-- Mathlib/RingTheory/MvPolynomial/Homogeneous.lean
lemma coeff_isHomogeneous_of_optionEquivLeft_symm
    [hσ : Finite σ] {p : Polynomial (MvPolynomial σ R)}
    (hp : ((optionEquivLeft R σ).symm p).IsHomogeneous n) (i j : ℕ) (h : i + j = n) :
    (p.coeff i).IsHomogeneous j

Proof state:
case intro.intro
σ : Type u_1
R : Type u_3
inst✝ : CommSemiring R
n : ℕ
hσ : Finite σ
p : Polynomial (MvPolynomial σ R)
hp : ((optionEquivLeft R σ).symm p).IsHomogeneous n
i j : ℕ
h : i + j = n
k : ℕ
e : σ ≃ Fin k
e' : Option σ ≃ Fin (k + 1) := e.optionCongr.trans (_root_.finSuccEquiv k).symm
F : MvPolynomial σ R ≃ₐ[R] MvPolynomial (Fin k) R := renameEquiv R e
F' : MvPolynomial (Option σ) R ≃ₐ[R] MvPolynomial (Fin (k + 1)) R := renameEquiv R e'
φ : MvPolynomial (Fin (k + 1)) R := F' ((optionEquivLeft R σ).symm p)
hφ : φ.IsHomogeneous n
⊢ (F (p.coeff i)).IsHomogeneous j

Tactic:
convert hφ.finSuccEquiv_coeff_isHomogeneous i j h using 1

Result:
case h.e'_4
σ : Type u_1
R : Type u_3
inst✝ : CommSemiring R
n : ℕ
hσ : Finite σ
p : Polynomial (MvPolynomial σ R)
hp : ((optionEquivLeft R σ).symm p).IsHomogeneous n
i j : ℕ
h : i + j = n
k : ℕ
e : σ ≃ Fin k
e' : Option σ ≃ Fin (k + 1) := e.optionCongr.trans (_root_.finSuccEquiv k).symm
F : MvPolynomial σ R ≃ₐ[R] MvPolynomial (Fin k) R := renameEquiv R e
F' : MvPolynomial (Option σ) R ≃ₐ[R] MvPolynomial (Fin (k + 1)) R := renameEquiv R e'
φ : MvPolynomial (Fin (k + 1)) R := F' ((optionEquivLeft R σ).symm p)
hφ : φ.IsHomogeneous n
⊢ F (p.coeff i) = ((finSuccEquiv R k) φ).coeff i

Proof state:
case h.e'_4
σ : Type u_1
R : Type u_3
inst✝ : CommSemiring R
n : ℕ
hσ : Finite σ
p : Polynomial (MvPolynomial σ R)
hp : ((optionEquivLeft R σ).symm p).IsHomogeneous n
i j : ℕ
h : i + j = n
k : ℕ
e : σ ≃ Fin k
e' : Option σ ≃ Fin (k + 1) := e.optionCongr.trans (_root_.finSuccEquiv k).symm
F : MvPolynomial σ R ≃ₐ[R] MvPolynomial (Fin k) R := renameEquiv R e
F' : MvPolynomial (Option σ) R ≃ₐ[R] MvPolynomial (Fin (k + 1)) R := renameEquiv R e'
φ : MvPolynomial (Fin (k + 1)) R := F' ((optionEquivLeft R σ).symm p)
hφ : φ.IsHomogeneous n
⊢ (rename ⇑e) (p.coeff i) = (Polynomial.map (rename ⇑e).toRingHom p).coeff i

Tactic:
simp

Result:
no goals

Proof state:
case intro.intro
σ : Type u_1
R : Type u_3
inst✝ : CommSemiring R
n : ℕ
hσ : Finite σ
p : Polynomial (MvPolynomial σ R)
hp : ((optionEquivLeft R σ).symm p).IsHomogeneous n
i j : ℕ
h : i + j = n
k : ℕ
e : σ ≃ Fin k
e' : Option σ ≃ Fin (k + 1) := e.optionCongr.trans (_root_.finSuccEquiv k).symm
⊢ (p.coeff i).IsHomogeneous j

Tactic:
let F := renameEquiv R e

Result:
case intro.intro
σ : Type u_1
R : Type u_3
inst✝ : CommSemiring R
n : ℕ
hσ : Finite σ
p : Polynomial (MvPolynomial σ R)
hp : ((optionEquivLeft R σ).symm p).IsHomogeneous n
i j : ℕ
h : i + j = n
k : ℕ
e : σ ≃ Fin k
e' : Option σ ≃ Fin (k + 1) := e.optionCongr.trans (_root_.finSuccEquiv k).symm
F : MvPolynomial σ R ≃ₐ[R] MvPolynomial (Fin k) R := renameEquiv R e
⊢ (p.coeff i).IsHomogeneous j

