-- Mathlib/RingTheory/Localization/Ideal.lean
theorem surjective_quotientMap_of_maximal_of_localization {I : Ideal S} [I.IsPrime] {J : Ideal R}
    {H : J ≤ I.comap (algebraMap R S)} (hI : (I.comap (algebraMap R S)).IsMaximal) :
    Function.Surjective (Ideal.quotientMap I (algebraMap R S) H)

Proof state:
case neg
R : Type u_1
inst✝⁴ : CommRing R
M : Submonoid R
S : Type u_2
inst✝³ : CommRing S
inst✝² : Algebra R S
inst✝¹ : IsLocalization M S
I : Ideal S
inst✝ : I.IsPrime
J : Ideal R
H : J ≤ Ideal.comap (algebraMap R S) I
hI : IsField (R ⧸ Ideal.comap (algebraMap R S) I)
r m : R
hm : m ∈ M
hM : ¬(Ideal.Quotient.mk (Ideal.comap (algebraMap R S) I)) m = 0
⊢ ∃ a, (Ideal.quotientMap I (algebraMap R S) H) a = (Ideal.Quotient.mk I) (mk' S r ⟨m, hm⟩)

Tactic:
obtain ⟨n, hn⟩ := hI.3 hM

Result:
case neg.intro
R : Type u_1
inst✝⁴ : CommRing R
M : Submonoid R
S : Type u_2
inst✝³ : CommRing S
inst✝² : Algebra R S
inst✝¹ : IsLocalization M S
I : Ideal S
inst✝ : I.IsPrime
J : Ideal R
H : J ≤ Ideal.comap (algebraMap R S) I
hI : IsField (R ⧸ Ideal.comap (algebraMap R S) I)
r m : R
hm : m ∈ M
hM : ¬(Ideal.Quotient.mk (Ideal.comap (algebraMap R S) I)) m = 0
n : R ⧸ Ideal.comap (algebraMap R S) I
hn : (Ideal.Quotient.mk (Ideal.comap (algebraMap R S) I)) m * n = 1
⊢ ∃ a, (Ideal.quotientMap I (algebraMap R S) H) a = (Ideal.Quotient.mk I) (mk' S r ⟨m, hm⟩)

Proof state:
case neg.intro.intro
R : Type u_1
inst✝⁴ : CommRing R
M : Submonoid R
S : Type u_2
inst✝³ : CommRing S
inst✝² : Algebra R S
inst✝¹ : IsLocalization M S
I : Ideal S
inst✝ : I.IsPrime
J : Ideal R
H : J ≤ Ideal.comap (algebraMap R S) I
hI : IsField (R ⧸ Ideal.comap (algebraMap R S) I)
r m : R
hm : m ∈ M
hM : ¬(Ideal.Quotient.mk (Ideal.comap (algebraMap R S) I)) m = 0
rn : R
hn :
  (Ideal.quotientMap I (algebraMap R S) ⋯) ((Ideal.Quotient.mk (Ideal.comap (algebraMap R S) I)) m) *
      (Ideal.quotientMap I (algebraMap R S) ⋯) ((Ideal.Quotient.mk (Ideal.comap (algebraMap R S) I)) rn) =
    1
⊢ (Ideal.Quotient.mk I) ((algebraMap R S) (r * rn)) = (Ideal.Quotient.mk I) ((algebraMap R S) r * mk' S 1 ⟨m, hm⟩)

Tactic:
simp only [mul_eq_mul_left_iff, RingHom.map_mul]

Result:
case neg.intro.intro
R : Type u_1
inst✝⁴ : CommRing R
M : Submonoid R
S : Type u_2
inst✝³ : CommRing S
inst✝² : Algebra R S
inst✝¹ : IsLocalization M S
I : Ideal S
inst✝ : I.IsPrime
J : Ideal R
H : J ≤ Ideal.comap (algebraMap R S) I
hI : IsField (R ⧸ Ideal.comap (algebraMap R S) I)
r m : R
hm : m ∈ M
hM : ¬(Ideal.Quotient.mk (Ideal.comap (algebraMap R S) I)) m = 0
rn : R
hn :
  (Ideal.quotientMap I (algebraMap R S) ⋯) ((Ideal.Quotient.mk (Ideal.comap (algebraMap R S) I)) m) *
      (Ideal.quotientMap I (algebraMap R S) ⋯) ((Ideal.Quotient.mk (Ideal.comap (algebraMap R S) I)) rn) =
    1
⊢ (Ideal.Quotient.mk I) ((algebraMap R S) rn) = (Ideal.Quotient.mk I) (mk' S 1 ⟨m, hm⟩) ∨
    (Ideal.Quotient.mk I) ((algebraMap R S) r) = 0

