448 lines
12 KiB
Plaintext
448 lines
12 KiB
Plaintext
import Bookshelf.Enderton.Set.OrderedPair
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/-! # Enderton.Set.Relation
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A representation of a relation, i.e. a set of ordered pairs. Like `Set`, it is
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assumed a relation is homogeneous.
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-/
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namespace Set
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/--
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A relation type as defined by Enderton.
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We choose to use tuples to represent our ordered pairs, as opposed to
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Kuratowski's definition of a set.
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Not to be confused with the Lean-provided `Rel`.
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-/
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abbrev Relation (α : Type _) := Set (α × α)
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namespace Relation
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/-! ## Domain and Range -/
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/--
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The domain of a `Relation`.
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-/
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def dom (R : Relation α) : Set α := Prod.fst '' R
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/--
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The first component of any pair in a `Relation` must be a member of the
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`Relation`'s domain.
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-/
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theorem mem_pair_imp_fst_mem_dom {R : Relation α} (h : (x, y) ∈ R)
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: x ∈ dom R := by
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unfold dom Prod.fst
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simp only [mem_image, Prod.exists, exists_and_right, exists_eq_right]
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exact ⟨y, h⟩
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/--
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If `x ∈ dom R`, there exists some `y` such that `⟨x, y⟩ ∈ R`.
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-/
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theorem dom_exists {R : Relation α} (hx : x ∈ R.dom)
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: ∃ y : α, (x, y) ∈ R := by
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unfold dom at hx
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simp only [mem_image, Prod.exists, exists_and_right, exists_eq_right] at hx
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exact hx
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/--
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The range of a `Relation`.
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-/
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def ran (R : Relation α) : Set α := Prod.snd '' R
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theorem mem_pair_imp_snd_mem_ran {R : Relation α} (h : (x, y) ∈ R)
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: y ∈ ran R := by
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unfold ran Prod.snd
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simp only [mem_image, Prod.exists, exists_eq_right]
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exact ⟨x, h⟩
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/--
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If `x ∈ ran R`, there exists some `t` such that `⟨t, x⟩ ∈ R`.
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-/
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theorem ran_exists {R : Relation α} (hx : x ∈ R.ran)
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: ∃ t : α, (t, x) ∈ R := by
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unfold ran at hx
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simp only [mem_image, Prod.exists, exists_eq_right] at hx
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exact hx
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/--
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The field of a `Relation`.
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-/
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def fld (R : Relation α) : Set α := dom R ∪ ran R
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/--
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The inverse of a `Relation`.
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-/
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def inv (R : Relation α) : Relation α := { (p.2, p.1) | p ∈ R }
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/--
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`(x, y)` is a member of relation `R` **iff** `(y, x)` is a member of `R⁻¹`.
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-/
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@[simp]
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theorem mem_self_comm_mem_inv {R : Relation α}
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: (y, x) ∈ R.inv ↔ (x, y) ∈ R := by
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unfold inv
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simp only [Prod.exists, mem_setOf_eq, Prod.mk.injEq]
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apply Iff.intro
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· intro ⟨x', y', hxy⟩
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rw [← hxy.right.left, ← hxy.right.right]
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exact hxy.left
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· intro hxy
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exact ⟨x, y, hxy, rfl, rfl⟩
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/--
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The inverse of the inverse of a `Relation` is the `Relation`.
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-/
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@[simp]
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theorem inv_inv_eq_self (R : Relation α)
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: R.inv.inv = R := by
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unfold inv
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simp only [Prod.exists, Set.mem_setOf_eq, Prod.mk.injEq]
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ext x
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apply Iff.intro
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· intro hx
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have ⟨a₁, b₁, ⟨⟨a₂, b₂, h₁⟩, h₂⟩⟩ := hx
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rw [← h₂, ← h₁.right.right, ← h₁.right.left]
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exact h₁.left
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· intro hx
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have (p, q) := x
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refine ⟨q, p, ⟨?_, ?_⟩⟩
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· exact ⟨p, q, hx, rfl, rfl⟩
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· rfl
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/--
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For a set `F`, `dom F⁻¹ = ran F`.
