-- 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 h.e'_3
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 ((↑ULift.ringEquiv.symm).comp ((eval₂Hom (RingHom.id ℤ) x).comp C)) fun i =>
        (eval₂Hom ((↑ULift.ringEquiv.symm).comp (RingHom.id ℤ)) fun i => ↑ULift.ringEquiv.symm (x i)) (ψ i))
      (wittPolynomial p ℤ k) =
    (eval₂Hom (algebraMap ℤ (ULift.{u, 0} ℤ)) fun n => (aeval (mk p fun i => { down := x i }).coeff) (ψ n))
      (wittPolynomial p ℤ k)

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✝ : 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)
⊢ (fun i => (eval₂Hom ((↑ULift.ringEquiv.symm).comp (RingHom.id ℤ)) fun i => ↑ULift.ringEquiv.symm (x i)) (ψ i)) =
    fun n => (aeval (mk p fun i => { down := x i }).coeff) (ψ n)

Proof state:
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

Tactic:
simp only [coeff_mk]

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)) = fun i => { down := x i }

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:
intros

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.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)
⊢ (eval₂Hom ((↑ULift.ringEquiv.symm).comp ((MvPolynomial.eval x).comp C)) fun i =>
        ↑ULift.ringEquiv.symm ((MvPolynomial.eval x) (φ i)))
      (wittPolynomial p ℤ k) =
    (eval₂Hom ((↑ULift.ringEquiv.symm).comp ((MvPolynomial.eval x).comp C)) fun i =>
        ↑ULift.ringEquiv.symm ((MvPolynomial.eval x) (ψ i)))
      (wittPolynomial p ℤ k)

Tactic:
convert h using 1

Result:
case h.e'_2
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 ((↑ULift.ringEquiv.symm).comp ((MvPolynomial.eval x).comp C)) fun i =>
        ↑ULift.ringEquiv.symm ((MvPolynomial.eval x) (φ i)))
      (wittPolynomial p ℤ k) =
    (eval₂Hom (algebraMap ℤ (ULift.{u, 0} ℤ)) (f (mk p fun i => { down := x i })).coeff) (wittPolynomial p ℤ k)

case h.e'_3
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 ((↑ULift.ringEquiv.symm).comp ((MvPolynomial.eval x).comp C)) fun i =>
        ↑ULift.ringEquiv.symm ((MvPolynomial.eval x) (ψ i)))
      (wittPolynomial p ℤ k) =
    (eval₂Hom (algebraMap ℤ (ULift.{u, 0} ℤ)) (g (mk p fun i => { down := x i })).coeff) (wittPolynomial p ℤ k)