-- Mathlib/RingTheory/WittVector/IsPoly.lean
theorem ext [Fact p.Prime] {f g} (hf : IsPoly p f) (hg : IsPoly p g)
    (h : ∀ (R : Type u) [_Rcr : CommRing R] (x : 𝕎 R) (n : ℕ),
        ghostComponent n (f x) = ghostComponent n (g x)) :
    ∀ (R : Type u) [_Rcr : CommRing R] (x : 𝕎 R), f x = g x

Proof state:
case mk'.intro.mk'.intro
p : ℕ
inst✝ : Fact (Nat.Prime p)
f g : ⦃R : Type u⦄ → [inst : CommRing R] → 𝕎 R → 𝕎 R
h : ∀ (R : Type u) [_Rcr : CommRing R] (x : 𝕎 R) (n : ℕ), (ghostComponent n) (f x) = (ghostComponent n) (g x)
φ : ℕ → MvPolynomial ℕ ℤ
hf : ∀ ⦃R : Type u⦄ [inst : CommRing R] (x : 𝕎 R), (f x).coeff = fun n => (aeval x.coeff) (φ n)
ψ : ℕ → MvPolynomial ℕ ℤ
hg : ∀ ⦃R : Type u⦄ [inst : CommRing R] (x : 𝕎 R), (g x).coeff = fun n => (aeval x.coeff) (ψ n)
R✝ : Type u
_Rcr✝ : CommRing R✝
x✝ : 𝕎 R✝
⊢ f x✝ = g x✝

Tactic:
ext n

Result:
case mk'.intro.mk'.intro.h
p : ℕ
inst✝ : Fact (Nat.Prime p)
f g : ⦃R : Type u⦄ → [inst : CommRing R] → 𝕎 R → 𝕎 R
h : ∀ (R : Type u) [_Rcr : CommRing R] (x : 𝕎 R) (n : ℕ), (ghostComponent n) (f x) = (ghostComponent n) (g x)
φ : ℕ → MvPolynomial ℕ ℤ
hf : ∀ ⦃R : Type u⦄ [inst : CommRing R] (x : 𝕎 R), (f x).coeff = fun n => (aeval x.coeff) (φ n)
ψ : ℕ → MvPolynomial ℕ ℤ
hg : ∀ ⦃R : Type u⦄ [inst : CommRing R] (x : 𝕎 R), (g x).coeff = fun n => (aeval x.coeff) (ψ n)
R✝ : Type u
_Rcr✝ : CommRing R✝
x✝ : 𝕎 R✝
n : ℕ
⊢ (f x✝).coeff n = (g x✝).coeff n

Proof state:
case mk'.intro
p : ℕ
inst✝ : Fact (Nat.Prime p)
f g : ⦃R : Type u⦄ → [inst : CommRing R] → 𝕎 R → 𝕎 R
hg : IsPoly p g
h : ∀ (R : Type u) [_Rcr : CommRing R] (x : 𝕎 R) (n : ℕ), (ghostComponent n) (f x) = (ghostComponent n) (g x)
φ : ℕ → MvPolynomial ℕ ℤ
hf : ∀ ⦃R : Type u⦄ [inst : CommRing R] (x : 𝕎 R), (f x).coeff = fun n => (aeval x.coeff) (φ n)
⊢ ∀ (R : Type u) [_Rcr : CommRing R] (x : 𝕎 R), f x = g x

Tactic:
obtain ⟨ψ, hg⟩ := hg

Result:
case mk'.intro.mk'.intro
p : ℕ
inst✝ : Fact (Nat.Prime p)
f g : ⦃R : Type u⦄ → [inst : CommRing R] → 𝕎 R → 𝕎 R
h : ∀ (R : Type u) [_Rcr : CommRing R] (x : 𝕎 R) (n : ℕ), (ghostComponent n) (f x) = (ghostComponent n) (g x)
φ : ℕ → MvPolynomial ℕ ℤ
hf : ∀ ⦃R : Type u⦄ [inst : CommRing R] (x : 𝕎 R), (f x).coeff = fun n => (aeval x.coeff) (φ n)
ψ : ℕ → MvPolynomial ℕ ℤ
hg : ∀ ⦃R : Type u⦄ [inst : CommRing R] (x : 𝕎 R), (g x).coeff = fun n => (aeval x.coeff) (ψ n)
⊢ ∀ (R : Type u) [_Rcr : CommRing R] (x : 𝕎 R), f x = g x

