bookshelf/Bookshelf/Enderton/Set/Chapter_2.lean

997 lines
29 KiB
Plaintext
Raw Blame History

This file contains ambiguous Unicode characters!

This file contains ambiguous Unicode characters that may be confused with others in your current locale. If your use case is intentional and legitimate, you can safely ignore this warning. Use the Escape button to highlight these characters.

import Bookshelf.Enderton.Set.Chapter_1
import Common.Set.Basic
import Mathlib.Data.Set.Lattice
import Mathlib.Order.SymmDiff
/-! # Enderton.Set.Chapter_2
Axioms and Operations
-/
namespace Enderton.Set.Chapter_2
/-! #### Commutative Laws
For any sets `A` and `B`,
```
A B = B A
A ∩ B = B ∩ A
```
-/
theorem commutative_law_i (A B : Set α)
: A B = B A := calc A B
_ = { x | x ∈ A x ∈ B } := rfl
_ = { x | x ∈ B x ∈ A } := by
ext
exact or_comm
_ = B A := rfl
#check Set.union_comm
theorem commutative_law_ii (A B : Set α)
: A ∩ B = B ∩ A := calc A ∩ B
_ = { x | x ∈ A ∧ x ∈ B } := rfl
_ = { x | x ∈ B ∧ x ∈ A } := by
ext
exact and_comm
_ = B ∩ A := rfl
#check Set.inter_comm
/-! #### Associative Laws
For any sets `A`, `B`, and `C`,
```
A (B C) = (A B) C
A ∩ (B ∩ C) = (A ∩ B) ∩ C
```
-/
theorem associative_law_i (A B C : Set α)
: A (B C) = (A B) C := calc A (B C)
_ = { x | x ∈ A x ∈ B C } := rfl
_ = { x | x ∈ A (x ∈ B x ∈ C) } := rfl
_ = { x | (x ∈ A x ∈ B) x ∈ C } := by
ext _
simp only [Set.mem_setOf_eq]
rw [← or_assoc]
_ = { x | x ∈ A B x ∈ C } := rfl
_ = (A B) C := rfl
#check Set.union_assoc
theorem associative_law_ii (A B C : Set α)
: A ∩ (B ∩ C) = (A ∩ B) ∩ C := calc A ∩ (B ∩ C)
_ = { x | x ∈ A ∧ (x ∈ B ∩ C) } := rfl
_ = { x | x ∈ A ∧ (x ∈ B ∧ x ∈ C) } := rfl
_ = { x | (x ∈ A ∧ x ∈ B) ∧ x ∈ C } := by
ext _
simp only [Set.mem_setOf_eq]
rw [← and_assoc]
_ = { x | x ∈ A ∩ B ∧ x ∈ C } := rfl
_ = (A ∩ B) ∩ C := rfl
#check Set.inter_assoc
/-! #### Distributive Laws
For any sets `A`, `B`, and `C`,
```
A ∩ (B C) = (A ∩ B) (A ∩ C)
A (B ∩ C) = (A B) ∩ (A C)
```
-/
theorem distributive_law_i (A B C : Set α)
: A ∩ (B C) = (A ∩ B) (A ∩ C) := calc A ∩ (B C)
_ = { x | x ∈ A ∧ x ∈ B C } := rfl
_ = { x | x ∈ A ∧ (x ∈ B x ∈ C) } := rfl
_ = { x | (x ∈ A ∧ x ∈ B) (x ∈ A ∧ x ∈ C) } := by
ext _
exact and_or_left
_ = { x | x ∈ A ∩ B x ∈ A ∩ C } := rfl
_ = (A ∩ B) (A ∩ C) := rfl
#check Set.inter_distrib_left
theorem distributive_law_ii (A B C : Set α)
: A (B ∩ C) = (A B) ∩ (A C) := calc A (B ∩ C)
_ = { x | x ∈ A x ∈ B ∩ C } := rfl
_ = { x | x ∈ A (x ∈ B ∧ x ∈ C) } := rfl
_ = { x | (x ∈ A x ∈ B) ∧ (x ∈ A x ∈ C) } := by
ext _
exact or_and_left
_ = { x | x ∈ A B ∧ x ∈ A C } := rfl
_ = (A B) ∩ (A C) := rfl
#check Set.union_distrib_left
/-! #### De Morgan's Laws
For any sets `A`, `B`, and `C`,
```
C - (A B) = (C - A) ∩ (C - B)
C - (A ∩ B) = (C - A) (C - B)
```
-/
theorem de_morgans_law_i (A B C : Set α)
: C \ (A B) = (C \ A) ∩ (C \ B) := calc C \ (A B)
_ = { x | x ∈ C ∧ x ∉ A B } := rfl
_ = { x | x ∈ C ∧ ¬(x ∈ A x ∈ B) } := rfl
_ = { x | x ∈ C ∧ (x ∉ A ∧ x ∉ B) } := by
ext _
simp only [Set.mem_setOf_eq]
rw [not_or_de_morgan]
_ = { x | (x ∈ C ∧ x ∉ A) ∧ (x ∈ C ∧ x ∉ B) } := by
ext _
exact and_and_left
_ = { x | x ∈ C \ A ∧ x ∈ C \ B } := rfl
_ = (C \ A) ∩ (C \ B) := rfl
