From 6cac38968b199c56c174cc69e01f4986d2ad8038 Mon Sep 17 00:00:00 2001
From: Joseph Myers <jsm@polyomino.org.uk>
Date: Sun, 18 Aug 2024 19:05:45 +0000
Subject: [PATCH] feat(Data/Setoid/Basic): `prod` and `piSetoid` lemmas and
 equivs

Add lemmas about applying `Setoid.r` for `Setoid.prod` and `piSetoid`,
and equivalences between a (pairwise or indexed) product of quotients
and the corresponding quotient of the product by `Setoid.prod` or
`piSetoid`.

From AperiodicMonotilesLean.
---
 Mathlib/Data/Setoid/Basic.lean | 41 ++++++++++++++++++++++++++++++++++
 1 file changed, 41 insertions(+)

diff --git a/Mathlib/Data/Setoid/Basic.lean b/Mathlib/Data/Setoid/Basic.lean
index 294f426f6d013..1bcd5c482481d 100644
--- a/Mathlib/Data/Setoid/Basic.lean
+++ b/Mathlib/Data/Setoid/Basic.lean
@@ -111,6 +111,47 @@ protected def prod (r : Setoid α) (s : Setoid β) :
     ⟨fun x => ⟨r.refl' x.1, s.refl' x.2⟩, fun h => ⟨r.symm' h.1, s.symm' h.2⟩,
       fun h₁ h₂ => ⟨r.trans' h₁.1 h₂.1, s.trans' h₁.2 h₂.2⟩⟩
 
+lemma prod_apply {r : Setoid α} {s : Setoid β} {x₁ x₂ : α} {y₁ y₂ : β} :
+    @Setoid.r _ (r.prod s) (x₁, y₁) (x₂, y₂) ↔ (@Setoid.r _ r x₁ x₂ ∧ @Setoid.r _ s y₁ y₂) :=
+  Iff.rfl
+
+lemma piSetoid_apply {ι : Sort*} {α : ι → Sort*} {r : ∀ i, Setoid (α i)} {x y : ∀ i, α i} :
+    @Setoid.r _ (@piSetoid _ _ r) x y ↔ ∀ i, @Setoid.r _ (r i) (x i) (y i) :=
+  Iff.rfl
+
+/-- A bijection between the product of two quotients and the quotient by the product of the
+equivalence relations. -/
+def prodQuotientEquiv (r : Setoid α) (s : Setoid β) :
+    Quotient r × Quotient s ≃ Quotient (r.prod s) where
+  toFun := fun (x, y) ↦ Quotient.map₂' Prod.mk (fun _ _ hx _ _ hy ↦ ⟨hx, hy⟩) x y
+  invFun := fun q ↦ Quotient.liftOn' q (fun xy ↦ (Quotient.mk'' xy.1, Quotient.mk'' xy.2))
+    fun x y hxy ↦ Prod.ext (by simpa using hxy.1) (by simpa using hxy.2)
+  left_inv := fun q ↦ by
+    rcases q with ⟨qa, qb⟩
+    exact Quotient.inductionOn₂' qa qb fun _ _ ↦ rfl
+  right_inv := fun q ↦ by
+    simp only
+    refine Quotient.inductionOn' q fun _ ↦ rfl
+
+/-- A bijection between an indexed product of quotients and the quotient by the product of the
+equivalence relations. -/
+noncomputable def piQuotientEquiv {ι : Sort*} {α : ι → Sort*} (r : ∀ i, Setoid (α i)) :
+    (∀ i, Quotient (r i)) ≃ Quotient (@piSetoid _ _ r) where
+  toFun := fun x ↦ Quotient.mk'' fun i ↦ (x i).out'
+  invFun := fun q ↦ Quotient.liftOn' q (fun x i ↦ Quotient.mk'' (x i)) fun x y hxy ↦ by
+    ext i
+    simpa using hxy i
+  left_inv := fun q ↦ by
+    ext i
+    simp
+  right_inv := fun q ↦ by
+    refine Quotient.inductionOn' q fun _ ↦ ?_
+    simp only [Quotient.liftOn'_mk'', Quotient.eq'']
+    intro i
+    change Setoid.r _ _
+    rw [← Quotient.eq'']
+    simp
+
 /-- The infimum of two equivalence relations. -/
 instance : Inf (Setoid α) :=
   ⟨fun r s =>