-- Mathlib/LinearAlgebra/AffineSpace/Independent.lean
theorem centroid_eq_iff [CharZero k] {n : ℕ} (s : Simplex k P n) {fs₁ fs₂ : Finset (Fin (n + 1))}
    {m₁ m₂ : ℕ} (h₁ : #fs₁ = m₁ + 1) (h₂ : #fs₂ = m₂ + 1) :
    fs₁.centroid k s.points = fs₂.centroid k s.points ↔ fs₁ = fs₂

Proof state:
k : Type u_1
V : Type u_2
P : Type u_3
inst✝⁴ : DivisionRing k
inst✝³ : AddCommGroup V
inst✝² : Module k V
inst✝¹ : AffineSpace V P
inst✝ : CharZero k
n : ℕ
s : Simplex k P n
fs₁ fs₂ : Finset (Fin (n + 1))
m₁ m₂ : ℕ
h₁ : #fs₁ = m₁ + 1
h₂ : #fs₂ = m₂ + 1
h :
  (affineCombination k univ s.points) (centroidWeightsIndicator k fs₁) =
    (affineCombination k univ s.points) (centroidWeightsIndicator k fs₂)
⊢ fs₁ = fs₂

Tactic:
have ha :=
  (affineIndependent_iff_indicator_eq_of_affineCombination_eq k s.points).1 s.independent _ _ _ _
    (fs₁.sum_centroidWeightsIndicator_eq_one_of_card_eq_add_one k h₁)
    (fs₂.sum_centroidWeightsIndicator_eq_one_of_card_eq_add_one k h₂) h

Result:
k : Type u_1
V : Type u_2
P : Type u_3
inst✝⁴ : DivisionRing k
inst✝³ : AddCommGroup V
inst✝² : Module k V
inst✝¹ : AffineSpace V P
inst✝ : CharZero k
n : ℕ
s : Simplex k P n
fs₁ fs₂ : Finset (Fin (n + 1))
m₁ m₂ : ℕ
h₁ : #fs₁ = m₁ + 1
h₂ : #fs₂ = m₂ + 1
h :
  (affineCombination k univ s.points) (centroidWeightsIndicator k fs₁) =
    (affineCombination k univ s.points) (centroidWeightsIndicator k fs₂)
ha : (↑univ).indicator (centroidWeightsIndicator k fs₁) = (↑univ).indicator (centroidWeightsIndicator k fs₂)
⊢ fs₁ = fs₂

Proof state:
k : Type u_1
V : Type u_2
P : Type u_3
inst✝⁴ : DivisionRing k
inst✝³ : AddCommGroup V
inst✝² : Module k V
inst✝¹ : AffineSpace V P
inst✝ : CharZero k
n : ℕ
s : Simplex k P n
fs₁ fs₂ : Finset (Fin (n + 1))
m₁ m₂ : ℕ
h₁ : #fs₁ = m₁ + 1
h₂ : #fs₂ = m₂ + 1
⊢ centroid k fs₁ s.points = centroid k fs₂ s.points ↔ fs₁ = fs₂

Tactic:
refine ⟨fun h => ?_, @congrArg _ _ fs₁ fs₂ (fun z => Finset.centroid k z s.points)⟩

Result:
k : Type u_1
V : Type u_2
P : Type u_3
inst✝⁴ : DivisionRing k
inst✝³ : AddCommGroup V
inst✝² : Module k V
inst✝¹ : AffineSpace V P
inst✝ : CharZero k
n : ℕ
s : Simplex k P n
fs₁ fs₂ : Finset (Fin (n + 1))
m₁ m₂ : ℕ
h₁ : #fs₁ = m₁ + 1
h₂ : #fs₂ = m₂ + 1
h : centroid k fs₁ s.points = centroid k fs₂ s.points
⊢ fs₁ = fs₂