#check Set.diff_inter_diff
theorem de_morgans_law_ii (A B C : Set α)
: C \ (A ∩ B) = (C \ A) (C \ B) := calc C \ (A ∩ B)
_ = { x | x ∈ C ∧ x ∉ A ∩ B } := rfl
_ = { x | x ∈ C ∧ ¬(x ∈ A ∧ x ∈ B) } := rfl
_ = { x | x ∈ C ∧ (x ∉ A x ∉ B) } := by
ext _
simp only [Set.mem_setOf_eq]
rw [not_and_de_morgan]
_ = { x | (x ∈ C ∧ x ∉ A) (x ∈ C ∧ x ∉ B) } := by
ext _
exact and_or_left
_ = { x | x ∈ C \ A x ∈ C \ B } := rfl
_ = (C \ A) (C \ B) := rfl
#check Set.diff_inter
/-! #### Identities Involving ∅
For any set `A`,
```
A ∅ = A
A ∩ ∅ = ∅
A ∩ (C - A) = ∅
```
-/
theorem emptyset_identity_i (A : Set α)
: A ∅ = A := calc A
_ = { x | x ∈ A x ∈ ∅ } := rfl
_ = { x | x ∈ A False } := rfl
_ = { x | x ∈ A } := by simp
_ = A := rfl
#check Set.union_empty
theorem emptyset_identity_ii (A : Set α)
: A ∩ ∅ = ∅ := calc A ∩ ∅
_ = { x | x ∈ A ∧ x ∈ ∅ } := rfl
_ = { x | x ∈ A ∧ False } := rfl
_ = ∅ := by simp
#check Set.inter_empty
theorem emptyset_identity_iii (A C : Set α)
: A ∩ (C \ A) = ∅ := calc A ∩ (C \ A)
_ = { x | x ∈ A ∧ x ∈ C \ A } := rfl
_ = { x | x ∈ A ∧ (x ∈ C ∧ x ∉ A) } := rfl
_ = { x | x ∈ C ∧ False } := by simp
_ = ∅ := by simp
#check Set.inter_diff_self
/-! #### Monotonicity
For any sets `A`, `B`, and `C`,
```
A ⊆ B ⇒ A C ⊆ B C
A ⊆ B ⇒ A ∩ C ⊆ B ∩ C
A ⊆ B ⇒ A ⊆ B
```
-/
theorem monotonicity_i (A B C : Set α) (h : A ⊆ B)
: A C ⊆ B C := by
show ∀ x, x ∈ A C → x ∈ B C
intro x hx
apply Or.elim hx
· intro hA
have := h hA
left
exact this
· intro hC
right
exact hC
#check Set.union_subset_union_left
theorem monotonicity_ii (A B C : Set α) (h : A ⊆ B)
: A ∩ C ⊆ B ∩ C := by
show ∀ x, x ∈ A ∩ C → x ∈ B ∩ C
intro x hx
have := h hx.left
exact ⟨this, hx.right⟩
#check Set.inter_subset_inter_left
theorem monotonicity_iii (A B : Set (Set α)) (h : A ⊆ B)
: ⋃₀ A ⊆ ⋃₀ B := by
show ∀ x, x ∈ ⋃₀ A → x ∈ ⋃₀ B
intro x hx
have ⟨b, hb⟩ := hx
have := h hb.left
exact ⟨b, this, hb.right⟩
#check Set.sUnion_mono
/-! #### Anti-monotonicity
For any sets `A`, `B`, and `C`,
```
A ⊆ B ⇒ C - B ⊆ C - A
∅ ≠ A ⊆ B ⇒ ⋂ B ⊆ ⋂ A
```
-/
theorem anti_monotonicity_i (A B C : Set α) (h : A ⊆ B)
: C \ B ⊆ C \ A := by
show ∀ x, x ∈ C \ B → x ∈ C \ A
intro x hx
have : x ∉ A := by
by_contra nh
have := h nh
exact absurd this hx.right
exact ⟨hx.left, this⟩
#check Set.diff_subset_diff_right
theorem anti_monotonicity_ii (A B : Set (Set α)) (h : A ⊆ B)
: ⋂₀ B ⊆ ⋂₀ A := by
show ∀ x, x ∈ ⋂₀ B → x ∈ ⋂₀ A
intro x hx
have : ∀ b, b ∈ B → x ∈ b := hx
show ∀ a, a ∈ A → x ∈ a
intro a ha
exact this a (h ha)
#check Set.sInter_subset_sInter
/-- #### Intersection/Difference Associativity
Let `A`, `B`, and `C` be sets. Then `A ∩ (B - C) = (A ∩ B) - C`.
-/
theorem inter_diff_assoc (A B C : Set α)
: A ∩ (B \ C) = (A ∩ B) \ C := calc A ∩ (B \ C)
_ = { x | x ∈ A ∧ x ∈ (B \ C) } := rfl
_ = { x | x ∈ A ∧ (x ∈ B ∧ x ∉ C) } := rfl
_ = { x | (x ∈ A ∧ x ∈ B) ∧ x ∉ C } := by
ext _
simp only [Set.mem_setOf_eq]
rw [and_assoc]
_ = { x | x ∈ A ∩ B ∧ x ∉ C } := rfl
_ = (A ∩ B) \ C := rfl
#check Set.inter_diff_assoc
/-- #### Exercise 2.1
Assume that `A` is the set of integers divisible by `4`. Similarly assume that
`B` and `C` are the sets of integers divisible by `9` and `10`, respectively.
