-- Mathlib/RingTheory/FinitePresentation.lean
theorem mvPolynomial_of_finitePresentation [FinitePresentation.{w₁, w₂} R A]
    (ι : Type v) [Finite ι] :
    FinitePresentation.{w₁, max v w₂} R (MvPolynomial ι A)

Proof state:
case intro.intro.intro.intro
R : Type w₁
A : Type w₂
inst✝⁴ : CommRing R
inst✝³ : CommRing A
inst✝² : Algebra R A
inst✝¹ : FinitePresentation R A
ι : Type v
inst✝ : Finite ι
ι' : Type v
w✝ : Fintype ι'
f : MvPolynomial ι' R →ₐ[R] A
hf_surj : Surjective ⇑f
hf_ker : (RingHom.ker f.toRingHom).FG
g : MvPolynomial (ι ⊕ ι') R →ₐ[R] MvPolynomial ι A := (MvPolynomial.mapAlgHom f).comp ↑(MvPolynomial.sumAlgEquiv R ι ι')
⊢ ∃ ι_1 x f, Surjective ⇑f ∧ (RingHom.ker f.toRingHom).FG

Tactic:
cases nonempty_fintype (ι ⊕ ι')

Result:
case intro.intro.intro.intro.intro
R : Type w₁
A : Type w₂
inst✝⁴ : CommRing R
inst✝³ : CommRing A
inst✝² : Algebra R A
inst✝¹ : FinitePresentation R A
ι : Type v
inst✝ : Finite ι
ι' : Type v
w✝ : Fintype ι'
f : MvPolynomial ι' R →ₐ[R] A
hf_surj : Surjective ⇑f
hf_ker : (RingHom.ker f.toRingHom).FG
g : MvPolynomial (ι ⊕ ι') R →ₐ[R] MvPolynomial ι A := (MvPolynomial.mapAlgHom f).comp ↑(MvPolynomial.sumAlgEquiv R ι ι')
val✝ : Fintype (ι ⊕ ι')
⊢ ∃ ι_1 x f, Surjective ⇑f ∧ (RingHom.ker f.toRingHom).FG

Proof state:
R : Type w₁
A : Type w₂
inst✝⁴ : CommRing R
inst✝³ : CommRing A
inst✝² : Algebra R A
inst✝¹ : FinitePresentation R A
ι : Type v
inst✝ : Finite ι
hfp : FinitePresentation R A
⊢ FinitePresentation R (MvPolynomial ι A)

Tactic:
rw [iff_quotient_mvPolynomial'] at hfp ⊢

Result:
R : Type w₁
A : Type w₂
inst✝⁴ : CommRing R
inst✝³ : CommRing A
inst✝² : Algebra R A
inst✝¹ : FinitePresentation R A
ι : Type v
inst✝ : Finite ι
hfp : ∃ ι x f, Surjective ⇑f ∧ (RingHom.ker f.toRingHom).FG
⊢ ∃ ι_1 x f, Surjective ⇑f ∧ (RingHom.ker f.toRingHom).FG

Proof state:
case intro.intro.intro.intro.intro.refine_2
R : Type w₁
A : Type w₂
inst✝⁴ : CommRing R
inst✝³ : CommRing A
inst✝² : Algebra R A
inst✝¹ : FinitePresentation R A
ι : Type v
inst✝ : Finite ι
ι' : Type v
w✝ : Fintype ι'
f : MvPolynomial ι' R →ₐ[R] A
hf_surj : Surjective ⇑f
hf_ker : (RingHom.ker f.toRingHom).FG
g : MvPolynomial (ι ⊕ ι') R →ₐ[R] MvPolynomial ι A := (MvPolynomial.mapAlgHom f).comp ↑(MvPolynomial.sumAlgEquiv R ι ι')
val✝ : Fintype (ι ⊕ ι')
⊢ (RingHom.ker (MvPolynomial.mapAlgHom f).toRingHom).FG

Tactic:
rw [AlgHom.toRingHom_eq_coe, MvPolynomial.mapAlgHom_coe_ringHom, MvPolynomial.ker_map]

