-- Mathlib/RingTheory/Algebraic/MvPolynomial.lean
theorem transcendental_supported_polynomial_aeval_X {i : σ} {s : Set σ} (h : i ∉ s)
    {f : R[X]} (hf : Transcendental R f) :
    Transcendental (supported R s) (Polynomial.aeval (X i : MvPolynomial σ R) f)

Proof state:
σ : Type u_1
R : Type u_2
inst✝ : CommRing R
i : σ
s : Set σ
h : i ∉ s
f : R[X]
g : MvPolynomial (↑s) R[X] →ₐ[R] MvPolynomial (↑s) R[X] := mapAlgHom (Polynomial.aeval f)
u : (MvPolynomial (↑s) R)[X] →ₐ[R] MvPolynomial σ R :=
  (((supported R (insert i s)).val.comp
            ↑(((optionEquivRight R ↑s).symm.trans (renameEquiv R (Set.subtypeInsertEquivOption h).symm)).trans
                (supportedEquivMvPolynomial (insert i s)).symm)).comp
        g).comp
    ↑((optionEquivLeft R ↑s).symm.trans (optionEquivRight R ↑s))
v : (MvPolynomial (↑s) R)[X] →ₐ[R] MvPolynomial σ R :=
  (AlgHom.restrictScalars R (Polynomial.aeval ((Polynomial.aeval (X i)) f))).comp
    ↑(Polynomial.mapAlgEquiv (supportedEquivMvPolynomial s).symm)
hf : Function.Injective ⇑u
⊢ Function.Injective ⇑(Polynomial.aeval ((Polynomial.aeval (X i)) f))

Tactic:
have h1 : Polynomial.aeval (X i : MvPolynomial σ R) = ((Subalgebra.val _).comp
    (supportedEquivMvPolynomial _).symm.toAlgHom |>.comp
    (Polynomial.aeval (X ⟨i, s.mem_insert i⟩ : MvPolynomial ↑(insert i s) R))) := by
  ext1; simp

Result:
σ : Type u_1
R : Type u_2
inst✝ : CommRing R
i : σ
s : Set σ
h : i ∉ s
f : R[X]
g : MvPolynomial (↑s) R[X] →ₐ[R] MvPolynomial (↑s) R[X] := mapAlgHom (Polynomial.aeval f)
u : (MvPolynomial (↑s) R)[X] →ₐ[R] MvPolynomial σ R :=
  (((supported R (insert i s)).val.comp
            ↑(((optionEquivRight R ↑s).symm.trans (renameEquiv R (Set.subtypeInsertEquivOption h).symm)).trans
                (supportedEquivMvPolynomial (insert i s)).symm)).comp
        g).comp
    ↑((optionEquivLeft R ↑s).symm.trans (optionEquivRight R ↑s))
v : (MvPolynomial (↑s) R)[X] →ₐ[R] MvPolynomial σ R :=
  (AlgHom.restrictScalars R (Polynomial.aeval ((Polynomial.aeval (X i)) f))).comp
    ↑(Polynomial.mapAlgEquiv (supportedEquivMvPolynomial s).symm)
hf : Function.Injective ⇑u
h1 :
  Polynomial.aeval (X i) =
    ((supported R (insert i s)).val.comp ↑(supportedEquivMvPolynomial (insert i s)).symm).comp
      (Polynomial.aeval (X ⟨i, ⋯⟩))
⊢ Function.Injective ⇑(Polynomial.aeval ((Polynomial.aeval (X i)) f))