What is in `A ∩ B ∩ C`?
-/
theorem exercise_2_1 {A B C : Set }
(hA : A = { x | Dvd.dvd 4 x })
(hB : B = { x | Dvd.dvd 9 x })
(hC : C = { x | Dvd.dvd 10 x })
: ∀ x ∈ (A ∩ B ∩ C), (4 x) ∧ (9 x) ∧ (10 x) := by
intro x h
rw [Set.mem_inter_iff] at h
have ⟨⟨ha, hb⟩, hc⟩ := h
refine ⟨?_, ⟨?_, ?_⟩⟩
· rw [hA] at ha
exact Set.mem_setOf.mp ha
· rw [hB] at hb
exact Set.mem_setOf.mp hb
· rw [hC] at hc
exact Set.mem_setOf.mp hc
/-- #### Exercise 2.2
Give an example of sets `A` and `B` for which ` A = B` but `A ≠ B`.
-/
theorem exercise_2_2 {A B : Set (Set )}
(hA : A = {{1}, {2}}) (hB : B = {{1, 2}})
: Set.sUnion A = Set.sUnion B ∧ A ≠ B := by
apply And.intro
· show ⋃₀ A = ⋃₀ B
ext x
show (∃ t, t ∈ A ∧ x ∈ t) ↔ ∃ t, t ∈ B ∧ x ∈ t
apply Iff.intro
· intro ⟨t, ⟨ht, hx⟩⟩
rw [hA] at ht
refine ⟨{1, 2}, ⟨by rw [hB]; simp, ?_⟩⟩
apply Or.elim ht <;>
· intro ht'
rw [ht'] at hx
rw [hx]
simp
· intro ⟨t, ⟨ht, hx⟩⟩
rw [hB] at ht
rw [ht] at hx
apply Or.elim hx
· intro hx'
exact ⟨{1}, ⟨by rw [hA]; simp, by rw [hx']; simp⟩⟩
· intro hx'
exact ⟨{2}, ⟨by rw [hA]; simp, by rw [hx']; simp⟩⟩
· show A ≠ B
-- Find an element that exists in `B` but not in `A`. Extensionality
-- concludes the proof.
intro h
rw [hA, hB, Set.ext_iff] at h
have h₁ := h {1, 2}
simp at h₁
rw [Set.ext_iff] at h₁
have h₂ := h₁ 2
simp at h₂
/-- #### Exercise 2.3
Show that every member of a set `A` is a subset of `U A`. (This was stated as an
example in this section.)
-/
theorem exercise_2_3 {A : Set (Set α)}
: ∀ x ∈ A, x ⊆ ⋃₀ A := by
intro x hx
show ∀ y ∈ x, y ∈ { a | ∃ t, t ∈ A ∧ a ∈ t }
intro y hy
rw [Set.mem_setOf_eq]
exact ⟨x, ⟨hx, hy⟩⟩
/-- #### Exercise 2.4
Show that if `A ⊆ B`, then ` A ⊆ B`.
-/
theorem exercise_2_4 {A B : Set (Set α)} (h : A ⊆ B) : ⋃₀ A ⊆ ⋃₀ B := by
show ∀ x ∈ { a | ∃ t, t ∈ A ∧ a ∈ t }, x ∈ { a | ∃ t, t ∈ B ∧ a ∈ t }
intro x hx
rw [Set.mem_setOf_eq] at hx
have ⟨t, ⟨ht, hxt⟩⟩ := hx
rw [Set.mem_setOf_eq]
exact ⟨t, ⟨h ht, hxt⟩⟩
/-- #### Exercise 2.5
Assume that every member of `𝓐` is a subset of `B`. Show that ` 𝓐 ⊆ B`.
-/
theorem exercise_2_5 {𝓐 : Set (Set α)} (h : ∀ x ∈ 𝓐, x ⊆ B)
: ⋃₀ 𝓐 ⊆ B := by
show ∀ y ∈ { a | ∃ t, t ∈ 𝓐 ∧ a ∈ t }, y ∈ B
intro y hy
rw [Set.mem_setOf_eq] at hy
have ⟨t, ⟨ht𝓐, hyt⟩⟩ := hy
exact (h t ht𝓐) hyt
/-- #### Exercise 2.6a
Show that for any set `A`, ` 𝓟 A = A`.
-/
theorem exercise_2_6a : ⋃₀ (𝒫 A) = A := by
show { a | ∃ t, t ∈ { t | t ⊆ A } ∧ a ∈ t } = A
ext x
apply Iff.intro
· intro hx
rw [Set.mem_setOf_eq] at hx
have ⟨t, ⟨htl, htr⟩⟩ := hx
rw [Set.mem_setOf_eq] at htl
exact htl htr
· intro hx
rw [Set.mem_setOf_eq]
exact ⟨A, ⟨by rw [Set.mem_setOf_eq], hx⟩⟩
/-- #### Exercise 2.6b
Show that `A ⊆ 𝓟 A`. Under what conditions does equality hold?
