261 lines
8.1 KiB
Plaintext
261 lines
8.1 KiB
Plaintext
import Mathlib.Data.Set.Basic
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import Common.Logic.Basic
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import Common.Set.Basic
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/-! # Enderton.Chapter_3
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Relations and Functions
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-/
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namespace Enderton.Set.Chapter_3
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/-! ## Ordered Pairs -/
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/--
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Kazimierz Kuratowski's definition of an ordered pair.
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-/
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def OrderedPair (x : α) (y : β) : Set (Set (α ⊕ β)) :=
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{{Sum.inl x}, {Sum.inl x, Sum.inr y}}
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namespace OrderedPair
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/--
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For any sets `x`, `y`, `u`, and `v`, `⟨u, v⟩ = ⟨x, y⟩` **iff** `u = x ∧ v = y`.
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-/
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theorem ext_iff {x u : α} {y v : β}
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: (OrderedPair x y = OrderedPair u v) ↔ (x = u ∧ y = v) := by
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unfold OrderedPair
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apply Iff.intro
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· intro h
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have hu := Set.ext_iff.mp h {Sum.inl u}
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have huv := Set.ext_iff.mp h {Sum.inl u, Sum.inr v}
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simp only [
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Set.mem_singleton_iff,
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Set.mem_insert_iff,
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true_or,
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iff_true
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] at hu
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simp only [
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Set.mem_singleton_iff,
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Set.mem_insert_iff,
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or_true,
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iff_true
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] at huv
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apply Or.elim hu
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· apply Or.elim huv
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· -- #### Case 1
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-- `{u} = {x}` and `{u, v} = {x}`.
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intro huv_x hu_x
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rw [Set.singleton_eq_singleton_iff] at hu_x
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rw [hu_x] at huv_x
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have hx_v := Set.pair_eq_singleton_mem_imp_eq_self huv_x
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rw [hu_x, hx_v] at h
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simp only [Set.mem_singleton_iff, Set.insert_eq_of_mem] at h
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have := Set.pair_eq_singleton_mem_imp_eq_self $
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Set.pair_eq_singleton_mem_imp_eq_self h
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rw [← hx_v] at this
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injection hu_x with p
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injection this with q
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exact ⟨p.symm, q⟩
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· -- #### Case 2
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-- `{u} = {x}` and `{u, v} = {x, y}`.
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intro huv_xy hu_x
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rw [Set.singleton_eq_singleton_iff] at hu_x
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rw [hu_x] at huv_xy
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by_cases hx_v : Sum.inl x = Sum.inr v
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· rw [hx_v] at huv_xy
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simp at huv_xy
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have := Set.pair_eq_singleton_mem_imp_eq_self huv_xy.symm
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injection hu_x with p
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injection this with q
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exact ⟨p.symm, q⟩
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· rw [Set.ext_iff] at huv_xy
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have := huv_xy (Sum.inr v)
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simp at this
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injection hu_x with p
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exact ⟨p.symm, this.symm⟩
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· apply Or.elim huv
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· -- #### Case 3
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-- `{u} = {x, y}` and `{u, v} = {x}`.
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intro huv_x _
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rw [Set.ext_iff] at huv_x
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have hv_x := huv_x (Sum.inr v)
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simp only [
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Set.mem_singleton_iff,
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Set.mem_insert_iff,
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or_true,
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true_iff
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] at hv_x
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· -- #### Case 4
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-- `{u} = {x, y}` and `{u, v} = {x, y}`.
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intro _ hu_xy
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rw [Set.ext_iff] at hu_xy
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have hy_u := hu_xy (Sum.inr y)
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simp only [
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Set.mem_singleton_iff,
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Set.mem_insert_iff,
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or_true,
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iff_true
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] at hy_u
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· intro h
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rw [h.left, h.right]
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end OrderedPair
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/-- ### Theorem 3B
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If `x ∈ C` and `y ∈ C`, then `⟨x, y⟩ ∈ 𝒫 𝒫 C`.
