-- Mathlib/RingTheory/Localization/Away/Basic.lean
theorem existsUnique_algebraMap_eq_of_span_eq_top (s : Set R) (span_eq : Ideal.span s = ⊤)
    (f : Π a : s, Away a.1) (h : ∀ a b : s,
      Away.awayToAwayRight (P := Away (a * b : R)) a.1 b (f a) = Away.awayToAwayLeft b.1 a (f b)) :
    ∃! r : R, ∀ a : s, algebraMap R _ r = f a

Proof state:
case inr
R : Type u_1
inst✝ : CommSemiring R
s : Set R
f : (a : ↑s) → Away ↑a
h : ∀ (a b : ↑s), (Away.awayToAwayRight ↑a ↑b) (f a) = (Away.awayToAwayLeft ↑b ↑a) (f b)
mem✝ : 1 ∈ Ideal.span s
this :
  ∀ (s : Set R) (f : (a : ↑s) → Away ↑a),
    (∀ (a b : ↑s), (Away.awayToAwayRight ↑a ↑b) (f a) = (Away.awayToAwayLeft ↑b ↑a) (f b)) →
      1 ∈ Ideal.span s → (∃ t, ↑t = s) → ∃! r, ∀ (a : ↑s), (algebraMap R (Away ↑a)) r = f a
finset_eq : ¬∃ t, ↑t = s
t : Finset R
hts : ↑t ⊆ s
mem : 1 ∈ Submodule.span R ↑t
r : R
eq : (fun r => ∀ (a : ↑↑t), (algebraMap R (Away ↑a)) r = f ⟨↑a, ⋯⟩) r
uniq : ∀ (y : R), (fun r => ∀ (a : ↑↑t), (algebraMap R (Away ↑a)) r = f ⟨↑a, ⋯⟩) y → y = r
⊢ ∃! r, ∀ (a : ↑s), (algebraMap R (Away ↑a)) r = f a

Tactic:
refine ⟨r, fun a ↦ ?_, fun _ eq ↦ uniq _ fun a ↦ eq ⟨a, hts a.2⟩⟩

Result:
case inr
R : Type u_1
inst✝ : CommSemiring R
s : Set R
f : (a : ↑s) → Away ↑a
h : ∀ (a b : ↑s), (Away.awayToAwayRight ↑a ↑b) (f a) = (Away.awayToAwayLeft ↑b ↑a) (f b)
mem✝ : 1 ∈ Ideal.span s
this :
  ∀ (s : Set R) (f : (a : ↑s) → Away ↑a),
    (∀ (a b : ↑s), (Away.awayToAwayRight ↑a ↑b) (f a) = (Away.awayToAwayLeft ↑b ↑a) (f b)) →
      1 ∈ Ideal.span s → (∃ t, ↑t = s) → ∃! r, ∀ (a : ↑s), (algebraMap R (Away ↑a)) r = f a
finset_eq : ¬∃ t, ↑t = s
t : Finset R
hts : ↑t ⊆ s
mem : 1 ∈ Submodule.span R ↑t
r : R
eq : (fun r => ∀ (a : ↑↑t), (algebraMap R (Away ↑a)) r = f ⟨↑a, ⋯⟩) r
uniq : ∀ (y : R), (fun r => ∀ (a : ↑↑t), (algebraMap R (Away ↑a)) r = f ⟨↑a, ⋯⟩) y → y = r
a : ↑s
⊢ (algebraMap R (Away ↑a)) r = f a

