ▸ le_bc,
have eq_ab : a = b, from le.antisymm (le_of_lt lt_ab) le_ba,
show false, from ne_of_lt lt_ab eq_ab,
show a < c, from lt_of_le_of_ne le_ac ne_ac
theorem lt_of_le_of_lt : a ≤ b → b < c → a < c :=
assume le_ab : a ≤ b,
assume lt_bc : b < c,
have le_ac : a ≤ c, from le.trans le_ab (le_of_lt lt_bc),
have ne_ac : a ≠ c, from
assume eq_ac : a = c,
have le_cb : c ≤ b, from eq_ac ▸ le_ab,
have eq_bc : b = c, from le.antisymm (le_of_lt lt_bc) le_cb,
show false, from ne_of_lt lt_bc eq_bc,
show a < c, from lt_of_le_of_ne le_ac ne_ac
theorem gt_of_gt_of_ge (H1 : a > b) (H2 : b ≥ c) : a > c := lt_of_le_of_lt H2 H1
theorem gt_of_ge_of_gt (H1 : a ≥ b) (H2 : b > c) : a > c := lt_of_lt_of_le H2 H1
calc_trans lt_of_lt_of_le
calc_trans lt_of_le_of_lt
calc_trans gt_of_gt_of_ge
calc_trans gt_of_ge_of_gt
theorem not_le_of_lt (H : a < b) : ¬ b ≤ a :=
assume H1 : b ≤ a,
lt.irrefl _ (lt_of_lt_of_le H H1)
theorem not_lt_of_le (H : a ≤ b) : ¬ b < a :=
assume H1 : b < a,
lt.irrefl _ (lt_of_le_of_lt H H1)
end
structure strong_order_pair [class] (A : Type) extends order_pair A :=
(le_iff_lt_or_eq : ∀a b, le a b ↔ lt a b a = b)
theorem le_iff_lt_or_eq [s : strong_order_pair A] {a b : A} : a ≤ b ↔ a < b a = b :=
!strong_order_pair.le_iff_lt_or_eq
theorem lt_or_eq_of_le [s : strong_order_pair A] {a b : A} (le_ab : a ≤ b) : a < b a = b :=
iff.mp le_iff_lt_or_eq le_ab
-- We can also construct a strong order pair by defining a strict order, and then defining
-- x ≤ y ↔ x < y x = y
structure strict_order_with_le [class] (A : Type) extends strict_order A, has_le A :=
(le_iff_lt_or_eq : ∀a b, le a b ↔ lt a b a = b)
private theorem le_refl (s : strict_order_with_le A) (a : A) : a ≤ a :=
iff.mp (iff.symm !strict_order_with_le.le_iff_lt_or_eq) (or.intro_right _ rfl)
private theorem le_trans (s : strict_order_with_le A) (a b c : A) (le_ab : a ≤ b) (le_bc : b ≤ c) : a ≤ c :=
or.elim (iff.mp !strict_order_with_le.le_iff_lt_or_eq le_ab)
(assume lt_ab : a < b,
or.elim (iff.mp !strict_order_with_le.le_iff_lt_or_eq le_bc)
(assume lt_bc : b < c,
iff.elim_right
!strict_order_with_le.le_iff_lt_or_eq (or.intro_left _ (lt.trans lt_ab lt_bc)))
(assume eq_bc : b = c, eq_bc ▸ le_ab))
(assume eq_ab : a = b,
eq_ab⁻¹ ▸ le_bc)
private theorem le_antisymm (s : strict_order_with_le A) (a b : A) (le_ab : a ≤ b) (le_ba : b ≤ a) : a = b :=
or.elim (iff.mp !strict_order_with_le.le_iff_lt_or_eq le_ab)
(assume lt_ab : a < b,
or.elim (iff.mp !strict_order_with_le.le_iff_lt_or_eq le_ba)
(assume lt_ba : b < a, absurd (lt.trans lt_ab lt_ba) (lt.irrefl a))
(assume eq_ba : b = a, eq_ba⁻¹))
(assume eq_ab : a = b, eq_ab)
private theorem lt_iff_le_ne (s : strict_order_with_le A) (a b : A) : a < b ↔ a ≤ b ∧ a ≠ b :=
iff.intro
(assume lt_ab : a < b,
have le_ab : a ≤ b, from
iff.elim_right !strict_order_with_le.le_iff_lt_or_eq (or.intro_left _ lt_ab),
show a ≤ b ∧ a ≠ b, from and.intro le_ab (ne_of_lt lt_ab))
(assume H : a ≤ b ∧ a ≠ b,
have H1 : a < b a = b, from
iff.mp !strict_order_with_le.le_iff_lt_or_eq (and.elim_left H),
show a < b, from or_resolve_left H1 (and.elim_right H))
definition strict_order_with_le.to_order_pair [instance] [coercion] [reducible] [s : strict_order_with_le A] :
strong_order_pair A :=
⦃ strong_order_pair, s,
le_refl := le_refl s,
le_trans := le_trans s,
le_antisymm := le_antisymm s,
lt_iff_le_and_ne := lt_iff_le_ne s ⦄
/- linear orders -/