Proof state:
σ : Type u_1
R : Type u_2
inst✝ : CommRing R
i : σ
s : Set σ
h : i ∉ s
f : R[X]
g : MvPolynomial (↑s) R[X] →ₐ[R] MvPolynomial (↑s) R[X] := mapAlgHom (Polynomial.aeval f)
u : (MvPolynomial (↑s) R)[X] →ₐ[R] MvPolynomial σ R :=
  (((supported R (insert i s)).val.comp
            ↑(((optionEquivRight R ↑s).symm.trans (renameEquiv R (Set.subtypeInsertEquivOption h).symm)).trans
                (supportedEquivMvPolynomial (insert i s)).symm)).comp
        g).comp
    ↑((optionEquivLeft R ↑s).symm.trans (optionEquivRight R ↑s))
v : (MvPolynomial (↑s) R)[X] →ₐ[R] MvPolynomial σ R :=
  (AlgHom.restrictScalars R (Polynomial.aeval ((Polynomial.aeval (X i)) f))).comp
    ↑(Polynomial.mapAlgEquiv (supportedEquivMvPolynomial s).symm)
hf : Function.Injective ⇑u
h1 :
  Polynomial.aeval (X i) =
    ((supported R (insert i s)).val.comp ↑(supportedEquivMvPolynomial (insert i s)).symm).comp
      (Polynomial.aeval (X ⟨i, ⋯⟩))
⊢ u = v

Tactic:
simp only [u, v, g]

Result:
σ : Type u_1
R : Type u_2
inst✝ : CommRing R
i : σ
s : Set σ
h : i ∉ s
f : R[X]
g : MvPolynomial (↑s) R[X] →ₐ[R] MvPolynomial (↑s) R[X] := mapAlgHom (Polynomial.aeval f)
u : (MvPolynomial (↑s) R)[X] →ₐ[R] MvPolynomial σ R :=
  (((supported R (insert i s)).val.comp
            ↑(((optionEquivRight R ↑s).symm.trans (renameEquiv R (Set.subtypeInsertEquivOption h).symm)).trans
                (supportedEquivMvPolynomial (insert i s)).symm)).comp
        g).comp
    ↑((optionEquivLeft R ↑s).symm.trans (optionEquivRight R ↑s))
v : (MvPolynomial (↑s) R)[X] →ₐ[R] MvPolynomial σ R :=
  (AlgHom.restrictScalars R (Polynomial.aeval ((Polynomial.aeval (X i)) f))).comp
    ↑(Polynomial.mapAlgEquiv (supportedEquivMvPolynomial s).symm)
hf : Function.Injective ⇑u
h1 :
  Polynomial.aeval (X i) =
    ((supported R (insert i s)).val.comp ↑(supportedEquivMvPolynomial (insert i s)).symm).comp
      (Polynomial.aeval (X ⟨i, ⋯⟩))
⊢ (((supported R (insert i s)).val.comp
              ↑(((optionEquivRight R ↑s).symm.trans (renameEquiv R (Set.subtypeInsertEquivOption h).symm)).trans
                  (supportedEquivMvPolynomial (insert i s)).symm)).comp
          (mapAlgHom (Polynomial.aeval f))).comp
      ↑((optionEquivLeft R ↑s).symm.trans (optionEquivRight R ↑s)) =
    (AlgHom.restrictScalars R (Polynomial.aeval ((Polynomial.aeval (X i)) f))).comp
      ↑(Polynomial.mapAlgEquiv (supportedEquivMvPolynomial s).symm)

Proof state:
σ : Type u_1
R : Type u_2
inst✝ : CommRing R
i : σ
s : Set σ
h : i ∉ s
f : R[X]
g : MvPolynomial (↑s) R[X] →ₐ[R] MvPolynomial (↑s) R[X] := mapAlgHom (Polynomial.aeval f)
hf : Function.Injective ⇑g
u : (MvPolynomial (↑s) R)[X] →ₐ[R] MvPolynomial σ R :=
  (((supported R (insert i s)).val.comp
            ↑(((optionEquivRight R ↑s).symm.trans (renameEquiv R (Set.subtypeInsertEquivOption h).symm)).trans
                (supportedEquivMvPolynomial (insert i s)).symm)).comp
        g).comp
    ↑((optionEquivLeft R ↑s).symm.trans (optionEquivRight R ↑s))
⊢ Function.Injective ⇑(Polynomial.aeval ((Polynomial.aeval (X i)) f))

