feat(data/equiv/basic): perm_congr group lemmas (#6982)

Co-authored-by: Yakov Pechersky <pechersky@users.noreply.github.com>

Author: pechersky

src/data/equiv/basic.lean

@@ -317,22 +317,36 @@ by { ext, refl }
 @[simp] lemma equiv_congr_apply_apply {δ} (ab : α ≃ β) (cd : γ ≃ δ) (e : α ≃ γ) (x) :
   ab.equiv_congr cd e x = cd (e (ab.symm x)) := rfl
 
+section perm_congr
+
+variables {α' β' : Type*} (e : α' ≃ β')
+
 /-- If `α` is equivalent to `β`, then `perm α` is equivalent to `perm β`. -/
-def perm_congr {α : Type*} {β : Type*} (e : α ≃ β) : perm α ≃ perm β :=
+def perm_congr : perm α' ≃ perm β' :=
 equiv_congr e e
 
-lemma perm_congr_def {α β : Type*} (e : α ≃ β) (p : equiv.perm α) :
+lemma perm_congr_def (p : equiv.perm α') :
   e.perm_congr p = (e.symm.trans p).trans e := rfl
 
-@[simp] lemma perm_congr_symm {α β : Type*} (e : α ≃ β) :
+@[simp] lemma perm_congr_refl :
+  e.perm_congr (equiv.refl _) = equiv.refl _ :=
+by simp [perm_congr_def]
+
+@[simp] lemma perm_congr_symm :
   e.perm_congr.symm = e.symm.perm_congr := rfl
 
-@[simp] lemma perm_congr_apply {α β : Type*} (e : α ≃ β) (p : equiv.perm α) (x) :
+@[simp] lemma perm_congr_apply (p : equiv.perm α') (x) :
   e.perm_congr p x = e (p (e.symm x)) := rfl
 
-lemma perm_congr_symm_apply {α β : Type*} (e : α ≃ β) (p : equiv.perm β) (x) :
+lemma perm_congr_symm_apply (p : equiv.perm β') (x) :
   e.perm_congr.symm p x = e.symm (p (e x)) := rfl
 
+lemma perm_congr_trans (p p' : equiv.perm α') :
+  (e.perm_congr p).trans (e.perm_congr p') = e.perm_congr (p.trans p') :=
+by { ext, simp }
+
+end perm_congr
+
 protected lemma image_eq_preimage {α β} (e : α ≃ β) (s : set α) : e '' s = e.symm ⁻¹' s :=
 set.ext $ assume x, set.mem_image_iff_of_inverse e.left_inv e.right_inv
 

src/group_theory/perm/basic.lean

@@ -176,6 +176,11 @@ begin
   simpa using equiv.congr_fun h i
 end
 
+/-- If `e` is also a permutation, we can write `perm_congr` 
+completely in terms of the group structure. -/
+@[simp] lemma perm_congr_eq_mul (e p : perm α) :
+  e.perm_congr p = e * p * e⁻¹ := rfl
+
 /-- If the permutation `f` fixes the subtype `{x // p x}`, then this returns the permutation
   on `{x // p x}` induced by `f`. -/
 def subtype_perm (f : perm α) {p : α → Prop} (h : ∀ x, p x ↔ p (f x)) : perm {x // p x} :=