Proof state:
case intro
R : Type u_1
inst✝ : CommSemiring R
s : Finset R
f : (a : ↑↑s) → Away ↑a
h : ∀ (a b : ↑↑s), (Away.awayToAwayRight ↑a ↑b) (f a) = (Away.awayToAwayLeft ↑b ↑a) (f b)
mem : 1 ∈ Ideal.span ↑s
span_eq : Ideal.span ↑s = ⊤
n : ↑↑s → ℕ
r✝ : ↑↑s → R
eq✝ : ∀ (a : ↑↑s), f a * (algebraMap R (Away ↑a)) ↑a ^ n a = (algebraMap R (Away ↑a)) (r✝ a)
N✝ : ℕ := s.attach.sup n
r : ↑↑s → R := fun a => ↑a ^ (N✝ - n a) * r✝ a
eq : ∀ (a : ↑↑s), f a * (algebraMap R (Away ↑a)) (↑a ^ N✝) = (algebraMap R (Away ↑a)) (r a)
N'✝ : ↑↑s → ↑↑s → ℕ
hN' : ∀ (a b : ↑↑s), (↑a * ↑b) ^ N'✝ a b * (r a * ↑b ^ N✝) = (↑a * ↑b) ^ N'✝ a b * (r b * ↑a ^ N✝)
N' : ℕ := (s ×ˢ s).attach.sup fun a => N'✝ ⟨(↑a).1, ⋯⟩ ⟨(↑a).2, ⋯⟩
eq2 : ∀ (a b : ↑↑s), (↑a * ↑b) ^ N' * (r a * ↑b ^ N✝) = (↑a * ↑b) ^ N' * (r b * ↑a ^ N✝)
N : ℕ := N' + N✝
⊢ ∃ x, ∀ (a : ↑↑s), (algebraMap R (Away ↑a)) x = f a

Tactic:
let r a := a ^ N' * r a

Result:
case intro
R : Type u_1
inst✝ : CommSemiring R
s : Finset R
f : (a : ↑↑s) → Away ↑a
h : ∀ (a b : ↑↑s), (Away.awayToAwayRight ↑a ↑b) (f a) = (Away.awayToAwayLeft ↑b ↑a) (f b)
mem : 1 ∈ Ideal.span ↑s
span_eq : Ideal.span ↑s = ⊤
n : ↑↑s → ℕ
r✝¹ : ↑↑s → R
eq✝ : ∀ (a : ↑↑s), f a * (algebraMap R (Away ↑a)) ↑a ^ n a = (algebraMap R (Away ↑a)) (r✝¹ a)
N✝ : ℕ := s.attach.sup n
r✝ : ↑↑s → R := fun a => ↑a ^ (N✝ - n a) * r✝¹ a
eq : ∀ (a : ↑↑s), f a * (algebraMap R (Away ↑a)) (↑a ^ N✝) = (algebraMap R (Away ↑a)) (r✝ a)
N'✝ : ↑↑s → ↑↑s → ℕ
hN' : ∀ (a b : ↑↑s), (↑a * ↑b) ^ N'✝ a b * (r✝ a * ↑b ^ N✝) = (↑a * ↑b) ^ N'✝ a b * (r✝ b * ↑a ^ N✝)
N' : ℕ := (s ×ˢ s).attach.sup fun a => N'✝ ⟨(↑a).1, ⋯⟩ ⟨(↑a).2, ⋯⟩
eq2 : ∀ (a b : ↑↑s), (↑a * ↑b) ^ N' * (r✝ a * ↑b ^ N✝) = (↑a * ↑b) ^ N' * (r✝ b * ↑a ^ N✝)
N : ℕ := N' + N✝
r : ↑↑s → R := fun a => ↑a ^ N' * r✝ a
⊢ ∃ x, ∀ (a : ↑↑s), (algebraMap R (Away ↑a)) x = f a