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-/
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theorem theorem_3b {C : Set (α ⊕ α)} (hx : Sum.inl x ∈ C) (hy : Sum.inr y ∈ C)
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: OrderedPair x y ∈ 𝒫 𝒫 C := by
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have hxs : {Sum.inl x} ⊆ C := Set.singleton_subset_iff.mpr hx
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have hxys : {Sum.inl x, Sum.inr y} ⊆ C := Set.mem_mem_imp_pair_subset hx hy
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exact Set.mem_mem_imp_pair_subset hxs hxys
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/-- ### Exercise 5.1
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Suppose that we attempted to generalize the Kuratowski definitions of ordered
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pairs to ordered triples by defining
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```
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⟨x, y, z⟩* = {{x}, {x, y}, {x, y, z}}.open Set
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```
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Show that this definition is unsuccessful by giving examples of objects `u`,
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`v`, `w`, `x`, `y`, `z` with `⟨x, y, z⟩* = ⟨u, v, w⟩*` but with either `y ≠ v`
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or `z ≠ w` (or both).
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-/
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theorem exercise_5_1 {x y z u v w : ℕ}
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(hx : x = 1) (hy : y = 1) (hz : z = 2)
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(hu : u = 1) (hv : v = 2) (hw : w = 2)
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: ({{x}, {x, y}, {x, y, z}} : Set (Set ℕ)) = {{u}, {u, v}, {u, v, w}}
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∧ y ≠ v := by
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apply And.intro
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· rw [hx, hy, hz, hu, hv, hw]
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simp
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· rw [hy, hv]
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simp only
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/-- ### Exercise 5.2a
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Show that `A × (B ∪ C) = (A × B) ∪ (A × C)`.
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-/
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theorem exercise_5_2a {A : Set α} {B C : Set β}
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: Set.prod A (B ∪ C) = (Set.prod A B) ∪ (Set.prod A C) := by
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calc Set.prod A (B ∪ C)
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_ = { p | p.1 ∈ A ∧ p.2 ∈ B ∪ C } := rfl
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_ = { p | p.1 ∈ A ∧ (p.2 ∈ B ∨ p.2 ∈ C) } := rfl
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_ = { p | (p.1 ∈ A ∧ p.2 ∈ B) ∨ (p.1 ∈ A ∧ p.2 ∈ C) } := by
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ext x
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rw [Set.mem_setOf_eq]
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conv => lhs; rw [and_or_left]
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_ = { p | p ∈ Set.prod A B ∨ (p ∈ Set.prod A C) } := rfl
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_ = (Set.prod A B) ∪ (Set.prod A C) := rfl
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/-- ### Exercise 5.2b
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Show that if `A × B = A × C` and `A ≠ ∅`, then `B = C`.
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-/
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theorem exercise_5_2b {A : Set α} {B C : Set β}
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(h : Set.prod A B = Set.prod A C) (hA : Set.Nonempty A)
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: B = C := by
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by_cases hB : Set.Nonempty B
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· suffices B ⊆ C ∧ C ⊆ B from Set.Subset.antisymm_iff.mpr this
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have ⟨a, ha⟩ := hA
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apply And.intro
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· show ∀ t, t ∈ B → t ∈ C
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intro t ht
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have : (a, t) ∈ Set.prod A B := ⟨ha, ht⟩
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rw [h] at this
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exact this.right
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· show ∀ t, t ∈ C → t ∈ B
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intro t ht
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have : (a, t) ∈ Set.prod A C := ⟨ha, ht⟩
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rw [← h] at this
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exact this.right
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· have nB : B = ∅ := Set.not_nonempty_iff_eq_empty.mp hB
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rw [nB, Set.prod_right_emptyset_eq_emptyset, Set.ext_iff] at h
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rw [nB]
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by_contra nC
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have ⟨a, ha⟩ := hA
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have ⟨c, hc⟩ := Set.nonempty_iff_ne_empty.mpr (Ne.symm nC)
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exact (h (a, c)).mpr ⟨ha, hc⟩
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/-- ### Exercise 5.3
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Show that `A × ⋃ 𝓑 = ⋃ {A × X | X ∈ 𝓑}`.