Proof state:
case mk'.intro.mk'.intro.h.h
p : ℕ
inst✝ : Fact (Nat.Prime p)
f g : ⦃R : Type u⦄ → [inst : CommRing R] → 𝕎 R → 𝕎 R
φ : ℕ → MvPolynomial ℕ ℤ
hf : ∀ ⦃R : Type u⦄ [inst : CommRing R] (x : 𝕎 R), (f x).coeff = fun n => (aeval x.coeff) (φ n)
ψ : ℕ → MvPolynomial ℕ ℤ
hg : ∀ ⦃R : Type u⦄ [inst : CommRing R] (x : 𝕎 R), (g x).coeff = fun n => (aeval x.coeff) (ψ n)
R✝ : Type u
_Rcr✝ : CommRing R✝
x✝ : 𝕎 R✝
n k : ℕ
x : ℕ → ℤ
h :
  (eval₂Hom (algebraMap ℤ (ULift.{u, 0} ℤ)) (f (mk p fun i => { down := x i })).coeff) (wittPolynomial p ℤ k) =
    (eval₂Hom (algebraMap ℤ (ULift.{u, 0} ℤ)) (g (mk p fun i => { down := x i })).coeff) (wittPolynomial p ℤ k)
⊢ (eval₂Hom ((MvPolynomial.eval x).comp C) fun i => (MvPolynomial.eval x) (φ i)) (wittPolynomial p ℤ k) =
    (eval₂Hom ((MvPolynomial.eval x).comp C) fun i => (MvPolynomial.eval x) (ψ i)) (wittPolynomial p ℤ k)

Tactic:
apply (ULift.ringEquiv.symm : ℤ ≃+* _).injective

Result:
case mk'.intro.mk'.intro.h.h.a
p : ℕ
inst✝ : Fact (Nat.Prime p)
f g : ⦃R : Type u⦄ → [inst : CommRing R] → 𝕎 R → 𝕎 R
φ : ℕ → MvPolynomial ℕ ℤ
hf : ∀ ⦃R : Type u⦄ [inst : CommRing R] (x : 𝕎 R), (f x).coeff = fun n => (aeval x.coeff) (φ n)
ψ : ℕ → MvPolynomial ℕ ℤ
hg : ∀ ⦃R : Type u⦄ [inst : CommRing R] (x : 𝕎 R), (g x).coeff = fun n => (aeval x.coeff) (ψ n)
R✝ : Type u
_Rcr✝ : CommRing R✝
x✝ : 𝕎 R✝
n k : ℕ
x : ℕ → ℤ
h :
  (eval₂Hom (algebraMap ℤ (ULift.{u, 0} ℤ)) (f (mk p fun i => { down := x i })).coeff) (wittPolynomial p ℤ k) =
    (eval₂Hom (algebraMap ℤ (ULift.{u, 0} ℤ)) (g (mk p fun i => { down := x i })).coeff) (wittPolynomial p ℤ k)
⊢ ULift.ringEquiv.symm
      ((eval₂Hom ((MvPolynomial.eval x).comp C) fun i => (MvPolynomial.eval x) (φ i)) (wittPolynomial p ℤ k)) =
    ULift.ringEquiv.symm
      ((eval₂Hom ((MvPolynomial.eval x).comp C) fun i => (MvPolynomial.eval x) (ψ i)) (wittPolynomial p ℤ k))

Proof state:
case h
p : ℕ
inst✝ : Fact (Nat.Prime p)
f g : ⦃R : Type u⦄ → [inst : CommRing R] → 𝕎 R → 𝕎 R
φ : ℕ → MvPolynomial ℕ ℤ
hf : ∀ ⦃R : Type u⦄ [inst : CommRing R] (x : 𝕎 R), (f x).coeff = fun n => (aeval x.coeff) (φ n)
ψ : ℕ → MvPolynomial ℕ ℤ
hg : ∀ ⦃R : Type u⦄ [inst : CommRing R] (x : 𝕎 R), (g x).coeff = fun n => (aeval x.coeff) (ψ n)
R✝ : Type u
_Rcr✝ : CommRing R✝
x✝¹ : 𝕎 R✝
n k : ℕ
x : ℕ → ℤ
h :
  (eval₂Hom (algebraMap ℤ (ULift.{u, 0} ℤ)) (f (mk p fun i => { down := x i })).coeff) (wittPolynomial p ℤ k) =
    (eval₂Hom (algebraMap ℤ (ULift.{u, 0} ℤ)) (g (mk p fun i => { down := x i })).coeff) (wittPolynomial p ℤ k)
x✝ : ℕ
⊢ (eval₂Hom ((↑ULift.ringEquiv.symm).comp (RingHom.id ℤ)) fun i => ↑ULift.ringEquiv.symm (x i)) (ψ x✝) =
    (aeval (mk p fun i => { down := x i }).coeff) (ψ x✝)