Proof state:
case h
k : Type u_1
V : Type u_2
P : Type u_3
inst✝⁴ : DivisionRing k
inst✝³ : AddCommGroup V
inst✝² : Module k V
inst✝¹ : AffineSpace V P
inst✝ : CharZero k
n : ℕ
s : Simplex k P n
fs₁ fs₂ : Finset (Fin (n + 1))
m₁ m₂ : ℕ
h₁ : #fs₁ = m₁ + 1
h₂ : #fs₂ = m₂ + 1
h :
  (affineCombination k univ s.points) (centroidWeightsIndicator k fs₁) =
    (affineCombination k univ s.points) (centroidWeightsIndicator k fs₂)
ha :
  ∀ (x : Fin (n + 1)),
    (↑fs₁).indicator (const (Fin (n + 1)) (↑(m₁ + 1))⁻¹) x = (↑fs₂).indicator (const (Fin (n + 1)) (↑(m₂ + 1))⁻¹) x
i : Fin (n + 1)
⊢ i ∈ fs₁ ↔ i ∈ fs₂

Tactic:
specialize ha i

Result:
case h
k : Type u_1
V : Type u_2
P : Type u_3
inst✝⁴ : DivisionRing k
inst✝³ : AddCommGroup V
inst✝² : Module k V
inst✝¹ : AffineSpace V P
inst✝ : CharZero k
n : ℕ
s : Simplex k P n
fs₁ fs₂ : Finset (Fin (n + 1))
m₁ m₂ : ℕ
h₁ : #fs₁ = m₁ + 1
h₂ : #fs₂ = m₂ + 1
h :
  (affineCombination k univ s.points) (centroidWeightsIndicator k fs₁) =
    (affineCombination k univ s.points) (centroidWeightsIndicator k fs₂)
i : Fin (n + 1)
ha : (↑fs₁).indicator (const (Fin (n + 1)) (↑(m₁ + 1))⁻¹) i = (↑fs₂).indicator (const (Fin (n + 1)) (↑(m₂ + 1))⁻¹) i
⊢ i ∈ fs₁ ↔ i ∈ fs₂

Proof state:
case h.mp
k : Type u_1
V : Type u_2
P : Type u_3
inst✝⁴ : DivisionRing k
inst✝³ : AddCommGroup V
inst✝² : Module k V
inst✝¹ : AffineSpace V P
inst✝ : CharZero k
n : ℕ
s : Simplex k P n
fs₁ fs₂ : Finset (Fin (n + 1))
m₁ m₂ : ℕ
h₁ : #fs₁ = m₁ + 1
h₂ : #fs₂ = m₂ + 1
h :
  (affineCombination k univ s.points) (centroidWeightsIndicator k fs₁) =
    (affineCombination k univ s.points) (centroidWeightsIndicator k fs₂)
i : Fin (n + 1)
ha : (↑fs₁).indicator (const (Fin (n + 1)) (↑(m₁ + 1))⁻¹) i = (↑fs₂).indicator (const (Fin (n + 1)) (↑(m₂ + 1))⁻¹) i
key : ∀ (n : ℕ), ↑n + 1 ≠ 0
hi : i ∈ fs₁
hni : i ∉ fs₂
⊢ False

Tactic:
simp [hni, hi, key] at ha

Result:
no goals

Proof state:
k : Type u_1
V : Type u_2
P : Type u_3
inst✝⁴ : DivisionRing k
inst✝³ : AddCommGroup V
inst✝² : Module k V
inst✝¹ : AffineSpace V P
inst✝ : CharZero k
n : ℕ
s : Simplex k P n
fs₁ fs₂ : Finset (Fin (n + 1))
m₁ m₂ : ℕ
h₁ : #fs₁ = m₁ + 1
h₂ : #fs₂ = m₂ + 1
h : centroid k fs₁ s.points = centroid k fs₂ s.points
⊢ fs₁ = fs₂

Tactic:
rw [Finset.centroid_eq_affineCombination_fintype,
  Finset.centroid_eq_affineCombination_fintype] at h

Result:
k : Type u_1
V : Type u_2
P : Type u_3
inst✝⁴ : DivisionRing k
inst✝³ : AddCommGroup V
inst✝² : Module k V
inst✝¹ : AffineSpace V P
inst✝ : CharZero k
n : ℕ
s : Simplex k P n
fs₁ fs₂ : Finset (Fin (n + 1))
m₁ m₂ : ℕ
h₁ : #fs₁ = m₁ + 1
h₂ : #fs₂ = m₂ + 1
h :
  (affineCombination k univ s.points) (centroidWeightsIndicator k fs₁) =
    (affineCombination k univ s.points) (centroidWeightsIndicator k fs₂)
⊢ fs₁ = fs₂