Proof state:
case inr
R : Type u_1
inst✝ : CommSemiring R
s : Set R
f : (a : ↑s) → Away ↑a
h : ∀ (a b : ↑s), (Away.awayToAwayRight ↑a ↑b) (f a) = (Away.awayToAwayLeft ↑b ↑a) (f b)
mem✝ : 1 ∈ Ideal.span s
this :
  ∀ (s : Set R) (f : (a : ↑s) → Away ↑a),
    (∀ (a b : ↑s), (Away.awayToAwayRight ↑a ↑b) (f a) = (Away.awayToAwayLeft ↑b ↑a) (f b)) →
      1 ∈ Ideal.span s → (∃ t, ↑t = s) → ∃! r, ∀ (a : ↑s), (algebraMap R (Away ↑a)) r = f a
finset_eq : ¬∃ t, ↑t = s
t : Finset R
hts✝ : ↑t ⊆ s
mem : 1 ∈ Submodule.span R ↑t
r : R
eq : (fun r => ∀ (a : ↑↑t), (algebraMap R (Away ↑a)) r = f ⟨↑a, ⋯⟩) r
uniq : ∀ (y : R), (fun r => ∀ (a : ↑↑t), (algebraMap R (Away ↑a)) r = f ⟨↑a, ⋯⟩) y → y = r
a : ↑s
hts : insert ↑a ↑t ⊆ s
⊢ (algebraMap R (Away ↑a)) r = f a

Tactic:
classical
have ⟨r', eq, _⟩ := this ({a.1} ∪ t) (fun a ↦ f ⟨a, hts a.2⟩) (fun a b ↦
  h ⟨a, hts a.2⟩ ⟨b, hts b.2⟩) (Ideal.span_mono (fun _ ↦ .inr) mem) ⟨{a.1} ∪ t, by simp⟩
exact (congr_arg _ (uniq _ fun b ↦ eq ⟨b, .inr b.2⟩).symm).trans (eq ⟨a, .inl rfl⟩)

Result:
no goals

Proof state:
case intro
R : Type u_1
inst✝ : CommSemiring R
s : Finset R
f : (a : ↑↑s) → Away ↑a
h : ∀ (a b : ↑↑s), (Away.awayToAwayRight ↑a ↑b) (f a) = (Away.awayToAwayLeft ↑b ↑a) (f b)
mem : 1 ∈ Ideal.span ↑s
span_eq : Ideal.span ↑s = ⊤
n : ↑↑s → ℕ
r✝¹ : ↑↑s → R
eq✝¹ : ∀ (a : ↑↑s), f a * (algebraMap R (Away ↑a)) ↑a ^ n a = (algebraMap R (Away ↑a)) (r✝¹ a)
N✝ : ℕ := s.attach.sup n
r✝ : ↑↑s → R := fun a => ↑a ^ (N✝ - n a) * r✝¹ a
eq✝ : ∀ (a : ↑↑s), f a * (algebraMap R (Away ↑a)) (↑a ^ N✝) = (algebraMap R (Away ↑a)) (r✝ a)
N'✝ : ↑↑s → ↑↑s → ℕ
hN' : ∀ (a b : ↑↑s), (↑a * ↑b) ^ N'✝ a b * (r✝ a * ↑b ^ N✝) = (↑a * ↑b) ^ N'✝ a b * (r✝ b * ↑a ^ N✝)
N' : ℕ := (s ×ˢ s).attach.sup fun a => N'✝ ⟨(↑a).1, ⋯⟩ ⟨(↑a).2, ⋯⟩
eq2✝ : ∀ (a b : ↑↑s), (↑a * ↑b) ^ N' * (r✝ a * ↑b ^ N✝) = (↑a * ↑b) ^ N' * (r✝ b * ↑a ^ N✝)
N : ℕ := N' + N✝
r : ↑↑s → R := fun a => ↑a ^ N' * r✝ a
eq : ∀ (a : ↑↑s), f a * (algebraMap R (Away ↑a)) (↑a ^ N) = (algebraMap R (Away ↑a)) (r a)
eq2 : ∀ (a b : ↑↑s), r a * ↑b ^ N = r b * ↑a ^ N
⊢ ∃ x, ∀ (a : ↑↑s), (algebraMap R (Away ↑a)) x = f a

Tactic:
have ⟨c, eq1⟩ := (mem_span_range_iff_exists_fun _).mp <|
  (Ideal.eq_top_iff_one _).mp <| (Set.image_eq_range _ _ ▸ Ideal.span_pow_eq_top _ span_eq N)