Result:
case intro.intro.intro.intro.intro.refine_2
R : Type w₁
A : Type w₂
inst✝⁴ : CommRing R
inst✝³ : CommRing A
inst✝² : Algebra R A
inst✝¹ : FinitePresentation R A
ι : Type v
inst✝ : Finite ι
ι' : Type v
w✝ : Fintype ι'
f : MvPolynomial ι' R →ₐ[R] A
hf_surj : Surjective ⇑f
hf_ker : (RingHom.ker f.toRingHom).FG
g : MvPolynomial (ι ⊕ ι') R →ₐ[R] MvPolynomial ι A := (MvPolynomial.mapAlgHom f).comp ↑(MvPolynomial.sumAlgEquiv R ι ι')
val✝ : Fintype (ι ⊕ ι')
⊢ (Ideal.map MvPolynomial.C (RingHom.ker ↑f)).FG

Proof state:
case intro.intro.intro.intro
R : Type w₁
A : Type w₂
inst✝⁴ : CommRing R
inst✝³ : CommRing A
inst✝² : Algebra R A
inst✝¹ : FinitePresentation R A
ι : Type v
inst✝ : Finite ι
ι' : Type v
w✝ : Fintype ι'
f : MvPolynomial ι' R →ₐ[R] A
hf_surj : Surjective ⇑f
hf_ker : (RingHom.ker f.toRingHom).FG
⊢ ∃ ι_1 x f, Surjective ⇑f ∧ (RingHom.ker f.toRingHom).FG

Tactic:
let g := (MvPolynomial.mapAlgHom f).comp (MvPolynomial.sumAlgEquiv R ι ι').toAlgHom

Result:
case intro.intro.intro.intro
R : Type w₁
A : Type w₂
inst✝⁴ : CommRing R
inst✝³ : CommRing A
inst✝² : Algebra R A
inst✝¹ : FinitePresentation R A
ι : Type v
inst✝ : Finite ι
ι' : Type v
w✝ : Fintype ι'
f : MvPolynomial ι' R →ₐ[R] A
hf_surj : Surjective ⇑f
hf_ker : (RingHom.ker f.toRingHom).FG
g : MvPolynomial (ι ⊕ ι') R →ₐ[R] MvPolynomial ι A := (MvPolynomial.mapAlgHom f).comp ↑(MvPolynomial.sumAlgEquiv R ι ι')
⊢ ∃ ι_1 x f, Surjective ⇑f ∧ (RingHom.ker f.toRingHom).FG

Proof state:
R : Type w₁
A : Type w₂
inst✝⁴ : CommRing R
inst✝³ : CommRing A
inst✝² : Algebra R A
inst✝¹ : FinitePresentation R A
ι : Type v
inst✝ : Finite ι
⊢ FinitePresentation R (MvPolynomial ι A)

Tactic:
have hfp : FinitePresentation.{w₁, w₂} R A := inferInstance

Result:
R : Type w₁
A : Type w₂
inst✝⁴ : CommRing R
inst✝³ : CommRing A
inst✝² : Algebra R A
inst✝¹ : FinitePresentation R A
ι : Type v
inst✝ : Finite ι
hfp : FinitePresentation R A
⊢ FinitePresentation R (MvPolynomial ι A)

Proof state:
case intro.intro.intro.intro.intro.refine_1
R : Type w₁
A : Type w₂
inst✝⁴ : CommRing R
inst✝³ : CommRing A
inst✝² : Algebra R A
inst✝¹ : FinitePresentation R A
ι : Type v
inst✝ : Finite ι
ι' : Type v
w✝ : Fintype ι'
f : MvPolynomial ι' R →ₐ[R] A
hf_surj : Surjective ⇑f
hf_ker : (RingHom.ker f.toRingHom).FG
g : MvPolynomial (ι ⊕ ι') R →ₐ[R] MvPolynomial ι A := (MvPolynomial.mapAlgHom f).comp ↑(MvPolynomial.sumAlgEquiv R ι ι')
val✝ : Fintype (ι ⊕ ι')
⊢ (RingHom.ker ↑↑(MvPolynomial.sumAlgEquiv R ι ι')).FG

Tactic:
erw [RingHom.ker_coe_equiv (MvPolynomial.sumAlgEquiv R ι ι').toRingEquiv]

