lean2/library/algebra/complete_lattice.lean

/-
Copyright (c) 2015 Microsoft Corporation. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Author: Leonardo de Moura

Complete lattices

TODO: define dual complete lattice and simplify proof of dual theorems.
-/
import algebra.lattice data.set.basic algebra.monotone
open set

variable {A : Type}

structure complete_lattice [class] (A : Type) extends lattice A :=
(Inf : set A → A)
(Sup : set A → A)
(Inf_le : ∀ {a : A} {s : set A}, a ∈ s → le (Inf s) a)
(le_Inf : ∀ {b : A} {s : set A}, (∀ (a : A), a ∈ s → le b a) → le b (Inf s))
(le_Sup : ∀ {a : A} {s : set A}, a ∈ s → le a (Sup s))
(Sup_le : ∀ {b : A} {s : set A} (h : ∀ (a : A), a ∈ s → le a b), le (Sup s) b)

section
  variable [complete_lattice A]

  definition Inf (S : set A) : A := complete_lattice.Inf S
  prefix `⨅ `:70 := Inf

  definition Sup (S : set A) : A := complete_lattice.Sup S
  prefix `⨆ `:65 := Sup

  theorem Inf_le {a : A} {s : set A} (H : a ∈ s) : (Inf s) ≤ a := complete_lattice.Inf_le H

  theorem le_Inf {b : A} {s : set A} (H : ∀ (a : A), a ∈ s → b ≤ a) : b ≤ Inf s :=
    complete_lattice.le_Inf H

  theorem le_Sup {a : A} {s : set A} (H : a ∈ s) : a ≤ Sup s := complete_lattice.le_Sup H

  theorem Sup_le {b : A} {s : set A} (H : ∀ (a : A), a ∈ s → a ≤ b) : Sup s ≤ b :=
    complete_lattice.Sup_le H
end

-- Minimal complete_lattice definition based just on Inf.
-- We later show that complete_lattice_Inf is a complete_lattice.
structure complete_lattice_Inf [class] (A : Type) extends weak_order A :=
(Inf : set A → A)
(Inf_le : ∀ {a : A} {s : set A}, a ∈ s → le (Inf s) a)
(le_Inf : ∀ {b : A} {s : set A}, (∀ (a : A), a ∈ s → le b a) → le b (Inf s))

-- Minimal complete_lattice definition based just on Sup.
-- We later show that complete_lattice_Sup is a complete_lattice.
structure complete_lattice_Sup [class] (A : Type) extends weak_order A :=
(Sup : set A → A)
(le_Sup : ∀ {a : A} {s : set A}, a ∈ s → le a (Sup s))
(Sup_le : ∀ {b : A} {s : set A} (h : ∀ (a : A), a ∈ s → le a b), le (Sup s) b)

namespace complete_lattice_Inf
variable [C : complete_lattice_Inf A]
include C
definition Sup (s : set A) : A :=
Inf {b | ∀ a, a ∈ s → a ≤ b}

local prefix `⨅`:70 := Inf
local prefix `⨆`:65 := Sup

lemma le_Sup {a : A} {s : set A} : a ∈ s → a ≤ ⨆ s :=
suppose a ∈ s, le_Inf
  (show ∀ (b : A), (∀ (a : A), a ∈ s → a ≤ b) → a ≤ b, from
   take b, assume h, h a `a ∈ s`)

lemma Sup_le {b : A} {s : set A} (h : ∀ (a : A), a ∈ s → a ≤ b) : ⨆ s ≤ b :=
Inf_le h

definition inf (a b : A) := ⨅ '{a, b}
definition sup (a b : A) := ⨆ '{a, b}

local infix `⊓` := inf
local infix `⊔` := sup

lemma inf_le_left (a b : A) : a ⊓ b ≤ a :=
Inf_le !mem_insert

lemma inf_le_right (a b : A) : a ⊓ b ≤ b :=
Inf_le (!mem_insert_of_mem !mem_insert)

lemma le_inf {a b c : A} : c ≤ a → c ≤ b → c ≤ a ⊓ b :=
assume h₁ h₂,
le_Inf (take x, suppose x ∈ '{a, b},
  or.elim (eq_or_mem_of_mem_insert this)
    (suppose x = a, begin subst x, exact h₁ end)
    (suppose x ∈ '{b},
      have x = b, from !eq_of_mem_singleton this,
      begin subst x, exact h₂ end))