-/
theorem exercise_2_6b
: A ⊆ 𝒫 (⋃₀ A)
∧ (A = 𝒫 (⋃₀ A) ↔ ∃ B, A = 𝒫 B) := by
apply And.intro
· show ∀ x ∈ A, x ∈ { t | t ⊆ ⋃₀ A }
intro x hx
rw [Set.mem_setOf]
exact exercise_2_3 x hx
· apply Iff.intro
· intro hA
exact ⟨⋃₀ A, hA⟩
· intro ⟨B, hB⟩
conv => rhs; rw [hB, exercise_2_6a]
exact hB
/-- #### Exercise 2.7a
Show that for any sets `A` and `B`, `𝓟 A ∩ 𝓟 B = 𝓟 (A ∩ B)`.
-/
theorem exercise_2_7A
: 𝒫 A ∩ 𝒫 B = 𝒫 (A ∩ B) := by
suffices 𝒫 A ∩ 𝒫 B ⊆ 𝒫 (A ∩ B) ∧ 𝒫 (A ∩ B) ⊆ 𝒫 A ∩ 𝒫 B from
subset_antisymm this.left this.right
apply And.intro
· unfold Set.powerset
intro x hx
simp only [Set.mem_inter_iff, Set.mem_setOf_eq] at hx
rwa [Set.mem_setOf, Set.subset_inter_iff]
· unfold Set.powerset
simp
intro x hA _
exact hA
/-- #### Exercise 2.7b (i)
Show that `𝓟 A 𝓟 B ⊆ 𝓟 (A B)`.
-/
theorem exercise_2_7b_i
: 𝒫 A 𝒫 B ⊆ 𝒫 (A B) := by
unfold Set.powerset
intro x hx
simp at hx
apply Or.elim hx
· intro hA
rw [Set.mem_setOf_eq]
exact Set.subset_union_of_subset_left hA B
· intro hB
rw [Set.mem_setOf_eq]
exact Set.subset_union_of_subset_right hB A
/-- #### Exercise 2.7b (ii)
Under what conditions does `𝓟 A 𝓟 B = 𝓟 (A B)`.?
-/
theorem exercise_2_7b_ii
: 𝒫 A 𝒫 B = 𝒫 (A B) ↔ A ⊆ B B ⊆ A := by
unfold Set.powerset
apply Iff.intro
· intro h
by_contra nh
rw [not_or_de_morgan] at nh
have ⟨a, hA⟩ := Set.not_subset.mp nh.left
have ⟨b, hB⟩ := Set.not_subset.mp nh.right
rw [Set.ext_iff] at h
have hz := h {a, b}
-- `hz` states that `{a, b} ⊆ A {a, b} ⊆ B ↔ {a, b} ⊆ A B`. We show the
-- left-hand side is false but the right-hand side is true, yielding our
-- contradiction.
suffices ¬({a, b} ⊆ A {a, b} ⊆ B) by
have hz₁ : a ∈ A B := by
rw [Set.mem_union]
exact Or.inl hA.left
have hz₂ : b ∈ A B := by
rw [Set.mem_union]
exact Or.inr hB.left
exact absurd (hz.mpr $ Set.mem_mem_imp_pair_subset hz₁ hz₂) this
intro hAB
exact Or.elim hAB
(fun y => absurd (y $ show b ∈ {a, b} by simp) hB.right)
(fun y => absurd (y $ show a ∈ {a, b} by simp) hA.right)
· intro h
ext x
apply Or.elim h
· intro hA
apply Iff.intro
· intro hx
exact Or.elim hx
(Set.subset_union_of_subset_left · B)
(Set.subset_union_of_subset_right · A)
· intro hx
refine Or.inr (Set.Subset.trans hx ?_)
exact subset_of_eq (Set.left_subset_union_eq_self hA)
· intro hB
apply Iff.intro
· intro hx
exact Or.elim hx
(Set.subset_union_of_subset_left · B)
(Set.subset_union_of_subset_right · A)
· intro hx
refine Or.inl (Set.Subset.trans hx ?_)
exact subset_of_eq (Set.right_subset_union_eq_self hB)
/-- #### Exercise 2.9
Give an example of sets `a` and `B` for which `a ∈ B` but `𝓟 a ∉ 𝓟 B`.
-/
theorem exercise_2_9 (ha : a = {1}) (hB : B = {{1}})
: a ∈ B ∧ 𝒫 a ∉ 𝒫 B := by
apply And.intro
· rw [ha, hB]
simp
· intro h
have h₁ : 𝒫 a = {∅, {1}} := by
rw [ha]
exact Set.powerset_singleton 1
have h₂ : 𝒫 B = {∅, {{1}}} := by
rw [hB]
exact Set.powerset_singleton {1}
rw [h₁, h₂] at h
simp at h
apply Or.elim h
· intro h
rw [Set.ext_iff] at h
have := h ∅
simp at this
· intro h
rw [Set.ext_iff] at h
have := h 1
simp at this
/-- #### Exercise 2.10
Show that if `a ∈ B`, then `𝓟 a ∈ 𝓟 𝓟 B`.