Tactic:
let v := ((Polynomial.aeval (R := supported R s)
  (Polynomial.aeval (X i : MvPolynomial σ R) f)).restrictScalars R).comp
    (Polynomial.mapAlgEquiv (supportedEquivMvPolynomial s).symm).toAlgHom

Result:
σ : Type u_1
R : Type u_2
inst✝ : CommRing R
i : σ
s : Set σ
h : i ∉ s
f : R[X]
g : MvPolynomial (↑s) R[X] →ₐ[R] MvPolynomial (↑s) R[X] := mapAlgHom (Polynomial.aeval f)
hf : Function.Injective ⇑g
u : (MvPolynomial (↑s) R)[X] →ₐ[R] MvPolynomial σ R :=
  (((supported R (insert i s)).val.comp
            ↑(((optionEquivRight R ↑s).symm.trans (renameEquiv R (Set.subtypeInsertEquivOption h).symm)).trans
                (supportedEquivMvPolynomial (insert i s)).symm)).comp
        g).comp
    ↑((optionEquivLeft R ↑s).symm.trans (optionEquivRight R ↑s))
v : (MvPolynomial (↑s) R)[X] →ₐ[R] MvPolynomial σ R :=
  (AlgHom.restrictScalars R (Polynomial.aeval ((Polynomial.aeval (X i)) f))).comp
    ↑(Polynomial.mapAlgEquiv (supportedEquivMvPolynomial s).symm)
⊢ Function.Injective ⇑(Polynomial.aeval ((Polynomial.aeval (X i)) f))

Proof state:
case hX
σ : Type u_1
R : Type u_2
inst✝ : CommRing R
i : σ
s : Set σ
h : i ∉ s
f : R[X]
g : MvPolynomial (↑s) R[X] →ₐ[R] MvPolynomial (↑s) R[X] := mapAlgHom (Polynomial.aeval f)
u : (MvPolynomial (↑s) R)[X] →ₐ[R] MvPolynomial σ R :=
  (((supported R (insert i s)).val.comp
            ↑(((optionEquivRight R ↑s).symm.trans (renameEquiv R (Set.subtypeInsertEquivOption h).symm)).trans
                (supportedEquivMvPolynomial (insert i s)).symm)).comp
        g).comp
    ↑((optionEquivLeft R ↑s).symm.trans (optionEquivRight R ↑s))
v : (MvPolynomial (↑s) R)[X] →ₐ[R] MvPolynomial σ R :=
  (AlgHom.restrictScalars R (Polynomial.aeval ((Polynomial.aeval (X i)) f))).comp
    ↑(Polynomial.mapAlgEquiv (supportedEquivMvPolynomial s).symm)
hf : Function.Injective ⇑u
h1 :
  Polynomial.aeval (X i) =
    ((supported R (insert i s)).val.comp ↑(supportedEquivMvPolynomial (insert i s)).symm).comp
      (Polynomial.aeval (X ⟨i, ⋯⟩))
⊢ ((((supported R (insert i s)).val.comp
                ↑(((optionEquivRight R ↑s).symm.trans (renameEquiv R (Set.subtypeInsertEquivOption h).symm)).trans
                    (supportedEquivMvPolynomial (insert i s)).symm)).comp
            (mapAlgHom (Polynomial.aeval f))).comp
        ↑((optionEquivLeft R ↑s).symm.trans (optionEquivRight R ↑s)))
      Polynomial.X =
    ((AlgHom.restrictScalars R (Polynomial.aeval ((Polynomial.aeval (X i)) f))).comp
        ↑(Polynomial.mapAlgEquiv (supportedEquivMvPolynomial s).symm))
      Polynomial.X

Tactic:
simp [Set.subtypeInsertEquivOption, h1]

Result:
no goals