lemma le_sup_left (a b : A) : a ≤ a ⊔ b :=
le_Sup !mem_insert

lemma le_sup_right (a b : A) : b ≤ a ⊔ b :=
le_Sup (!mem_insert_of_mem !mem_insert)

lemma sup_le {a b c : A} : a ≤ c → b ≤ c → a ⊔ b ≤ c :=
assume h₁ h₂,
Sup_le (take x, suppose x ∈ '{a, b},
  or.elim (eq_or_mem_of_mem_insert this)
    (suppose x = a, by subst x; assumption)
    (suppose x ∈ '{b},
      have x = b, from !eq_of_mem_singleton this,
      by subst x; assumption))
end complete_lattice_Inf

-- Every complete_lattice_Inf is a complete_lattice_Sup
definition complete_lattice_Inf_to_complete_lattice_Sup [C : complete_lattice_Inf A] :
  complete_lattice_Sup A :=
⦃ complete_lattice_Sup, C ⦄

-- Every complete_lattice_Inf is a complete_lattice
definition complete_lattice_Inf_to_complete_lattice [trans_instance] [C : complete_lattice_Inf A] :
  complete_lattice A :=
⦃ complete_lattice, C ⦄

namespace complete_lattice_Sup
variable [C : complete_lattice_Sup A]
include C
definition Inf (s : set A) : A :=
Sup {b | ∀ a, a ∈ s → b ≤ a}

lemma Inf_le {a : A} {s : set A} : a ∈ s → Inf s ≤ a :=
suppose a ∈ s, Sup_le
  (show ∀ (b : A), (∀ (a : A), a ∈ s → b ≤ a) → b ≤ a, from
   take b, assume h, h a `a ∈ s`)

lemma le_Inf {b : A} {s : set A} (h : ∀ (a : A), a ∈ s → b ≤ a) : b ≤ Inf s :=
le_Sup h

local prefix `⨅`:70 := Inf
local prefix `⨆`:65 := Sup

definition inf (a b : A) := ⨅ '{a, b}
definition sup (a b : A) := ⨆ '{a, b}

local infix `⊓` := inf
local infix `⊔` := sup

lemma inf_le_left (a b : A) : a ⊓ b ≤ a :=
Inf_le !mem_insert

lemma inf_le_right (a b : A) : a ⊓ b ≤ b :=
Inf_le (!mem_insert_of_mem !mem_insert)

lemma le_inf {a b c : A} : c ≤ a → c ≤ b → c ≤ a ⊓ b :=
assume h₁ h₂,
le_Inf (take x, suppose x ∈ '{a, b},
  or.elim (eq_or_mem_of_mem_insert this)
    (suppose x = a, begin subst x, exact h₁ end)
    (suppose x ∈ '{b},
      have x = b, from !eq_of_mem_singleton this,
      begin subst x, exact h₂ end))

lemma le_sup_left (a b : A) : a ≤ a ⊔ b :=
le_Sup !mem_insert

lemma le_sup_right (a b : A) : b ≤ a ⊔ b :=
le_Sup (!mem_insert_of_mem !mem_insert)

lemma sup_le {a b c : A} : a ≤ c → b ≤ c → a ⊔ b ≤ c :=
assume h₁ h₂,
Sup_le (take x, suppose x ∈ '{a, b},
  or.elim (eq_or_mem_of_mem_insert this)
    (assume H : x = a, by subst x; exact h₁)
    (suppose x ∈ '{b},
      have x = b, from !eq_of_mem_singleton this,
      by subst x; exact h₂))

end complete_lattice_Sup

-- Every complete_lattice_Sup is a complete_lattice_Inf
definition complete_lattice_Sup_to_complete_lattice_Inf [C : complete_lattice_Sup A] :
  complete_lattice_Inf A :=
⦃ complete_lattice_Inf, C ⦄