-/
theorem exercise_2_10 {B : Set (Set α)} (ha : a ∈ B)
: 𝒫 a ∈ 𝒫 (𝒫 (⋃₀ B)) := by
have h₁ := exercise_2_3 a ha
have h₂ := Chapter_1.exercise_1_3 h₁
generalize hb : 𝒫 (⋃₀ B) = b
conv => rhs; unfold Set.powerset
rw [← hb, Set.mem_setOf_eq]
exact h₂
/-- #### Exercise 2.11 (i)
Show that for any sets `A` and `B`, `A = (A ∩ B) (A - B)`.
-/
theorem exercise_2_11_i {A B : Set α}
: A = (A ∩ B) (A \ B) := by
show A = fun a => A a ∧ B a A a ∧ ¬B a
suffices ∀ x, (A x ∧ (B x ¬B x)) = A x by
conv => rhs; ext x; rw [← and_or_left, this]
intro x
refine propext ?_
apply Iff.intro
· intro hx
exact hx.left
· intro hx
exact ⟨hx, em (B x)⟩
/-- #### Exercise 2.11 (ii)
Show that for any sets `A` and `B`, `A (B - A) = A B`.
-/
theorem exercise_2_11_ii {A B : Set α}
: A (B \ A) = A B := by
show (fun a => A a B a ∧ ¬A a) = fun a => A a B a
suffices ∀ x, ((A x B x) ∧ (A x ¬A x)) = (A x B x) by
conv => lhs; ext x; rw [or_and_left, this x]
intro x
refine propext ?_
apply Iff.intro
· intro hx
exact hx.left
· intro hx
exact ⟨hx, em (A x)⟩
section
variable {A B C : Set }
variable {hA : A = {1, 2, 3}}
variable {hB : B = {2, 3, 4}}
variable {hC : C = {3, 4, 5}}
lemma right_diff_eq_insert_one_three : A \ (B \ C) = {1, 3} := by
rw [hA, hB, hC]
ext x
apply Iff.intro
· intro hx
unfold SDiff.sdiff Set.instSDiffSet Set.diff at hx
unfold Membership.mem Set.instMembershipSet Set.Mem setOf at hx
unfold insert Set.instInsertSet Set.insert setOf at hx
have ⟨ha, hb⟩ := hx
rw [not_and_de_morgan, not_or_de_morgan] at hb
simp only [Set.mem_singleton_iff, not_not] at hb
refine Or.elim ha Or.inl ?_
intro hy
apply Or.elim hb
· intro hz
exact Or.elim hy (absurd · hz.left) Or.inr
· intro hz
refine Or.elim hz Or.inr ?_
intro hz₁
apply Or.elim hy <;> apply Or.elim hz₁ <;>
· intro hz₂ hz₃
rw [hz₂] at hz₃
simp at hz₃
· intro hx
unfold SDiff.sdiff Set.instSDiffSet Set.diff
unfold Membership.mem Set.instMembershipSet Set.Mem setOf
unfold insert Set.instInsertSet Set.insert setOf
apply Or.elim hx
· intro hy
refine ⟨Or.inl hy, ?_⟩
intro hz
rw [hy] at hz
unfold Membership.mem Set.instMembershipSet Set.Mem at hz
unfold singleton Set.instSingletonSet Set.singleton setOf at hz
simp only at hz
· intro hy
refine ⟨Or.inr (Or.inr hy), ?_⟩
intro hz
have hzr := hz.right
rw [not_or_de_morgan] at hzr
exact absurd hy hzr.left
lemma left_diff_eq_singleton_one : (A \ B) \ C = {1} := by
rw [hA, hB, hC]
ext x
apply Iff.intro
· intro hx
unfold SDiff.sdiff Set.instSDiffSet Set.diff at hx
unfold Membership.mem Set.instMembershipSet Set.Mem setOf at hx
unfold insert Set.instInsertSet Set.insert setOf at hx
have ⟨⟨ha, hb⟩, hc⟩ := hx
rw [not_or_de_morgan] at hb hc
apply Or.elim ha
· simp
· intro hy
apply Or.elim hy
· intro hz
exact absurd hz hb.left
· intro hz
exact absurd hz hc.left
· intro hx
refine ⟨⟨Or.inl hx, ?_⟩, ?_⟩ <;>
· intro hy
cases hy with
| inl y => rw [hx] at y; simp at y
| inr hz => cases hz with
| inl y => rw [hx] at y; simp at y
| inr y => rw [hx] at y; simp at y
/-- #### Exercise 2.14
Show by example that for some sets `A`, `B`, and `C`, the set `A - (B - C)` is
different from `(A - B) - C`.
-/
theorem exercise_2_14 : A \ (B \ C) ≠ (A \ B) \ C := by
rw [
@right_diff_eq_insert_one_three A B C hA hB hC,
@left_diff_eq_singleton_one A B C hA hB hC
]
intro h
rw [Set.ext_iff] at h
have := h 3
simp at this
end
/-- #### Exercise 2.15 (a)
Show that `A ∩ (B + C) = (A ∩ B) + (A ∩ C)`.
