126 lines
5.3 KiB
Text
126 lines
5.3 KiB
Text
/-
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Copyright (c) 2015 Microsoft Corporation. All rights reserved.
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Released under Apache 2.0 license as described in the file LICENSE.
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Author: Haitao Zhang
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Partitions of a type A into finite subsets of A. Such a partition is represented by
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a function f : A → finset A which maps every element a : A to its equivalence class.
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-/
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import .card
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open function eq.ops
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variable {A : Type}
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variable [deceqA : decidable_eq A]
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include deceqA
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namespace finset
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definition is_partition (f : A → finset A) := ∀ a b, a ∈ f b = (f a = f b)
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structure partition : Type :=
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(set : finset A) (part : A → finset A) (is_part : is_partition part)
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(complete : set = Union set part)
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attribute partition.part [coercion]
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namespace partition
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definition equiv_classes (f : partition) : finset (finset A) :=
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image (partition.part f) (partition.set f)
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lemma equiv_class_disjoint (f : partition) (a1 a2 : finset A) (Pa1 : a1 ∈ equiv_classes f)
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(Pa2 : a2 ∈ equiv_classes f) :
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a1 ≠ a2 → a1 ∩ a2 = ∅ :=
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assume Pne,
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assert Pe1 : _, from exists_of_mem_image Pa1, obtain g1 Pg1, from Pe1,
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assert Pe2 : _, from exists_of_mem_image Pa2, obtain g2 Pg2, from Pe2,
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begin
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apply inter_eq_empty_of_disjoint,
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apply disjoint.intro,
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rewrite [eq.symm (and.right Pg1), eq.symm (and.right Pg2)],
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intro x,
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rewrite [*partition.is_part f],
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intro Pxg1, rewrite [Pxg1, and.right Pg1, and.right Pg2],
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intro Pe, exact absurd Pe Pne
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end
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theorem class_equation (f : @partition A _) :
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card (partition.set f) = nat.Sum (equiv_classes f) card :=
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let s := (partition.set f), p := (partition.part f), img := image p s in
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calc
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card s = card (Union s p) : partition.complete f
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... = card (Union img id) : image_eq_Union_index_image s p
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... = card (Union (equiv_classes f) id) : rfl
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... = nat.Sum (equiv_classes f) card : card_Union_of_disjoint _ id (equiv_class_disjoint f)
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lemma equiv_class_refl {f : A → finset A} (Pequiv : is_partition f) : ∀ a, a ∈ f a :=
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take a, by rewrite [Pequiv a a]
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-- make it a little easier to prove union from restriction
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lemma restriction_imp_union {s : finset A} (f : A → finset A) (Pequiv : is_partition f)
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(Psub : ∀{a}, a ∈ s → f a ⊆ s) :
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s = Union s f :=
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ext (take a, iff.intro
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(assume Pains,
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begin
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rewrite [(Union_insert_of_mem f Pains)⁻¹, Union_insert],
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apply mem_union_l, exact equiv_class_refl Pequiv a
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end)
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(assume Painu,
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have Pclass : ∃ x, x ∈ s ∧ a ∈ f x,
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from iff.elim_left (mem_Union_iff s f _) Painu,
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obtain x Px, from Pclass,
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have Pfx : f x ⊆ s, from Psub (and.left Px),
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mem_of_subset_of_mem Pfx (and.right Px)))
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lemma binary_union (P : A → Prop) [decP : decidable_pred P] {S : finset A} :
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S = {a ∈ S | P a} ∪ {a ∈ S | ¬(P a)} :=
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ext take a, iff.intro
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(assume Pin, decidable.by_cases
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(λ Pa : P a, mem_union_l (mem_filter_of_mem Pin Pa))
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(λ nPa, mem_union_r (mem_filter_of_mem Pin nPa)))
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(assume Pinu, or.elim (mem_or_mem_of_mem_union Pinu)
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(assume Pin, mem_of_mem_filter Pin)
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(assume Pin, mem_of_mem_filter Pin))
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lemma binary_inter_empty {P : A → Prop} [decP : decidable_pred P] {S : finset A} :
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{a ∈ S | P a} ∩ {a ∈ S | ¬(P a)} = ∅ :=
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inter_eq_empty (take a, assume Pa nPa, absurd (of_mem_filter Pa) (of_mem_filter nPa))
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definition disjoint_sets (S : finset (finset A)) : Prop :=
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∀ s₁ s₂ (P₁ : s₁ ∈ S) (P₂ : s₂ ∈ S), s₁ ≠ s₂ → s₁ ∩ s₂ = ∅
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lemma disjoint_sets_filter_of_disjoint_sets {P : finset A → Prop} [decP : decidable_pred P] {S : finset (finset A)} :
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disjoint_sets S → disjoint_sets {s ∈ S | P s} :=
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assume Pds, take s₁ s₂, assume P₁ P₂, Pds s₁ s₂ (mem_of_mem_filter P₁) (mem_of_mem_filter P₂)
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lemma binary_inter_empty_Union_disjoint_sets {P : finset A → Prop} [decP : decidable_pred P] {S : finset (finset A)} :
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disjoint_sets S → Union {s ∈ S | P s} id ∩ Union {s ∈ S | ¬P s} id = ∅ :=
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assume Pds, inter_eq_empty (take a, assume Pa nPa,
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obtain s Psin Pains, from iff.elim_left !mem_Union_iff Pa,
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obtain t Ptin Paint, from iff.elim_left !mem_Union_iff nPa,
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assert Pneq : s ≠ t,
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from assume Peq, absurd (Peq ▸ of_mem_filter Psin) (of_mem_filter Ptin),
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Pds s t (mem_of_mem_filter Psin) (mem_of_mem_filter Ptin) Pneq ▸ mem_inter Pains Paint)
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section
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variables {B: Type} [deceqB : decidable_eq B]
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include deceqB
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lemma binary_Union (f : A → finset B) {P : A → Prop} [decP : decidable_pred P] {s : finset A} :
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Union s f = Union {a ∈ s | P a} f ∪ Union {a ∈ s | ¬P a} f :=
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begin rewrite [binary_union P at {1}], apply Union_union, exact binary_inter_empty end
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end
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open nat
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lemma card_binary_Union_disjoint_sets (P : finset A → Prop) [decP : decidable_pred P] {S : finset (finset A)} :
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disjoint_sets S → Sum S card = Sum {s ∈ S | P s} card + Sum {s ∈ S | ¬P s} card :=
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assume Pds, calc
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Sum S card = card (Union S id) : card_Union_of_disjoint S id Pds
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... = card (Union {s ∈ S | P s} id ∪ Union {s ∈ S | ¬P s} id) : binary_Union
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... = card (Union {s ∈ S | P s} id) + card (Union {s ∈ S | ¬P s} id) : card_union_of_disjoint (binary_inter_empty_Union_disjoint_sets Pds)
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... = Sum {s ∈ S | P s} card + Sum {s ∈ S | ¬P s} card : by rewrite [*(card_Union_of_disjoint _ id (disjoint_sets_filter_of_disjoint_sets Pds))]
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end partition
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end finset
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