-- Every complete_lattice_Sup is a complete_lattice
section
definition complete_lattice_Sup_to_complete_lattice [trans_instance] [C : complete_lattice_Sup A] :
  complete_lattice A :=
⦃ complete_lattice, C ⦄
end

section complete_lattice
variable [C : complete_lattice A]
include C

variable {f : A → A}
premise  (mono : nondecreasing f)

theorem knaster_tarski : ∃ a, f a = a ∧ ∀ b, f b = b → a ≤ b :=
let a := ⨅ {u | f u ≤ u} in
have h₁ : f a = a, from
  have ge : f a ≤ a, from
    have ∀ b, b ∈ {u | f u ≤ u} → f a ≤ b, from
      take b, suppose f b ≤ b,
      have a ≤ b,     from Inf_le this,
      have f a ≤ f b, from mono this,
      le.trans `f a ≤ f b` `f b ≤ b`,
    le_Inf this,
  have le : a ≤ f a, from
    have f (f a) ≤ f a,  from !mono ge,
    have f a ∈ {u | f u ≤ u}, from this,
    Inf_le this,
  le.antisymm ge le,
have h₂ : ∀ b, f b = b → a ≤ b, from
  take b,
  suppose f b = b,
  have b ∈ {u | f u ≤ u}, from
    show f b ≤ b, by rewrite this,
  Inf_le this,
exists.intro a (and.intro h₁ h₂)

theorem knaster_tarski_dual : ∃ a, f a = a ∧ ∀ b, f b = b → b ≤ a :=
let a := ⨆ {u | u ≤ f u} in
have h₁ : f a = a, from
  have le : a ≤ f a, from
    have ∀ b, b ∈ {u | u ≤ f u} → b ≤ f a, from
      take b, suppose b ≤ f b,
      have b ≤ a,     from le_Sup this,
      have f b ≤ f a, from mono this,
      le.trans `b ≤ f b` `f b ≤ f a`,
    Sup_le this,
  have ge : f a ≤ a, from
    have f a ≤ f (f a),  from !mono le,
    have f a ∈ {u | u ≤ f u}, from this,
    le_Sup this,
  le.antisymm ge le,
have h₂ : ∀ b, f b = b → b ≤ a, from
  take b,
  suppose f b = b,
  have b ≤ f b, by rewrite this,
  le_Sup this,
exists.intro a (and.intro h₁ h₂)

/- top and bot -/

definition bot : A := ⨅ univ
definition top : A := ⨆ univ
notation `⊥` := bot
notation `⊤` := top

lemma bot_le (a : A) : ⊥ ≤ a :=
Inf_le !mem_univ

lemma eq_bot {a : A} : (∀ b, a ≤ b) → a = ⊥ :=
assume h,
have a ≤ ⊥, from le_Inf (take b bin, h b),
le.antisymm this !bot_le

lemma le_top (a : A) : a ≤ ⊤ :=
le_Sup !mem_univ

lemma eq_top {a : A} : (∀ b, b ≤ a) → a = ⊤ :=
assume h,
have ⊤ ≤ a, from Sup_le (take b bin, h b),
le.antisymm !le_top this

/- general facts about complete lattices -/

lemma Inf_singleton {a : A} : ⨅'{a} = a :=
have ⨅'{a} ≤ a, from
  Inf_le !mem_insert,
have a ≤ ⨅'{a}, from
  le_Inf (take b, suppose b ∈ '{a}, have b = a, from eq_of_mem_singleton this, by rewrite this),
le.antisymm `⨅'{a} ≤ a` `a ≤ ⨅'{a}`

lemma Sup_singleton {a : A} : ⨆'{a} = a :=
have ⨆'{a} ≤ a, from
  Sup_le (take b, suppose b ∈ '{a}, have b = a, from eq_of_mem_singleton this, by rewrite this),
have a ≤ ⨆'{a}, from
  le_Sup !mem_insert,
le.antisymm `⨆'{a} ≤ a` `a ≤ ⨆'{a}`

lemma Inf_antimono {s₁ s₂ : set A} : s₁ ⊆ s₂ → ⨅ s₂ ≤ ⨅ s₁ :=
suppose s₁ ⊆ s₂, le_Inf (take a : A, suppose a ∈ s₁, Inf_le (mem_of_subset_of_mem `s₁ ⊆ s₂` `a ∈ s₁`))

lemma Sup_mono {s₁ s₂ : set A} : s₁ ⊆ s₂ → ⨆ s₁ ≤ ⨆ s₂ :=
suppose s₁ ⊆ s₂, Sup_le (take a : A, suppose a ∈ s₁, le_Sup (mem_of_subset_of_mem `s₁ ⊆ s₂` `a ∈ s₁`))

