-- Mathlib/GroupTheory/Perm/Sign.lean
theorem sign_subtypePerm (f : Perm α) {p : α → Prop} [DecidablePred p] (h₁ : ∀ x, p x ↔ p (f x))
    (h₂ : ∀ x, f x ≠ x → p x) : sign (subtypePerm f h₁) = sign f

Proof state:
α : Type u
inst✝² : DecidableEq α
inst✝¹ : Fintype α
f : Perm α
p : α → Prop
inst✝ : DecidablePred p
h₁ : ∀ (x : α), p x ↔ p (f x)
h₂ : ∀ (x : α), f x ≠ x → p x
l : { l // l.prod = f.subtypePerm h₁ ∧ ∀ g ∈ l, g.IsSwap } := (f.subtypePerm h₁).truncSwapFactors.out
hl' : ∀ g' ∈ List.map ⇑ofSubtype ↑l, g'.IsSwap
⊢ (List.map ⇑ofSubtype ↑l).prod = f

Tactic:
rw [l.1.prod_hom ofSubtype, l.2.1, ofSubtype_subtypePerm _ h₂]

Result:
no goals

Proof state:
α : Type u
inst✝² : DecidableEq α
inst✝¹ : Fintype α
f : Perm α
p : α → Prop
inst✝ : DecidablePred p
h₁ : ∀ (x : α), p x ↔ p (f x)
h₂ : ∀ (x : α), f x ≠ x → p x
l : { l // l.prod = f.subtypePerm h₁ ∧ ∀ g ∈ l, g.IsSwap } := (f.subtypePerm h₁).truncSwapFactors.out
hl' : ∀ g' ∈ List.map ⇑ofSubtype ↑l, g'.IsSwap
hl'₂ : (List.map ⇑ofSubtype ↑l).prod = f
⊢ sign (↑l).prod = sign (List.map ⇑ofSubtype ↑l).prod

Tactic:
rw [sign_prod_list_swap l.2.2, sign_prod_list_swap hl', List.length_map]

Result:
no goals

Proof state:
α : Type u
inst✝² : DecidableEq α
inst✝¹ : Fintype α
f : Perm α
p : α → Prop
inst✝ : DecidablePred p
h₁ : ∀ (x : α), p x ↔ p (f x)
h₂ : ∀ (x : α), f x ≠ x → p x
l : { l // l.prod = f.subtypePerm h₁ ∧ ∀ g ∈ l, g.IsSwap } := (f.subtypePerm h₁).truncSwapFactors.out
hl' : ∀ g' ∈ List.map ⇑ofSubtype ↑l, g'.IsSwap
⊢ sign (f.subtypePerm h₁) = sign f

Tactic:
have hl'₂ : (l.1.map ofSubtype).prod = f := by
  rw [l.1.prod_hom ofSubtype, l.2.1, ofSubtype_subtypePerm _ h₂]

Result:
α : Type u
inst✝² : DecidableEq α
inst✝¹ : Fintype α
f : Perm α
p : α → Prop
inst✝ : DecidablePred p
h₁ : ∀ (x : α), p x ↔ p (f x)
h₂ : ∀ (x : α), f x ≠ x → p x
l : { l // l.prod = f.subtypePerm h₁ ∧ ∀ g ∈ l, g.IsSwap } := (f.subtypePerm h₁).truncSwapFactors.out
hl' : ∀ g' ∈ List.map ⇑ofSubtype ↑l, g'.IsSwap
hl'₂ : (List.map ⇑ofSubtype ↑l).prod = f
⊢ sign (f.subtypePerm h₁) = sign f

Proof state:
α : Type u
inst✝² : DecidableEq α
inst✝¹ : Fintype α
f : Perm α
p : α → Prop
inst✝ : DecidablePred p
h₁ : ∀ (x : α), p x ↔ p (f x)
h₂ : ∀ (x : α), f x ≠ x → p x
l : { l // l.prod = f.subtypePerm h₁ ∧ ∀ g ∈ l, g.IsSwap } := (f.subtypePerm h₁).truncSwapFactors.out
⊢ sign (f.subtypePerm h₁) = sign f

Tactic:
have hl' : ∀ g' ∈ l.1.map ofSubtype, IsSwap g' := fun g' hg' =>
  let ⟨g, hg⟩ := List.mem_map.1 hg'
  hg.2 ▸ (l.2.2 _ hg.1).of_subtype_isSwap