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-/
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@[simp]
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theorem dom_inv_eq_ran_self {F : Relation α}
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: dom (F.inv) = ran F := by
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ext x
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unfold dom ran inv
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simp only [
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Prod.exists,
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Set.mem_image,
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Set.mem_setOf_eq,
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Prod.mk.injEq,
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exists_and_right,
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exists_eq_right
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]
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apply Iff.intro
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· intro ⟨y, a, _, h⟩
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rw [← h.right.left]
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exact ⟨a, h.left⟩
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· intro ⟨y, hy⟩
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exact ⟨y, y, x, hy, rfl, rfl⟩
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/--
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For a set `F`, `ran F⁻¹ = dom F`.
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-/
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@[simp]
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theorem ran_inv_eq_dom_self {F : Relation α}
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: ran (F.inv) = dom F := by
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ext x
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unfold dom ran inv
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simp only [
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Prod.exists,
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Set.mem_image,
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Set.mem_setOf_eq,
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Prod.mk.injEq,
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exists_eq_right,
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exists_and_right
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]
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apply Iff.intro
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· intro ⟨a, y, b, h⟩
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rw [← h.right.right]
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exact ⟨b, h.left⟩
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· intro ⟨y, hy⟩
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exact ⟨y, x, y, hy, rfl, rfl⟩
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/-! ## Restriction -/
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/--
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The restriction of a `Relation` to a `Set`.
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-/
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def restriction (R : Relation α) (A : Set α) : Relation α :=
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{ p ∈ R | p.1 ∈ A }
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/-! ## Image -/
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/--
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The image of a `Relation` under a `Set`.
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-/
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def image (R : Relation α) (A : Set α) : Set α :=
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{ y | ∃ u ∈ A, (u, y) ∈ R }
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/-! ## Single-Rooted and Single-Valued -/
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/--
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A `Relation` `R` is said to be single-rooted **iff** for all `y ∈ ran R`, there
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exists exactly one `x` such that `⟨x, y⟩ ∈ R`.
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-/
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def isSingleRooted (R : Relation α) : Prop :=
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∀ y ∈ ran R, ∃! x, x ∈ dom R ∧ (x, y) ∈ R
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/--
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A single-rooted `Relation` should map the same output to the same input.
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-/
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theorem single_rooted_eq_unique {R : Relation α} {x₁ x₂ y : α}
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(hR : R.isSingleRooted)
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: (x₁, y) ∈ R → (x₂, y) ∈ R → x₁ = x₂ := by
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intro hx₁ hx₂
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unfold isSingleRooted at hR
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have := hR y (mem_pair_imp_snd_mem_ran hx₁)
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have ⟨y₁, hy₁⟩ := this
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simp only [and_imp] at hy₁
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have h₁ := hy₁.right x₁ (mem_pair_imp_fst_mem_dom hx₁) hx₁
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have h₂ := hy₁.right x₂ (mem_pair_imp_fst_mem_dom hx₂) hx₂
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rw [h₁, h₂]
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/--
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A `Relation` `R` is said to be single-valued **iff** for all `x ∈ dom R`, there
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exists exactly one `y` such that `⟨x, y⟩ ∈ R`.
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Notice, a `Relation` that is single-valued is a function.
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-/
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def isSingleValued (R : Relation α) : Prop :=
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∀ x ∈ dom R, ∃! y, y ∈ ran R ∧ (x, y) ∈ R
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/--
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A single-valued `Relation` should map the same input to the same output.
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-/
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theorem single_valued_eq_unique {R : Relation α} {x y₁ y₂ : α}
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(hR : R.isSingleValued)
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: (x, y₁) ∈ R → (x, y₂) ∈ R → y₁ = y₂ := by
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intro hy₁ hy₂
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unfold isSingleValued at hR
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have := hR x (mem_pair_imp_fst_mem_dom hy₁)
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have ⟨x₁, hx₁⟩ := this
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simp only [and_imp] at hx₁
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have h₁ := hx₁.right y₁ (mem_pair_imp_snd_mem_ran hy₁) hy₁
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have h₂ := hx₁.right y₂ (mem_pair_imp_snd_mem_ran hy₂) hy₂
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rw [h₁, h₂]
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/--
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For a set `F`, `F⁻¹` is a function **iff** `F` is single-rooted.