-/
theorem exercise_2_15a (A B C : Set α)
: A ∩ (B ∆ C) = (A ∩ B) ∆ (A ∩ C) := Eq.symm $
calc (A ∩ B) ∆ (A ∩ C)
_ = ((A ∩ B) \ (A ∩ C)) ((A ∩ C) \ (A ∩ B)) := rfl
_ = ((A ∩ B) \ A)
((A ∩ B) \ C)
(((A ∩ C) \ A)
((A ∩ C) \ B)) := by
iterate 2 rw [Set.diff_inter]
_ = (A ∩ (B \ A))
(A ∩ (B \ C))
((A ∩ (C \ A))
(A ∩ (C \ B))) := by
iterate 4 rw [Set.inter_diff_assoc]
_ = ∅ (A ∩ (B \ C)) (∅ (A ∩ (C \ B))) := by
iterate 2 rw [Set.inter_diff_self]
_ = (A ∩ (B \ C)) (A ∩ (C \ B)) := by
simp only [Set.empty_union]
_ = A ∩ ((B \ C) (C \ B)) := by
rw [Set.inter_distrib_left]
_ = A ∩ (B ∆ C) := rfl
#check Set.inter_symmDiff_distrib_left
/-- #### Exercise 2.15 (b)
Show that `A + (B + C) = (A + B) + C`.
-/
theorem exercise_2_15b (A B C : Set α)
: A ∆ (B ∆ C) = (A ∆ B) ∆ C := by
rw [Set.Subset.antisymm_iff]
apply And.intro
· show ∀ x, x ∈ A ∆ (B ∆ C) → x ∈ (A ∆ B) ∆ C
intro x hx
apply Or.elim hx
· intro ⟨hA, nBC⟩
rw [Set.not_mem_symm_diff_inter_or_not_union] at nBC
apply Or.elim nBC
· intro h
have : x ∉ A ∆ B := Set.symm_diff_mem_both_not_mem hA h.left
exact Set.symm_diff_mem_right this h.right
· intro h
have ⟨nB, nC⟩ : x ∉ B ∧ x ∉ C := not_or_de_morgan.mp h
have : x ∈ A ∆ B := Set.symm_diff_mem_left hA nB
exact Set.symm_diff_mem_left this nC
· intro ⟨hx₁, hx₂⟩
apply Or.elim hx₁
· intro ⟨hB, nC⟩
have : x ∈ A ∆ B := Set.symm_diff_mem_right hx₂ hB
exact Set.symm_diff_mem_left this nC
· intro ⟨hC, nB⟩
have : x ∉ A ∆ B := Set.symm_diff_not_mem_both_not_mem hx₂ nB
exact Set.symm_diff_mem_right this hC
· show ∀ x, x ∈ (A ∆ B) ∆ C → x ∈ A ∆ (B ∆ C)
intro x hx
apply Or.elim hx
· intro ⟨hAB, nC⟩
apply Or.elim hAB
· intro ⟨hA, nB⟩
have : x ∉ B ∆ C := Set.symm_diff_not_mem_both_not_mem nB nC
exact Set.symm_diff_mem_left hA this
· intro ⟨hB, nA⟩
have : x ∈ B ∆ C := Set.symm_diff_mem_left hB nC
exact Set.symm_diff_mem_right nA this
· intro ⟨hC, nAB⟩
rw [Set.not_mem_symm_diff_inter_or_not_union] at nAB
apply Or.elim nAB
· intro ⟨hA, hB⟩
have : x ∉ B ∆ C := Set.symm_diff_mem_both_not_mem hB hC
exact Set.symm_diff_mem_left hA this
· intro h
have ⟨nA, nB⟩ : x ∉ A ∧ x ∉ B := not_or_de_morgan.mp h
have : x ∈ B ∆ C := Set.symm_diff_mem_right nB hC
exact Set.symm_diff_mem_right nA this
#check symmDiff_assoc
/-- #### Exercise 2.16
Simplify:
`[(A B C) ∩ (A B)] - [(A (B - C)) ∩ A]`
-/
theorem exercise_2_16 {A B C : Set α}
: ((A B C) ∩ (A B)) \ ((A (B \ C)) ∩ A) = B \ A := by
calc ((A B C) ∩ (A B)) \ ((A (B \ C)) ∩ A)
_ = (A B) \ ((A (B \ C)) ∩ A) := by rw [Set.union_inter_cancel_left]
_ = (A B) \ A := by rw [Set.union_inter_cancel_left]
_ = B \ A := by rw [Set.union_diff_left]
/-! #### Exercise 2.17
Show that the following four conditions are equivalent.