Proof state:
σ : Type u_1
R : Type u_3
inst✝ : CommSemiring R
n : ℕ
hσ : Finite σ
p : Polynomial (MvPolynomial σ R)
hp : ((optionEquivLeft R σ).symm p).IsHomogeneous n
i j : ℕ
h : i + j = n
⊢ (p.coeff i).IsHomogeneous j

Tactic:
obtain ⟨k, ⟨e⟩⟩ := Finite.exists_equiv_fin σ

Result:
case intro.intro
σ : Type u_1
R : Type u_3
inst✝ : CommSemiring R
n : ℕ
hσ : Finite σ
p : Polynomial (MvPolynomial σ R)
hp : ((optionEquivLeft R σ).symm p).IsHomogeneous n
i j : ℕ
h : i + j = n
k : ℕ
e : σ ≃ Fin k
⊢ (p.coeff i).IsHomogeneous j

Proof state:
case intro.intro
σ : Type u_1
R : Type u_3
inst✝ : CommSemiring R
n : ℕ
hσ : Finite σ
p : Polynomial (MvPolynomial σ R)
hp : ((optionEquivLeft R σ).symm p).IsHomogeneous n
i j : ℕ
h : i + j = n
k : ℕ
e : σ ≃ Fin k
⊢ (p.coeff i).IsHomogeneous j

Tactic:
let e' := e.optionCongr.trans (_root_.finSuccEquiv _).symm

Result:
case intro.intro
σ : Type u_1
R : Type u_3
inst✝ : CommSemiring R
n : ℕ
hσ : Finite σ
p : Polynomial (MvPolynomial σ R)
hp : ((optionEquivLeft R σ).symm p).IsHomogeneous n
i j : ℕ
h : i + j = n
k : ℕ
e : σ ≃ Fin k
e' : Option σ ≃ Fin (k + 1) := e.optionCongr.trans (_root_.finSuccEquiv k).symm
⊢ (p.coeff i).IsHomogeneous j

Proof state:
case h.e'_4
σ : Type u_1
R : Type u_3
inst✝ : CommSemiring R
n : ℕ
hσ : Finite σ
p : Polynomial (MvPolynomial σ R)
hp : ((optionEquivLeft R σ).symm p).IsHomogeneous n
i j : ℕ
h : i + j = n
k : ℕ
e : σ ≃ Fin k
e' : Option σ ≃ Fin (k + 1) := e.optionCongr.trans (_root_.finSuccEquiv k).symm
F : MvPolynomial σ R ≃ₐ[R] MvPolynomial (Fin k) R := renameEquiv R e
F' : MvPolynomial (Option σ) R ≃ₐ[R] MvPolynomial (Fin (k + 1)) R := renameEquiv R e'
φ : MvPolynomial (Fin (k + 1)) R := F' ((optionEquivLeft R σ).symm p)
hφ : φ.IsHomogeneous n
⊢ (rename ⇑e) (p.coeff i) = ((finSuccEquiv R k) ((rename ⇑e') ((optionEquivLeft R σ).symm p))).coeff i

Tactic:
rw [finSuccEquiv_rename_finSuccEquiv, AlgEquiv.apply_symm_apply]

Result:
case h.e'_4
σ : Type u_1
R : Type u_3
inst✝ : CommSemiring R
n : ℕ
hσ : Finite σ
p : Polynomial (MvPolynomial σ R)
hp : ((optionEquivLeft R σ).symm p).IsHomogeneous n
i j : ℕ
h : i + j = n
k : ℕ
e : σ ≃ Fin k
e' : Option σ ≃ Fin (k + 1) := e.optionCongr.trans (_root_.finSuccEquiv k).symm
F : MvPolynomial σ R ≃ₐ[R] MvPolynomial (Fin k) R := renameEquiv R e
F' : MvPolynomial (Option σ) R ≃ₐ[R] MvPolynomial (Fin (k + 1)) R := renameEquiv R e'
φ : MvPolynomial (Fin (k + 1)) R := F' ((optionEquivLeft R σ).symm p)
hφ : φ.IsHomogeneous n
⊢ (rename ⇑e) (p.coeff i) = (Polynomial.map (rename ⇑e).toRingHom p).coeff i