lemma Inf_union (s₁ s₂ : set A) : ⨅ (s₁ ∪ s₂) = (⨅s₁) ⊓ (⨅s₂) :=
have le₁ : ⨅ (s₁ ∪ s₂) ≤ (⨅s₁) ⊓ (⨅s₂), from
 !le_inf
     (le_Inf (take a : A, suppose a ∈ s₁, Inf_le (mem_unionl `a ∈ s₁`)))
     (le_Inf (take a : A, suppose a ∈ s₂, Inf_le (mem_unionr `a ∈ s₂`))),
have le₂ : (⨅s₁) ⊓ (⨅s₂) ≤ ⨅ (s₁ ∪ s₂), from
  le_Inf (take a : A, suppose a ∈ s₁ ∪ s₂,
    or.elim this
      (suppose a ∈ s₁,
        have (⨅s₁) ⊓ (⨅s₂) ≤ ⨅s₁, from !inf_le_left,
        have ⨅s₁ ≤ a, from Inf_le `a ∈ s₁`,
        le.trans `(⨅s₁) ⊓ (⨅s₂) ≤ ⨅s₁` `⨅s₁ ≤ a`)
      (suppose a ∈ s₂,
        have (⨅s₁) ⊓ (⨅s₂) ≤ ⨅s₂, from !inf_le_right,
        have ⨅s₂ ≤ a, from Inf_le `a ∈ s₂`,
        le.trans `(⨅s₁) ⊓ (⨅s₂) ≤ ⨅s₂` `⨅s₂ ≤ a`)),
le.antisymm le₁ le₂

lemma Sup_union (s₁ s₂ : set A) : ⨆ (s₁ ∪ s₂) = (⨆s₁) ⊔ (⨆s₂) :=
have le₁ : ⨆ (s₁ ∪ s₂) ≤ (⨆s₁) ⊔ (⨆s₂), from
  Sup_le (take a : A, suppose a ∈ s₁ ∪ s₂,
    or.elim this
      (suppose a ∈ s₁,
        have a ≤ ⨆s₁, from le_Sup `a ∈ s₁`,
        have ⨆s₁ ≤ (⨆s₁) ⊔ (⨆s₂), from !le_sup_left,
        le.trans `a ≤ ⨆s₁` `⨆s₁ ≤ (⨆s₁) ⊔ (⨆s₂)`)
      (suppose a ∈ s₂,
        have a ≤ ⨆s₂, from le_Sup `a ∈ s₂`,
        have ⨆s₂ ≤ (⨆s₁) ⊔ (⨆s₂), from !le_sup_right,
        le.trans `a ≤ ⨆s₂` `⨆s₂ ≤ (⨆s₁) ⊔ (⨆s₂)`)),
have le₂ : (⨆s₁) ⊔ (⨆s₂) ≤ ⨆ (s₁ ∪ s₂), from
  !sup_le
    (Sup_le (take a : A, suppose a ∈ s₁, le_Sup (mem_unionl `a ∈ s₁`)))
    (Sup_le (take a : A, suppose a ∈ s₂, le_Sup (mem_unionr `a ∈ s₂`))),
le.antisymm le₁ le₂

lemma Inf_empty_eq_Sup_univ : ⨅ (∅ : set A) = ⨆ univ :=
have le₁ : ⨅ (∅ : set A) ≤ ⨆ univ, from
  le_Sup !mem_univ,
have le₂ : ⨆ univ ≤ ⨅ ∅, from
  le_Inf (take a : A, suppose a ∈ ∅, absurd this !not_mem_empty),
le.antisymm le₁ le₂

lemma Sup_empty_eq_Inf_univ : ⨆ (∅ : set A) = ⨅ univ :=
have le₁ : ⨆ (∅ : set A) ≤ ⨅ univ, from
  Sup_le (take a, suppose a ∈ ∅, absurd this !not_mem_empty),
have le₂ : ⨅ univ ≤ ⨆ (∅ : set A), from
  Inf_le !mem_univ,
le.antisymm le₁ le₂

lemma Sup_pair (a b : A) : Sup '{a, b} = sup a b :=
by rewrite [insert_eq, Sup_union, *Sup_singleton]