Result:
α : Type u
inst✝² : DecidableEq α
inst✝¹ : Fintype α
f : Perm α
p : α → Prop
inst✝ : DecidablePred p
h₁ : ∀ (x : α), p x ↔ p (f x)
h₂ : ∀ (x : α), f x ≠ x → p x
l : { l // l.prod = f.subtypePerm h₁ ∧ ∀ g ∈ l, g.IsSwap } := (f.subtypePerm h₁).truncSwapFactors.out
hl' : ∀ g' ∈ List.map ⇑ofSubtype ↑l, g'.IsSwap
⊢ sign (f.subtypePerm h₁) = sign f

Proof state:
α : Type u
inst✝² : DecidableEq α
inst✝¹ : Fintype α
f : Perm α
p : α → Prop
inst✝ : DecidablePred p
h₁ : ∀ (x : α), p x ↔ p (f x)
h₂ : ∀ (x : α), f x ≠ x → p x
l : { l // l.prod = f.subtypePerm h₁ ∧ ∀ g ∈ l, g.IsSwap } := (f.subtypePerm h₁).truncSwapFactors.out
hl' : ∀ g' ∈ List.map ⇑ofSubtype ↑l, g'.IsSwap
hl'₂ : (List.map ⇑ofSubtype ↑l).prod = f
⊢ sign (f.subtypePerm h₁) = sign f

Tactic:
conv =>
  congr
  rw [← l.2.1]

Result:
α : Type u
inst✝² : DecidableEq α
inst✝¹ : Fintype α
f : Perm α
p : α → Prop
inst✝ : DecidablePred p
h₁ : ∀ (x : α), p x ↔ p (f x)
h₂ : ∀ (x : α), f x ≠ x → p x
l : { l // l.prod = f.subtypePerm h₁ ∧ ∀ g ∈ l, g.IsSwap } := (f.subtypePerm h₁).truncSwapFactors.out
hl' : ∀ g' ∈ List.map ⇑ofSubtype ↑l, g'.IsSwap
hl'₂ : (List.map ⇑ofSubtype ↑l).prod = f
⊢ sign (↑l).prod = sign f

Proof state:
α : Type u
inst✝² : DecidableEq α
inst✝¹ : Fintype α
f : Perm α
p : α → Prop
inst✝ : DecidablePred p
h₁ : ∀ (x : α), p x ↔ p (f x)
h₂ : ∀ (x : α), f x ≠ x → p x
⊢ sign (f.subtypePerm h₁) = sign f

Tactic:
let l := (truncSwapFactors (subtypePerm f h₁)).out

Result:
α : Type u
inst✝² : DecidableEq α
inst✝¹ : Fintype α
f : Perm α
p : α → Prop
inst✝ : DecidablePred p
h₁ : ∀ (x : α), p x ↔ p (f x)
h₂ : ∀ (x : α), f x ≠ x → p x
l : { l // l.prod = f.subtypePerm h₁ ∧ ∀ g ∈ l, g.IsSwap } := (f.subtypePerm h₁).truncSwapFactors.out
⊢ sign (f.subtypePerm h₁) = sign f

Proof state:
α : Type u
inst✝² : DecidableEq α
inst✝¹ : Fintype α
f : Perm α
p : α → Prop
inst✝ : DecidablePred p
h₁ : ∀ (x : α), p x ↔ p (f x)
h₂ : ∀ (x : α), f x ≠ x → p x
l : { l // l.prod = f.subtypePerm h₁ ∧ ∀ g ∈ l, g.IsSwap } := (f.subtypePerm h₁).truncSwapFactors.out
hl' : ∀ g' ∈ List.map ⇑ofSubtype ↑l, g'.IsSwap
hl'₂ : (List.map ⇑ofSubtype ↑l).prod = f
⊢ sign (↑l).prod = sign f

Tactic:
simp_rw [← hl'₂]

Result:
α : Type u
inst✝² : DecidableEq α
inst✝¹ : Fintype α
f : Perm α
p : α → Prop
inst✝ : DecidablePred p
h₁ : ∀ (x : α), p x ↔ p (f x)
h₂ : ∀ (x : α), f x ≠ x → p x
l : { l // l.prod = f.subtypePerm h₁ ∧ ∀ g ∈ l, g.IsSwap } := (f.subtypePerm h₁).truncSwapFactors.out
hl' : ∀ g' ∈ List.map ⇑ofSubtype ↑l, g'.IsSwap
hl'₂ : (List.map ⇑ofSubtype ↑l).prod = f
⊢ sign (↑l).prod = sign (List.map ⇑ofSubtype ↑l).prod