(a) `A ⊆ B`
(b) `A - B = ∅`
(c) `A B = B`
(d) `A ∩ B = A`
-/
theorem exercise_2_17_i {A B : Set α} (h : A ⊆ B)
: A \ B = ∅ := by
ext x
apply Iff.intro
· intro hx
exact absurd (h hx.left) hx.right
· intro hx
exact False.elim hx
theorem exercise_2_17_ii {A B : Set α} (h : A \ B = ∅)
: A B = B := by
suffices A ⊆ B from Set.left_subset_union_eq_self this
show ∀ t, t ∈ A → t ∈ B
intro t ht
rw [Set.ext_iff] at h
by_contra nt
exact (h t).mp ⟨ht, nt⟩
theorem exercise_2_17_iii {A B : Set α} (h : A B = B)
: A ∩ B = A := by
suffices A ⊆ B from Set.inter_eq_left.mpr this
exact Set.union_eq_right.mp h
theorem exercise_2_17_iv {A B : Set α} (h : A ∩ B = A)
: A ⊆ B := Set.inter_eq_left.mp h
/-- #### Exercise 2.19
Is `𝒫 (A - B)` always equal to `𝒫 A - 𝒫 B`? Is it ever equal to `𝒫 A - 𝒫 B`?
-/
theorem exercise_2_19 {A B : Set α}
: 𝒫 (A \ B) ≠ (𝒫 A) \ (𝒫 B) := by
intro h
have he : ∅ ∈ 𝒫 (A \ B) := by simp
have ne : ∅ ∉ (𝒫 A) \ (𝒫 B) := by simp
rw [Set.ext_iff] at h
have := h ∅
exact absurd (this.mp he) ne
/-- #### Exercise 2.20
Let `A`, `B`, and `C` be sets such that `A B = A C` and `A ∩ B = A ∩ C`.
Show that `B = C`.
-/
theorem exercise_2_20 {A B C : Set α}
(hu : A B = A C) (hi : A ∩ B = A ∩ C) : B = C := by
ext x
apply Iff.intro
· intro hB
by_cases hA : x ∈ A
· have : x ∈ A ∩ B := Set.mem_inter hA hB
rw [hi] at this
exact this.right
· have : x ∈ A B := Set.mem_union_right A hB
rw [hu] at this
exact Or.elim this (absurd · hA) (by simp)
· intro hC
by_cases hA : x ∈ A
· have : x ∈ A ∩ C := Set.mem_inter hA hC
rw [← hi] at this
exact this.right
· have : x ∈ A C := Set.mem_union_right A hC
rw [← hu] at this
exact Or.elim this (absurd · hA) (by simp)
/-- #### Exercise 2.21
Show that ` (A B) = ( A) ( B)`.
-/
theorem exercise_2_21 {A B : Set (Set α)}
: ⋃₀ (A B) = (⋃₀ A) (⋃₀ B) := by
ext x
apply Iff.intro
· intro hx
have ⟨t, ht⟩ : ∃ t, t ∈ A B ∧ x ∈ t := hx
apply Or.elim ht.left
· intro hA
exact Or.inl ⟨t, ⟨hA, ht.right⟩⟩
· intro hB
exact Or.inr ⟨t, ⟨hB, ht.right⟩⟩
· intro hx
apply Or.elim hx
· intro hA
have ⟨t, ht⟩ : ∃ t, t ∈ A ∧ x ∈ t := hA
exact ⟨t, ⟨Set.mem_union_left B ht.left, ht.right⟩⟩
· intro hB
have ⟨t, ht⟩ : ∃ t, t ∈ B ∧ x ∈ t := hB
exact ⟨t, ⟨Set.mem_union_right A ht.left, ht.right⟩⟩
/-- #### Exercise 2.22
Show that if `A` and `B` are nonempty sets, then `⋂ (A B) = ⋂ A ∩ ⋂ B`.
-/
theorem exercise_2_22 {A B : Set (Set α)}
: ⋂₀ (A B) = ⋂₀ A ∩ ⋂₀ B := by
ext x
apply Iff.intro
· intro hx
have : ∀ t : Set α, t ∈ A B → x ∈ t := hx
show (∀ t : Set α, t ∈ A → x ∈ t) ∧ (∀ t : Set α, t ∈ B → x ∈ t)
rw [← forall_and]
intro t
exact ⟨
fun ht => this t (Set.mem_union_left B ht),
fun ht => this t (Set.mem_union_right A ht)
· intro hx
have : ∀ t : Set α, (t ∈ A → x ∈ t) ∧ (t ∈ B → x ∈ t) := by
have : (∀ t : Set α, t ∈ A → x ∈ t) ∧ (∀ t : Set α, t ∈ B → x ∈ t) := hx
rwa [← forall_and] at this
show ∀ (t : Set α), t ∈ A B → x ∈ t
intro t ht
apply Or.elim ht
· intro hA
exact (this t).left hA
· intro hB
exact (this t).right hB
/-- #### Exercise 2.24a
Show that is `𝓐` is nonempty, then `𝒫 (⋂ 𝓐) = ⋂ { 𝒫 X | X ∈ 𝓐 }`.