Proof state:
R : Type u_1
inst✝⁴ : CommRing R
M : Submonoid R
S : Type u_2
inst✝³ : CommRing S
inst✝² : Algebra R S
inst✝¹ : IsLocalization M S
I : Ideal S
inst✝ : I.IsPrime
J : Ideal R
H : J ≤ Ideal.comap (algebraMap R S) I
hI : (Ideal.comap (algebraMap R S) I).IsMaximal
r m : R
hm : m ∈ M
hM : (Ideal.Quotient.mk (Ideal.comap (algebraMap R S) I)) m = 0
this : I = ⊤
⊢ (Ideal.Quotient.mk I) (mk' S r ⟨m, hm⟩) = (Ideal.quotientMap I (algebraMap R S) H) 0

Tactic:
simp [Ideal.Quotient.eq_zero_iff_mem, this]

Result:
no goals

Proof state:
case pos
R : Type u_1
inst✝⁴ : CommRing R
M : Submonoid R
S : Type u_2
inst✝³ : CommRing S
inst✝² : Algebra R S
inst✝¹ : IsLocalization M S
I : Ideal S
inst✝ : I.IsPrime
J : Ideal R
H : J ≤ Ideal.comap (algebraMap R S) I
hI : (Ideal.comap (algebraMap R S) I).IsMaximal
r m : R
hm : m ∈ M
hM : (Ideal.Quotient.mk (Ideal.comap (algebraMap R S) I)) m = 0
this : I = ⊤
⊢ ∃ a, (Ideal.quotientMap I (algebraMap R S) H) a = (Ideal.Quotient.mk I) (mk' S r ⟨m, hm⟩)

Tactic:
exact ⟨0, eq_comm.1 (by simp [Ideal.Quotient.eq_zero_iff_mem, this])⟩

Result:
no goals

Proof state:
case neg.intro.intro
R : Type u_1
inst✝⁴ : CommRing R
M : Submonoid R
S : Type u_2
inst✝³ : CommRing S
inst✝² : Algebra R S
inst✝¹ : IsLocalization M S
I : Ideal S
inst✝ : I.IsPrime
J : Ideal R
H : J ≤ Ideal.comap (algebraMap R S) I
hI : IsField (R ⧸ Ideal.comap (algebraMap R S) I)
r m : R
hm : m ∈ M
hM : ¬(Ideal.Quotient.mk (Ideal.comap (algebraMap R S) I)) m = 0
rn : R
hn :
  (Ideal.quotientMap I (algebraMap R S) ⋯) ((Ideal.Quotient.mk (Ideal.comap (algebraMap R S) I)) m) *
      (Ideal.quotientMap I (algebraMap R S) ⋯) ((Ideal.Quotient.mk (Ideal.comap (algebraMap R S) I)) rn) =
    1
⊢ (Ideal.quotientMap I (algebraMap R S) H) ((Ideal.Quotient.mk J) (r * rn)) = (Ideal.Quotient.mk I) (mk' S r ⟨m, hm⟩)

Tactic:
rw [Ideal.quotientMap_mk, ← sub_eq_zero, ← RingHom.map_sub, Ideal.Quotient.eq_zero_iff_mem, ←
  Ideal.Quotient.eq_zero_iff_mem, RingHom.map_sub, sub_eq_zero, mk'_eq_mul_mk'_one]