structure linear_order_pair [class] (A : Type) extends order_pair A, linear_weak_order A
structure linear_strong_order_pair [class] (A : Type) extends strong_order_pair A,
linear_weak_order A
section
variable [s : linear_strong_order_pair A]
variables (a b c : A)
include s
theorem lt.trichotomy : a < b a = b b < a :=
or.elim (le.total a b)
(assume H : a ≤ b,
or.elim (iff.mp !le_iff_lt_or_eq H) (assume H1, or.inl H1) (assume H1, or.inr (or.inl H1)))
(assume H : b ≤ a,
or.elim (iff.mp !le_iff_lt_or_eq H)
(assume H1, or.inr (or.inr H1))
(assume H1, or.inr (or.inl (H1⁻¹))))
theorem lt.by_cases {a b : A} {P : Prop}
(H1 : a < b → P) (H2 : a = b → P) (H3 : b < a → P) : P :=
or.elim !lt.trichotomy
(assume H, H1 H)
(assume H, or.elim H (assume H', H2 H') (assume H', H3 H'))
definition linear_strong_order_pair.to_linear_order_pair [instance] [coercion] [reducible]
: linear_order_pair A :=
⦃ linear_order_pair, s ⦄
theorem le_of_not_lt {a b : A} (H : ¬ a < b) : b ≤ a :=
lt.by_cases (assume H', absurd H' H) (assume H', H' ▸ !le.refl) (assume H', le_of_lt H')
theorem lt_of_not_le {a b : A} (H : ¬ a ≤ b) : b < a :=
lt.by_cases
(assume H', absurd (le_of_lt H') H)
(assume H', absurd (H' ▸ !le.refl) H)
(assume H', H')
theorem lt_or_ge : a < b a ≥ b :=
lt.by_cases
(assume H1 : a < b, or.inl H1)
(assume H1 : a = b, or.inr (H1 ▸ le.refl a))
(assume H1 : a > b, or.inr (le_of_lt H1))
theorem le_or_gt : a ≤ b a > b :=
!or.swap (lt_or_ge b a)
theorem lt_or_gt_of_ne {a b : A} (H : a ≠ b) : a < b a > b :=
lt.by_cases (assume H1, or.inl H1) (assume H1, absurd H1 H) (assume H1, or.inr H1)
end
structure decidable_linear_order [class] (A : Type) extends linear_strong_order_pair A :=
(decidable_lt : decidable_rel lt)
section
variable [s : decidable_linear_order A]
variables {a b c d : A}
include s
open decidable
definition decidable_lt [instance] : decidable (a < b) :=
@decidable_linear_order.decidable_lt _ _ _ _
definition decidable_le [instance] : decidable (a ≤ b) :=
by_cases
(assume H : a < b, inl (le_of_lt H))
(assume H : ¬ a < b,
have H1 : b ≤ a, from le_of_not_lt H,
by_cases
(assume H2 : b < a, inr (not_le_of_lt H2))
(assume H2 : ¬ b < a, inl (le_of_not_lt H2)))
definition decidable_eq [instance] : decidable (a = b) :=
by_cases
(assume H : a ≤ b,
by_cases
(assume H1 : b ≤ a, inl (le.antisymm H H1))
(assume H1 : ¬ b ≤ a, inr (assume H2 : a = b, H1 (H2 ▸ le.refl a))))
(assume H : ¬ a ≤ b,
(inr (assume H1 : a = b, H (H1 ▸ !le.refl))))
-- testing equality first may result in more definitional equalities
definition lt.cases {B : Type} (a b : A) (t_lt t_eq t_gt : B) : B :=
if a = b then t_eq else (if a < b then t_lt else t_gt)
theorem lt.cases_of_eq {B : Type} {a b : A} {t_lt t_eq t_gt : B} (H : a = b) :
lt.cases a b t_lt t_eq t_gt = t_eq := if_pos H
theorem lt.cases_of_lt {B : Type} {a b : A} {t_lt t_eq t_gt : B} (H : a < b) :
lt.cases a b t_lt t_eq t_gt = t_lt :=
if_neg (ne_of_lt H) ⬝ if_pos H
theorem lt.cases_of_gt {B : Type} {a b : A} {t_lt t_eq t_gt : B} (H : a > b) :
lt.cases a b t_lt t_eq t_gt = t_gt :=
if_neg (ne.symm (ne_of_lt H)) ⬝ if_neg (lt.asymm H)
end
end algebra
/-
For reference, these are all the transitivity rules defined in this file:
calc_trans le.trans
calc_trans lt.trans
calc_trans lt_of_lt_of_le
calc_trans lt_of_le_of_lt
calc_trans ge.trans
calc_trans gt.trans
calc_trans gt_of_gt_of_ge
calc_trans gt_of_ge_of_gt
-/