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-/
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theorem single_valued_inv_iff_single_rooted_self {F : Set.Relation α}
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: F.inv.isSingleValued ↔ F.isSingleRooted := by
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apply Iff.intro
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· intro hF
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unfold isSingleValued at hF
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simp only [
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dom_inv_eq_ran_self,
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ran_inv_eq_dom_self,
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mem_self_comm_mem_inv
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] at hF
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suffices ∀ x ∈ F.ran, ∃! y, (y, x) ∈ F from hF
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intro x hx
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have ⟨y, hy⟩ := hF x hx
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simp only [
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ran_inv_eq_dom_self,
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mem_self_comm_mem_inv,
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and_imp
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] at hy
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refine ⟨y, hy.left.right, ?_⟩
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intro y₁ hy₁
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exact hy.right y₁ (mem_pair_imp_fst_mem_dom hy₁) hy₁
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· intro hF
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unfold isSingleRooted at hF
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unfold isSingleValued
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simp only [
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dom_inv_eq_ran_self,
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ran_inv_eq_dom_self,
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mem_self_comm_mem_inv
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]
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exact hF
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/--
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For a relation `F`, `F` is a function **iff** `F⁻¹` is single-rooted.
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-/
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theorem single_valued_self_iff_single_rooted_inv {F : Set.Relation α}
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: F.isSingleValued ↔ F.inv.isSingleRooted := by
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conv => lhs; rw [← inv_inv_eq_self F]
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rw [single_valued_inv_iff_single_rooted_self]
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/--
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The subset of a function must also be a function.
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-/
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theorem single_valued_subset {F G : Set.Relation α}
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(hG : G.isSingleValued) (h : F ⊆ G)
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: F.isSingleValued := by
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unfold isSingleValued
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intro x hx
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have ⟨y, hy⟩ := dom_exists hx
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unfold ExistsUnique
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simp only
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refine ⟨y, ⟨mem_pair_imp_snd_mem_ran hy, hy⟩, ?_⟩
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intro y₁ hy₁
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exact single_valued_eq_unique hG (h hy₁.right) (h hy)
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/--
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A `Relation` `R` is one-to-one if it is a single-rooted function.
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-/
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def isOneToOne (R : Relation α) : Prop :=
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R.isSingleValued ∧ R.isSingleRooted
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/--
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A `Relation` is one-to-one **iff** it's inverse is.
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-/
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theorem one_to_one_self_iff_one_to_one_inv {R : Relation α}
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: R.isOneToOne ↔ R.inv.isOneToOne := by
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unfold isOneToOne isSingleValued isSingleRooted
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conv => rhs; simp only [
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dom_inv_eq_ran_self,
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ran_inv_eq_dom_self,
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mem_self_comm_mem_inv,
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eq_iff_iff
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]
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apply Iff.intro <;>
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· intro ⟨hx, hy⟩
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exact ⟨hy, hx⟩
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/-! ## Composition -/
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/--
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The composition of two `Relation`s.
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-/
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def comp (F G : Relation α) : Relation α :=
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{ p | ∃ t : α, (p.1, t) ∈ G ∧ (t, p.2) ∈ F}
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/--
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If `x ∈ dom (F ∘ G)`, then `x ∈ dom G`.
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-/
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theorem dom_comp_imp_dom_self {F G : Relation α}
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: x ∈ dom (F.comp G) → x ∈ dom G := by
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unfold dom comp
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simp only [
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mem_image,
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mem_setOf_eq,
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Prod.exists,
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exists_and_right,
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exists_eq_right,
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forall_exists_index
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]
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intro y t ht
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exact ⟨t, ht.left⟩
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/--
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If `y ∈ ran (F ∘ G)`, then `y ∈ ran F`.
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-/
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theorem ran_comp_imp_ran_self {F G : Relation α}
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: y ∈ ran (F.comp G) → y ∈ ran F := by
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unfold ran comp
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simp only [
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mem_image,
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mem_setOf_eq,
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Prod.exists,
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exists_eq_right,
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forall_exists_index
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]
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intro x t ht
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exact ⟨t, ht.right⟩
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/--
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The composition of two functions is itself a function.
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-/
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theorem single_valued_comp_is_single_valued {F G : Relation α}
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(hF : F.isSingleValued) (hG : G.isSingleValued)
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: (F.comp G).isSingleValued := by
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unfold isSingleValued
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intro x hx
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have ⟨y, hxy⟩ := dom_exists hx
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have hy := mem_pair_imp_snd_mem_ran hxy
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refine ⟨y, ⟨hy, hxy⟩, ?_⟩
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simp only [and_imp]
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intro y₁ _ hxy₁
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unfold comp at hxy hxy₁
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simp only [mem_setOf_eq] at hxy hxy₁
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have ⟨t₁, ht₁⟩ := hxy
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have ⟨t₂, ht₂⟩ := hxy₁
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-- First show `t₁ = t₂` and then show `y = y₁`.