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-/
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theorem exercise_5_3 {A : Set (Set α)} {𝓑 : Set (Set β)}
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: Set.prod A (⋃₀ 𝓑) = ⋃₀ {Set.prod A X | X ∈ 𝓑} := by
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calc Set.prod A (⋃₀ 𝓑)
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_ = { p | p.1 ∈ A ∧ p.2 ∈ ⋃₀ 𝓑} := rfl
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_ = { p | p.1 ∈ A ∧ ∃ b ∈ 𝓑, p.2 ∈ b } := rfl
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_ = { p | ∃ b ∈ 𝓑, p.1 ∈ A ∧ p.2 ∈ b } := by
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ext x
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rw [Set.mem_setOf_eq]
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apply Iff.intro
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· intro ⟨h₁, ⟨b, h₂⟩⟩
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exact ⟨b, ⟨h₂.left, ⟨h₁, h₂.right⟩⟩⟩
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· intro ⟨b, ⟨h₁, ⟨h₂, h₃⟩⟩⟩
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exact ⟨h₂, ⟨b, ⟨h₁, h₃⟩⟩⟩
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_ = ⋃₀ { Set.prod A p | p ∈ 𝓑 } := by
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ext x
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rw [Set.mem_setOf_eq]
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unfold Set.sUnion sSup Set.instSupSetSet
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simp only [Set.mem_setOf_eq, exists_exists_and_eq_and]
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apply Iff.intro
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· intro ⟨b, ⟨h₁, ⟨h₂, h₃⟩⟩⟩
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exact ⟨b, ⟨h₁, ⟨h₂, h₃⟩⟩⟩
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· intro ⟨b, ⟨h₁, ⟨h₂, h₃⟩⟩⟩
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exact ⟨b, ⟨h₁, ⟨h₂, h₃⟩⟩⟩
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/-- ### Exercise 5.5a
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Assume that `A` and `B` are given sets, and show that there exists a set `C`
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such that for any `y`,
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```
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y ∈ C ↔ y = {x} × B for some x in A.
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```
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In other words, show that `{{x} × B | x ∈ A}` is a set.
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-/
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theorem exercise_5_5a {A : Set α} {B : Set β}
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: ∃ C : Set (Set (α × β)),
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y ∈ C ↔ ∃ x ∈ A, y = Set.prod {x} B := by
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sorry
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/-- ### Exercise 5.5b
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With `A`, `B`, and `C` as above, show that `A × B = ∪ C`.
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-/
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theorem exercise_5_5b {A : Set α} (B : Set β)
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: Set.prod A B = ⋃₀ {Set.prod ({x} : Set α) B | x ∈ A} := by
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suffices Set.prod A B ⊆ ⋃₀ {Set.prod {x} B | x ∈ A} ∧
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⋃₀ {Set.prod {x} B | x ∈ A} ⊆ Set.prod A B from
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Set.Subset.antisymm_iff.mpr this
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apply And.intro
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· show ∀ t, t ∈ Set.prod A B → t ∈ ⋃₀ {Set.prod {x} B | x ∈ A}
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intro t h
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simp only [Set.mem_setOf_eq] at h
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unfold Set.sUnion sSup Set.instSupSetSet
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simp only [Set.mem_setOf_eq, exists_exists_and_eq_and]
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unfold Set.prod at h
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simp only [Set.mem_setOf_eq] at h
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refine ⟨t.fst, ⟨h.left, ?_⟩⟩
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unfold Set.prod
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simp only [Set.mem_singleton_iff, Set.mem_setOf_eq, true_and]
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exact h.right
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· show ∀ t, t ∈ ⋃₀ {Set.prod {x} B | x ∈ A} → t ∈ Set.prod A B
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unfold Set.prod
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intro t ht
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simp only [
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Set.mem_singleton_iff,
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Set.mem_sUnion,
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Set.mem_setOf_eq,
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exists_exists_and_eq_and
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] at ht
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have ⟨a, ⟨h, ⟨ha, hb⟩⟩⟩ := ht
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simp only [Set.mem_setOf_eq]
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rw [← ha] at h
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exact ⟨h, hb⟩
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end Enderton.Set.Chapter_3 |