Result:
case intro
R : Type u_1
inst✝ : CommSemiring R
s : Finset R
f : (a : ↑↑s) → Away ↑a
h : ∀ (a b : ↑↑s), (Away.awayToAwayRight ↑a ↑b) (f a) = (Away.awayToAwayLeft ↑b ↑a) (f b)
mem : 1 ∈ Ideal.span ↑s
span_eq : Ideal.span ↑s = ⊤
n : ↑↑s → ℕ
r✝¹ : ↑↑s → R
eq✝¹ : ∀ (a : ↑↑s), f a * (algebraMap R (Away ↑a)) ↑a ^ n a = (algebraMap R (Away ↑a)) (r✝¹ a)
N✝ : ℕ := s.attach.sup n
r✝ : ↑↑s → R := fun a => ↑a ^ (N✝ - n a) * r✝¹ a
eq✝ : ∀ (a : ↑↑s), f a * (algebraMap R (Away ↑a)) (↑a ^ N✝) = (algebraMap R (Away ↑a)) (r✝ a)
N'✝ : ↑↑s → ↑↑s → ℕ
hN' : ∀ (a b : ↑↑s), (↑a * ↑b) ^ N'✝ a b * (r✝ a * ↑b ^ N✝) = (↑a * ↑b) ^ N'✝ a b * (r✝ b * ↑a ^ N✝)
N' : ℕ := (s ×ˢ s).attach.sup fun a => N'✝ ⟨(↑a).1, ⋯⟩ ⟨(↑a).2, ⋯⟩
eq2✝ : ∀ (a b : ↑↑s), (↑a * ↑b) ^ N' * (r✝ a * ↑b ^ N✝) = (↑a * ↑b) ^ N' * (r✝ b * ↑a ^ N✝)
N : ℕ := N' + N✝
r : ↑↑s → R := fun a => ↑a ^ N' * r✝ a
eq : ∀ (a : ↑↑s), f a * (algebraMap R (Away ↑a)) (↑a ^ N) = (algebraMap R (Away ↑a)) (r a)
eq2 : ∀ (a b : ↑↑s), r a * ↑b ^ N = r b * ↑a ^ N
c : ↑↑s → R
eq1 : ∑ i : ↑↑s, c i • ↑i ^ N = 1
⊢ ∃ x, ∀ (a : ↑↑s), (algebraMap R (Away ↑a)) x = f a

Proof state:
case intro
R : Type u_1
inst✝ : CommSemiring R
s : Finset R
f : (a : ↑↑s) → Away ↑a
h : ∀ (a b : ↑↑s), (Away.awayToAwayRight ↑a ↑b) (f a) = (Away.awayToAwayLeft ↑b ↑a) (f b)
mem : 1 ∈ Ideal.span ↑s
span_eq : Ideal.span ↑s = ⊤
n : ↑↑s → ℕ
r : ↑↑s → R
eq : ∀ (a : ↑↑s), f a * (algebraMap R (Away ↑a)) ↑a ^ n a = (algebraMap R (Away ↑a)) (r a)
⊢ ∃ x, ∀ (a : ↑↑s), (algebraMap R (Away ↑a)) x = f a

Tactic:
let N := s.attach.sup n

Result:
case intro
R : Type u_1
inst✝ : CommSemiring R
s : Finset R
f : (a : ↑↑s) → Away ↑a
h : ∀ (a b : ↑↑s), (Away.awayToAwayRight ↑a ↑b) (f a) = (Away.awayToAwayLeft ↑b ↑a) (f b)
mem : 1 ∈ Ideal.span ↑s
span_eq : Ideal.span ↑s = ⊤
n : ↑↑s → ℕ
r : ↑↑s → R
eq : ∀ (a : ↑↑s), f a * (algebraMap R (Away ↑a)) ↑a ^ n a = (algebraMap R (Away ↑a)) (r a)
N : ℕ := s.attach.sup n
⊢ ∃ x, ∀ (a : ↑↑s), (algebraMap R (Away ↑a)) x = f a