lemma Inf_pair (a b : A) : Inf '{a, b} = inf a b :=
by rewrite [insert_eq, Inf_union, *Inf_singleton]

end complete_lattice

/- complete lattice instances  -/

section
open eq.ops complete_lattice

definition complete_lattice_fun [instance] (A B : Type) [complete_lattice B] :
    complete_lattice (A → B) :=
⦃ complete_lattice, lattice_fun A B,
  Inf := λS x, Inf ((λf, f x) ' S),
  le_Inf := take f S H x,
    le_Inf (take y Hy, obtain g `g ∈ S` `g x = y`, from Hy, `g x = y` ▸ H g `g ∈ S` x),
  Inf_le := take f S `f ∈ S` x,
    Inf_le (exists.intro f (and.intro `f ∈ S` rfl)),
  Sup := λS x, Sup ((λf, f x) ' S),
  le_Sup := take f S `f ∈ S` x,
    le_Sup (exists.intro f (and.intro `f ∈ S` rfl)),
  Sup_le := take f S H x,
    Sup_le (take y Hy, obtain g `g ∈ S` `g x = y`, from Hy, `g x = y` ▸ H g `g ∈ S` x)
⦄

section
open classical -- Prop and set are only in the classical setting a complete lattice

definition complete_lattice_Prop [instance] : complete_lattice Prop :=
⦃ complete_lattice, lattice_Prop,
  Inf := λS, false ∉ S,
  le_Inf := take x S H Hx Hf,
    H _ Hf Hx,
  Inf_le := take x S Hx Hf,
    (classical.cases_on x (take x, true.intro) Hf) Hx,
  Sup := λS, true ∈ S,
  le_Sup := take x S Hx H,
    iff_subst (iff.intro (take H, true.intro) (take H', H)) Hx,
  Sup_le := take x S H Ht,
    H _ Ht true.intro
⦄

lemma sInter_eq_Inf_fun {A : Type} (S : set (set A)) : ⋂₀ S = @Inf (A → Prop) _ S :=
funext (take x,
  calc
    (⋂₀ S) x = ∀₀ P ∈ S, P x : rfl
    ... = ¬ (∃₀ P ∈ S, P x = false) :
    begin
      rewrite not_bounded_exists,
      apply bounded_forall_congr,
      intros,
      rewrite eq_false,
      rewrite not_not_iff
    end
    ... = @Inf (A → Prop) _ S x : rfl)

lemma sUnion_eq_Sup_fun {A : Type} (S : set (set A)) : ⋃₀ S = @Sup (A → Prop) _ S :=
funext (take x,
  calc
    (⋃₀ S) x = ∃₀ P ∈ S, P x : rfl
    ... = (∃₀ P ∈ S, P x = true) :
    begin
      apply bounded_exists_congr,
      intros,
      rewrite eq_true
    end
    ... = @Sup (A → Prop) _ S x : rfl)

definition complete_lattice_set [instance] (A : Type) : complete_lattice (set A) :=
⦃ complete_lattice,
  le := subset,
  le_refl := @le_refl (A → Prop) _,
  le_trans := @le_trans (A → Prop) _,
  le_antisymm := @le_antisymm (A → Prop) _,
  inf := inter,
  sup := union,
  inf_le_left := @inf_le_left (A → Prop) _,
  inf_le_right := @inf_le_right (A → Prop) _,
  le_inf := @le_inf (A → Prop) _,
  le_sup_left := @le_sup_left (A → Prop) _,
  le_sup_right := @le_sup_right (A → Prop) _,
  sup_le := @sup_le (A → Prop) _,
  Inf := sInter,
  Sup := sUnion,
  le_Inf := begin intros X S H, rewrite sInter_eq_Inf_fun, apply (@le_Inf (A → Prop) _), exact H end,
  Inf_le := begin intros X S H, rewrite sInter_eq_Inf_fun, apply (@Inf_le (A → Prop) _), exact H end,
  le_Sup := begin intros X S H, rewrite sUnion_eq_Sup_fun, apply (@le_Sup (A → Prop) _), exact H end,
  Sup_le := begin intros X S H, rewrite sUnion_eq_Sup_fun, apply (@Sup_le (A → Prop) _), exact H end
⦄

end

end