import Common.Logic.Basic import Mathlib.Data.Set.Basic import Mathlib.Data.Set.Prod /-! # Common.Set.Basic Additional theorems and definitions useful in the context of `Set`s. -/ namespace Set /-! ## Minkowski Sum -/ /- The Minkowski sum of two `Set`s `s` and `t` is the set `s + t = { a + b : a ∈ s, b ∈ t }`. -/ def minkowskiSum {α : Type u} [Add α] (s t : Set α) := { x | ∃ a ∈ s, ∃ b ∈ t, x = a + b } /-- The sum of two `Set`s is nonempty **iff** the summands are nonempty. -/ theorem nonempty_minkowski_sum_iff_nonempty_add_nonempty {α : Type u} [Add α] {s t : Set α} : (minkowskiSum s t).Nonempty ↔ s.Nonempty ∧ t.Nonempty := by apply Iff.intro · intro h have ⟨x, hx⟩ := h have ⟨a, ⟨ha, ⟨b, ⟨hb, _⟩⟩⟩⟩ := hx apply And.intro · exact ⟨a, ha⟩ · exact ⟨b, hb⟩ · intro ⟨⟨a, ha⟩, ⟨b, hb⟩⟩ exact ⟨a + b, ⟨a, ⟨ha, ⟨b, ⟨hb, rfl⟩⟩⟩⟩⟩ /-! ## Characteristic Function -/ /-- The characteristic function of a `Set` `S`. It returns `1` if the specified input belongs to `S` and `0` otherwise. -/ def characteristic (S : Set α) (x : α) [Decidable (x ∈ S)] : Nat := if x ∈ S then 1 else 0 /-! ## Equality -/ /-- If `{x, y} = {x}` then `x = y`. -/ theorem pair_eq_singleton_mem_imp_eq_self {x y : α} (h : {x, y} = ({x} : Set α)) : y = x := by rw [Set.ext_iff] at h have := h y simp at this exact this /-- If `{x, y} = {z}` then `x = y = z`. -/ theorem pair_eq_singleton_mem_imp_eq_all {x y z : α} (h : {x, y} = ({z} : Set α)) : x = z ∧ y = z := by have h' := h rw [Set.ext_iff] at h' have hz := h' z simp at hz apply Or.elim hz · intro hzx rw [← hzx] at h have := pair_eq_singleton_mem_imp_eq_self h exact ⟨hzx.symm, this⟩ · intro hzy rw [← hzy, Set.pair_comm] at h have := pair_eq_singleton_mem_imp_eq_self h exact ⟨this, hzy.symm⟩ /-! ## Subsets -/ /-- Every `Set` is a subset of itself. -/ theorem subset_self (S : Set α) : S ⊆ S := by intro _ hs exact hs /-- If `Set` `A` is a subset of `Set` `B`, then `A ∪ B = B`. -/ theorem left_subset_union_eq_self {A B : Set α} (h : A ⊆ B) : A ∪ B = B := by rw [Set.ext_iff] intro x apply Iff.intro · intro hU apply Or.elim hU · intro hA exact h hA · simp · intro hB exact Or.inr hB /-- If `Set` `B` is a subset of `Set` `A`, then `A ∪ B = B`. -/ theorem right_subset_union_eq_self {A B : Set α} (h : B ⊆ A) : A ∪ B = A := by rw [Set.union_comm] exact left_subset_union_eq_self h /-- If `x` and `y` are members of `Set` `A`, it follows `{x, y}` is a subset of `A`. -/ theorem mem_mem_imp_pair_subset {x y : α} (hx : x ∈ A) (hy : y ∈ A) : ({x, y} : Set α) ⊆ A := by intro a ha apply Or.elim ha · intro hx' rwa [hx'] · intro hy' rwa [hy'] /-! ## Powerset -/ /-- Every `Set` is a member of its own powerset. -/ theorem self_mem_powerset_self {A : Set α} : A ∈ 𝒫 A := subset_self A /-! ## Cartesian Product -/ /-- For any `Set` `A`, `∅ × A = ∅`. -/ theorem prod_left_emptyset_eq_emptyset {A : Set α} : Set.prod (∅ : Set β) A = ∅ := by unfold prod simp only [mem_empty_iff_false, false_and, setOf_false] /-- For any `Set` `A`, `A × ∅ = ∅`. -/ theorem prod_right_emptyset_eq_emptyset {A : Set α} : Set.prod A (∅ : Set β) = ∅ := by unfold prod simp only [mem_empty_iff_false, and_false, setOf_false] /-- For any `Set`s `A` and `B`, if both `A` and `B` are nonempty, then `A × B` is also nonempty. -/ theorem prod_nonempty_nonempty_imp_nonempty_prod {A : Set α} {B : Set β} : A ≠ ∅ ∧ B ≠ ∅ ↔ Set.prod A B ≠ ∅ := by apply Iff.intro · intro nAB h have ⟨a, ha⟩ := nonempty_iff_ne_empty.mpr nAB.left have ⟨b, hb⟩ := nonempty_iff_ne_empty.mpr nAB.right rw [Set.ext_iff] at h exact (h (a, b)).mp ⟨ha, hb⟩ · intro h rw [← nonempty_iff_ne_empty] at h have ⟨(a, b), ⟨ha, hb⟩⟩ := h rw [← nonempty_iff_ne_empty, ← nonempty_iff_ne_empty] exact ⟨⟨a, ha⟩, ⟨b, hb⟩⟩ /-! ## Symmetric Difference -/ /-- `x` is a member of the `symmDiff` of `A` and `B` **iff** `x ∈ A ∧ x ∉ B` or `x ∉ A ∧ x ∈ B`. -/ theorem mem_symm_diff_iff_exclusive_mem {A B : Set α} : x ∈ (A ∆ B) ↔ (x ∈ A ∧ x ∉ B) ∨ (x ∉ A ∧ x ∈ B) := by unfold symmDiff apply Iff.intro · intro hx simp at hx conv => arg 2; rw [and_comm] exact hx · intro hx simp conv => arg 2; rw [and_comm] exact hx /-- `x` is not a member of the `symmDiff` of `A` and `B` **iff** `x ∈ A ∩ B` or `x ∉ A ∪ B`. This is the contraposition of `mem_symm_diff_iff_exclusive_mem`. -/ theorem not_mem_symm_diff_inter_or_not_union {A B : Set α} : x ∉ (A ∆ B) ↔ (x ∈ A ∩ B) ∨ (x ∉ A ∪ B) := by show ¬(x ∈ A ∧ ¬x ∈ B ∨ x ∈ B ∧ ¬x ∈ A) ↔ x ∈ A ∧ x ∈ B ∨ ¬(x ∈ A ∨ x ∈ B) rw [ not_or_de_morgan, not_and_de_morgan, not_and_de_morgan, not_not, not_not, not_or_de_morgan ] apply Iff.intro · intro nx apply Or.elim nx.left · intro nA exact Or.elim nx.right (Or.inr ⟨nA, ·⟩) (absurd · nA) · intro hB exact Or.elim nx.right (absurd hB ·) (Or.inl ⟨·, hB⟩) · intro hx apply Or.elim hx · intro hy exact ⟨Or.inr hy.right, Or.inr hy.left⟩ · intro hy exact ⟨Or.inl hy.left, Or.inl hy.right⟩ end Set