-/
theorem exercise_2_24a {𝓐 : Set (Set α)}
: 𝒫 (⋂₀ 𝓐) = ⋂₀ { 𝒫 X | X ∈ 𝓐 } := by
calc 𝒫 (⋂₀ 𝓐)
_ = { x | x ⊆ ⋂₀ 𝓐 } := rfl
_ = { x | x ⊆ { y | ∀ X, X ∈ 𝓐 → y ∈ X } } := rfl
_ = { x | ∀ t ∈ x, t ∈ { y | ∀ X, X ∈ 𝓐 → y ∈ X } } := rfl
_ = { x | ∀ t ∈ x, (∀ X, X ∈ 𝓐 → t ∈ X) } := rfl
_ = { x | ∀ X ∈ 𝓐, (∀ t, t ∈ x → t ∈ X) } := by
ext
rw [Set.mem_setOf, Set.mem_setOf, forall_mem_comm (· ∈ ·)]
_ = { x | ∀ X ∈ 𝓐, x ⊆ X} := rfl
_ = { x | ∀ X ∈ 𝓐, x ∈ 𝒫 X } := rfl
_ = { x | ∀ t ∈ { 𝒫 X | X ∈ 𝓐 }, x ∈ t} := by simp
_ = ⋂₀ { 𝒫 X | X ∈ 𝓐 } := rfl
/-- #### Exercise 2.24b
Show that
```
{𝒫 X | X ∈ 𝓐} ⊆ 𝒫 𝓐.
```
Under what conditions does equality hold?
-/
theorem exercise_2_24b {𝓐 : Set (Set α)}
: (⋃₀ { 𝒫 X | X ∈ 𝓐 } ⊆ 𝒫 ⋃₀ 𝓐)
∧ ((⋃₀ { 𝒫 X | X ∈ 𝓐 } = 𝒫 ⋃₀ 𝓐) ↔ (⋃₀ 𝓐𝓐)) := by
have hS : (⋃₀ { 𝒫 X | X ∈ 𝓐 } ⊆ 𝒫 ⋃₀ 𝓐) := by
simp
exact exercise_2_3
refine ⟨hS, ?_⟩
apply Iff.intro
· intro rS
have rS : 𝒫 ⋃₀ 𝓐 ⊆ ⋃₀ { 𝒫 X | X ∈ 𝓐 } :=
(Set.Subset.antisymm_iff.mp rS).right
have hA : ⋃₀ 𝓐 ∈ ⋃₀ { 𝒫 X | X ∈ 𝓐 } :=
rS Set.self_mem_powerset_self
conv at hA =>
rhs
unfold Set.sUnion sSup Set.instSupSetSet
simp only
have ⟨X, ⟨⟨x, hx⟩, ht⟩⟩ := Set.mem_setOf.mp hA
have : ⋃₀ 𝓐 = x := by
rw [← hx.right] at ht
have hl : ⋃₀ 𝓐 ⊆ x := ht
have hr : x ⊆ ⋃₀ 𝓐 := exercise_2_3 x hx.left
exact Set.Subset.antisymm hl hr
rw [← this] at hx
exact hx.left
· intro hA
suffices 𝒫 ⋃₀ 𝓐 ⊆ ⋃₀ { 𝒫 X | X ∈ 𝓐 } from
subset_antisymm hS this
show ∀ x, x ∈ 𝒫 ⋃₀ 𝓐 → x ∈ ⋃₀ { x | ∃ X, X ∈ 𝓐𝒫 X = x }
intro x hx
unfold Set.sUnion sSup Set.instSupSetSet
simp only [Set.mem_setOf_eq, exists_exists_and_eq_and, Set.mem_powerset_iff]
exact ⟨⋃₀ 𝓐, ⟨hA, hx⟩⟩
/-- #### Exercise 2.25
Is `A ( 𝓑)` always the same as ` { A X | X ∈ 𝓑 }`? If not, then under
what conditions does equality hold?
-/
theorem exercise_2_25 {A : Set α} (𝓑 : Set (Set α))
: (A (⋃₀ 𝓑) = ⋃₀ { A X | X ∈ 𝓑 }) ↔ (A = ∅ Set.Nonempty 𝓑) := by
apply Iff.intro
· intro h
by_cases h𝓑 : Set.Nonempty 𝓑
· exact Or.inr h𝓑
· have : 𝓑 = ∅ := Set.not_nonempty_iff_eq_empty.mp h𝓑
rw [this] at h
simp at h
exact Or.inl h
· intro h
apply Or.elim h
· intro hA
rw [hA]
simp
· intro h𝓑
calc A (⋃₀ 𝓑)
_ = { x | x ∈ A x ∈ ⋃₀ 𝓑} := rfl
_ = { x | x ∈ A (∃ b ∈ 𝓑, x ∈ b) } := rfl
_ = { x | ∃ b ∈ 𝓑, x ∈ A x ∈ b } := by
ext x
rw [Set.mem_setOf, Set.mem_setOf]
apply Iff.intro
· intro hx
apply Or.elim hx
· intro hA
have ⟨b, hb⟩ := Set.nonempty_def.mp h𝓑
exact ⟨b, ⟨hb, Or.inl hA⟩⟩
· intro ⟨b, hb⟩
exact ⟨b, ⟨hb.left, Or.inr hb.right⟩⟩
· intro ⟨b, ⟨hb, hx⟩⟩
apply Or.elim hx
· exact (Or.inl ·)
· intro h
exact Or.inr ⟨b, ⟨hb, h⟩⟩
_ = { x | ∃ b ∈ 𝓑, x ∈ A b } := rfl
_ = { x | ∃ t, t ∈ { y | ∃ X, X ∈ 𝓑 ∧ A X = y } ∧ x ∈ t } := by simp
_ = ⋃₀ { A X | X ∈ 𝓑 } := rfl
end Enderton.Set.Chapter_2