Result:
case neg.intro.intro
R : Type u_1
inst✝⁴ : CommRing R
M : Submonoid R
S : Type u_2
inst✝³ : CommRing S
inst✝² : Algebra R S
inst✝¹ : IsLocalization M S
I : Ideal S
inst✝ : I.IsPrime
J : Ideal R
H : J ≤ Ideal.comap (algebraMap R S) I
hI : IsField (R ⧸ Ideal.comap (algebraMap R S) I)
r m : R
hm : m ∈ M
hM : ¬(Ideal.Quotient.mk (Ideal.comap (algebraMap R S) I)) m = 0
rn : R
hn :
  (Ideal.quotientMap I (algebraMap R S) ⋯) ((Ideal.Quotient.mk (Ideal.comap (algebraMap R S) I)) m) *
      (Ideal.quotientMap I (algebraMap R S) ⋯) ((Ideal.Quotient.mk (Ideal.comap (algebraMap R S) I)) rn) =
    1
⊢ (Ideal.Quotient.mk I) ((algebraMap R S) (r * rn)) = (Ideal.Quotient.mk I) ((algebraMap R S) r * mk' S 1 ⟨m, hm⟩)

Proof state:
case h.e'_5
R : Type u_1
inst✝⁴ : CommRing R
M : Submonoid R
S : Type u_2
inst✝³ : CommRing S
inst✝² : Algebra R S
inst✝¹ : IsLocalization M S
I : Ideal S
inst✝ : I.IsPrime
J : Ideal R
H : J ≤ Ideal.comap (algebraMap R S) I
hI : (Ideal.comap (algebraMap R S) I).IsMaximal
r m : R
hm : m ∈ M
hM : (algebraMap R S) m ∈ I
⊢ 1 = (algebraMap R S) m * mk' S 1 ⟨m, hm⟩

Tactic:
rw [← mk'_eq_mul_mk'_one, mk'_self]

Result:
no goals

Proof state:
case neg.intro.intro
R : Type u_1
inst✝⁴ : CommRing R
M : Submonoid R
S : Type u_2
inst✝³ : CommRing S
inst✝² : Algebra R S
inst✝¹ : IsLocalization M S
I : Ideal S
inst✝ : I.IsPrime
J : Ideal R
H : J ≤ Ideal.comap (algebraMap R S) I
hI : IsField (R ⧸ Ideal.comap (algebraMap R S) I)
r m : R
hm : m ∈ M
hM : ¬(Ideal.Quotient.mk (Ideal.comap (algebraMap R S) I)) m = 0
rn : R
hn :
  (Ideal.quotientMap I (algebraMap R S) ⋯)
      ((Ideal.Quotient.mk (Ideal.comap (algebraMap R S) I)) m *
        (Ideal.Quotient.mk (Ideal.comap (algebraMap R S) I)) rn) =
    (Ideal.quotientMap I (algebraMap R S) ⋯) 1
⊢ (Ideal.quotientMap I (algebraMap R S) H) ((Ideal.Quotient.mk J) (r * rn)) = (Ideal.Quotient.mk I) (mk' S r ⟨m, hm⟩)

Tactic:
rw [RingHom.map_one, RingHom.map_mul] at hn

Result:
case neg.intro.intro
R : Type u_1
inst✝⁴ : CommRing R
M : Submonoid R
S : Type u_2
inst✝³ : CommRing S
inst✝² : Algebra R S
inst✝¹ : IsLocalization M S
I : Ideal S
inst✝ : I.IsPrime
J : Ideal R
H : J ≤ Ideal.comap (algebraMap R S) I
hI : IsField (R ⧸ Ideal.comap (algebraMap R S) I)
r m : R
hm : m ∈ M
hM : ¬(Ideal.Quotient.mk (Ideal.comap (algebraMap R S) I)) m = 0
rn : R
hn :
  (Ideal.quotientMap I (algebraMap R S) ⋯) ((Ideal.Quotient.mk (Ideal.comap (algebraMap R S) I)) m) *
      (Ideal.quotientMap I (algebraMap R S) ⋯) ((Ideal.Quotient.mk (Ideal.comap (algebraMap R S) I)) rn) =
    1
⊢ (Ideal.quotientMap I (algebraMap R S) H) ((Ideal.Quotient.mk J) (r * rn)) = (Ideal.Quotient.mk I) (mk' S r ⟨m, hm⟩)