Result:
case intro.intro.intro.intro.intro.refine_1
R : Type w₁
A : Type w₂
inst✝⁴ : CommRing R
inst✝³ : CommRing A
inst✝² : Algebra R A
inst✝¹ : FinitePresentation R A
ι : Type v
inst✝ : Finite ι
ι' : Type v
w✝ : Fintype ι'
f : MvPolynomial ι' R →ₐ[R] A
hf_surj : Surjective ⇑f
hf_ker : (RingHom.ker f.toRingHom).FG
g : MvPolynomial (ι ⊕ ι') R →ₐ[R] MvPolynomial ι A := (MvPolynomial.mapAlgHom f).comp ↑(MvPolynomial.sumAlgEquiv R ι ι')
val✝ : Fintype (ι ⊕ ι')
⊢ ⊥.FG

Proof state:
R : Type w₁
A : Type w₂
inst✝⁴ : CommRing R
inst✝³ : CommRing A
inst✝² : Algebra R A
inst✝¹ : FinitePresentation R A
ι : Type v
inst✝ : Finite ι
ι' : Type v
w✝ : Fintype ι'
f : MvPolynomial ι' R →ₐ[R] A
hf_surj : Surjective ⇑f
hf_ker : (RingHom.ker f.toRingHom).FG
g : MvPolynomial (ι ⊕ ι') R →ₐ[R] MvPolynomial ι A := (MvPolynomial.mapAlgHom f).comp ↑(MvPolynomial.sumAlgEquiv R ι ι')
val✝ : Fintype (ι ⊕ ι')
⊢ Fintype (ι ⊕ ι')

Tactic:
infer_instance

Result:
no goals

Proof state:
case intro.intro.intro.intro.intro
R : Type w₁
A : Type w₂
inst✝⁴ : CommRing R
inst✝³ : CommRing A
inst✝² : Algebra R A
inst✝¹ : FinitePresentation R A
ι : Type v
inst✝ : Finite ι
ι' : Type v
w✝ : Fintype ι'
f : MvPolynomial ι' R →ₐ[R] A
hf_surj : Surjective ⇑f
hf_ker : (RingHom.ker f.toRingHom).FG
g : MvPolynomial (ι ⊕ ι') R →ₐ[R] MvPolynomial ι A := (MvPolynomial.mapAlgHom f).comp ↑(MvPolynomial.sumAlgEquiv R ι ι')
val✝ : Fintype (ι ⊕ ι')
⊢ ∃ ι_1 x f, Surjective ⇑f ∧ (RingHom.ker f.toRingHom).FG

Tactic:
refine
  ⟨ι ⊕ ι', by infer_instance, g,
    (MvPolynomial.map_surjective f.toRingHom hf_surj).comp (AlgEquiv.surjective _),
    Ideal.fg_ker_comp _ _ ?_ ?_ (AlgEquiv.surjective _)⟩

Result:
case intro.intro.intro.intro.intro.refine_1
R : Type w₁
A : Type w₂
inst✝⁴ : CommRing R
inst✝³ : CommRing A
inst✝² : Algebra R A
inst✝¹ : FinitePresentation R A
ι : Type v
inst✝ : Finite ι
ι' : Type v
w✝ : Fintype ι'
f : MvPolynomial ι' R →ₐ[R] A
hf_surj : Surjective ⇑f
hf_ker : (RingHom.ker f.toRingHom).FG
g : MvPolynomial (ι ⊕ ι') R →ₐ[R] MvPolynomial ι A := (MvPolynomial.mapAlgHom f).comp ↑(MvPolynomial.sumAlgEquiv R ι ι')
val✝ : Fintype (ι ⊕ ι')
⊢ (RingHom.ker ↑↑(MvPolynomial.sumAlgEquiv R ι ι')).FG

case intro.intro.intro.intro.intro.refine_2
R : Type w₁
A : Type w₂
inst✝⁴ : CommRing R
inst✝³ : CommRing A
inst✝² : Algebra R A
inst✝¹ : FinitePresentation R A
ι : Type v
inst✝ : Finite ι
ι' : Type v
w✝ : Fintype ι'
f : MvPolynomial ι' R →ₐ[R] A
hf_surj : Surjective ⇑f
hf_ker : (RingHom.ker f.toRingHom).FG
g : MvPolynomial (ι ⊕ ι') R →ₐ[R] MvPolynomial ι A := (MvPolynomial.mapAlgHom f).comp ↑(MvPolynomial.sumAlgEquiv R ι ι')
val✝ : Fintype (ι ⊕ ι')
⊢ (RingHom.ker (MvPolynomial.mapAlgHom f).toRingHom).FG