Tactic:
apply eval₂Hom_congr (RingHom.ext_int _ _) _ rfl

Result:
p : ℕ
inst✝ : Fact (Nat.Prime p)
f g : ⦃R : Type u⦄ → [inst : CommRing R] → 𝕎 R → 𝕎 R
φ : ℕ → MvPolynomial ℕ ℤ
hf : ∀ ⦃R : Type u⦄ [inst : CommRing R] (x : 𝕎 R), (f x).coeff = fun n => (aeval x.coeff) (φ n)
ψ : ℕ → MvPolynomial ℕ ℤ
hg : ∀ ⦃R : Type u⦄ [inst : CommRing R] (x : 𝕎 R), (g x).coeff = fun n => (aeval x.coeff) (ψ n)
R✝ : Type u
_Rcr✝ : CommRing R✝
x✝¹ : 𝕎 R✝
n k : ℕ
x : ℕ → ℤ
h :
  (eval₂Hom (algebraMap ℤ (ULift.{u, 0} ℤ)) (f (mk p fun i => { down := x i })).coeff) (wittPolynomial p ℤ k) =
    (eval₂Hom (algebraMap ℤ (ULift.{u, 0} ℤ)) (g (mk p fun i => { down := x i })).coeff) (wittPolynomial p ℤ k)
x✝ : ℕ
⊢ (fun i => ↑ULift.ringEquiv.symm (x i)) = (mk p fun i => { down := x i }).coeff

Proof state:
case mk'.intro.mk'.intro.h
p : ℕ
inst✝ : Fact (Nat.Prime p)
f g : ⦃R : Type u⦄ → [inst : CommRing R] → 𝕎 R → 𝕎 R
h : ∀ (R : Type u) [_Rcr : CommRing R] (x : 𝕎 R) (n : ℕ), (ghostComponent n) (f x) = (ghostComponent n) (g x)
φ : ℕ → MvPolynomial ℕ ℤ
hf : ∀ ⦃R : Type u⦄ [inst : CommRing R] (x : 𝕎 R), (f x).coeff = fun n => (aeval x.coeff) (φ n)
ψ : ℕ → MvPolynomial ℕ ℤ
hg : ∀ ⦃R : Type u⦄ [inst : CommRing R] (x : 𝕎 R), (g x).coeff = fun n => (aeval x.coeff) (ψ n)
R✝ : Type u
_Rcr✝ : CommRing R✝
x✝ : 𝕎 R✝
n k : ℕ
⊢ (bind₁ φ) (wittPolynomial p ℤ k) = (bind₁ ψ) (wittPolynomial p ℤ k)

Tactic:
apply MvPolynomial.funext

Result:
case mk'.intro.mk'.intro.h.h
p : ℕ
inst✝ : Fact (Nat.Prime p)
f g : ⦃R : Type u⦄ → [inst : CommRing R] → 𝕎 R → 𝕎 R
h : ∀ (R : Type u) [_Rcr : CommRing R] (x : 𝕎 R) (n : ℕ), (ghostComponent n) (f x) = (ghostComponent n) (g x)
φ : ℕ → MvPolynomial ℕ ℤ
hf : ∀ ⦃R : Type u⦄ [inst : CommRing R] (x : 𝕎 R), (f x).coeff = fun n => (aeval x.coeff) (φ n)
ψ : ℕ → MvPolynomial ℕ ℤ
hg : ∀ ⦃R : Type u⦄ [inst : CommRing R] (x : 𝕎 R), (g x).coeff = fun n => (aeval x.coeff) (ψ n)
R✝ : Type u
_Rcr✝ : CommRing R✝
x✝ : 𝕎 R✝
n k : ℕ
⊢ ∀ (x : ℕ → ℤ),
    (MvPolynomial.eval x) ((bind₁ φ) (wittPolynomial p ℤ k)) = (MvPolynomial.eval x) ((bind₁ ψ) (wittPolynomial p ℤ k))