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have t_eq : t₁ = t₂ := by
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unfold isSingleValued at hG
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have ⟨t', ht'⟩ := hG x (mem_pair_imp_fst_mem_dom ht₁.left)
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simp only [and_imp] at ht'
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have ht₁' := ht'.right t₁ (mem_pair_imp_snd_mem_ran ht₁.left) ht₁.left
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have ht₂' := ht'.right t₂ (mem_pair_imp_snd_mem_ran ht₂.left) ht₂.left
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rw [ht₁', ht₂']
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unfold isSingleValued at hF
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rw [t_eq] at ht₁
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have ⟨y', hy'⟩ := hF t₂ (mem_pair_imp_fst_mem_dom ht₁.right)
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simp only [and_imp] at hy'
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have hk₁ := hy'.right y (mem_pair_imp_snd_mem_ran ht₁.right) ht₁.right
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have hk₂ := hy'.right y₁ (mem_pair_imp_snd_mem_ran ht₂.right) ht₂.right
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rw [hk₁, hk₂]
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/--
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For `Relation`s `F` and `G`, `(F ∘ G)⁻¹ = G⁻¹ ∘ F⁻¹`.
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-/
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theorem comp_inv_eq_inv_comp_inv {F G : Relation α}
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: (F.comp G).inv = G.inv.comp F.inv := by
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calc (F.comp G).inv
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_ = {p | ∃ t, (p.2, t) ∈ G ∧ (t, p.1) ∈ F} := by
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rw [Set.Subset.antisymm_iff]
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apply And.intro
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· unfold inv comp
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intro t ht
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simp only [mem_setOf_eq, Prod.exists] at ht
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have ⟨a, b, ⟨⟨p, hp⟩, hab⟩⟩ := ht
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rw [← hab]
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exact ⟨p, hp⟩
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· unfold inv comp
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intro (a, b) ⟨p, hp⟩
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simp only [mem_setOf_eq, Prod.exists, Prod.mk.injEq]
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exact ⟨b, a, ⟨p, hp⟩, rfl, rfl⟩
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_ = {p | ∃ t, (t, p.1) ∈ F ∧ (p.2, t) ∈ G} := by
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rw [Set.Subset.antisymm_iff]
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apply And.intro
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· intro (a, b) ht
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simp only [mem_setOf_eq] at *
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have ⟨t, p, q⟩ := ht
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exact ⟨t, q, p⟩
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· intro (a, b) ht
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simp only [mem_setOf_eq] at *
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have ⟨t, p, q⟩ := ht
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exact ⟨t, q, p⟩
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_ = {p | ∃ t, (p.1, t) ∈ F.inv ∧ (t, p.2) ∈ G.inv } := by
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rw [Set.Subset.antisymm_iff]
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apply And.intro
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· intro (a, b) ht
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simp only [mem_setOf_eq] at *
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have ⟨t, p, q⟩ := ht
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refine ⟨t, ?_, ?_⟩ <;> rwa [mem_self_comm_mem_inv]
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· intro (a, b) ht
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simp only [mem_setOf_eq] at *
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have ⟨t, p, q⟩ := ht
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refine ⟨t, ?_, ?_⟩ <;> rwa [← mem_self_comm_mem_inv]
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_ = comp (inv G) (inv F) := rfl
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/--
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Indicates `Relation` `F` is a function from `A` to `B`.
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This is usually denoted as `F : A → B`.
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-/
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def mapsInto (F : Relation α) (A B : Set α) :=
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F.isSingleValued ∧ dom F = A ∧ ran F ⊆ B
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/--
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Indicates `Relation` `F` is a function from `A` to `ran F = B`.
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-/
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def mapsOnto (F : Relation α) (A B : Set α) :=
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F.isSingleValued ∧ dom F = A ∧ ran F = B
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/-! ## Ordered Pairs -/
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/--
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Convert a `Relation` into an equivalent representation using `OrderedPair`s.
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-/
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def toOrderedPairs (R : Relation α) : Set (Set (Set α)) :=
|
||
-- Notice here we are using `Set.image` and *not* `Set.Relation.image`.
|
||
Set.image (fun (x, y) => OrderedPair x y) R
|
||
|
||
end Relation
|
||
|
||
end Set |