l3lab/ntp-mathlib huggingface dataset:
/-- Nonzero integral ideals in a Dedekind domain are invertible.

We will use this to show that nonzero fractional ideals are invertible,
and finally conclude that fractional ideals in a Dedekind domain form a group with zero.
-/
theorem coe_ideal_mul_inv [h : IsDedekindDomain A] (I : Ideal A) (hI0 : I ≠ ⊥) :
    I * (I : FractionalIdeal A⁰ K)⁻¹ = 1 := by
  -- We'll show `1 ≤ J⁻¹ = (I * I⁻¹)⁻¹ ≤ 1`.
  apply mul_inv_cancel_of_le_one hI0
  by_cases hJ0 : I * (I : FractionalIdeal A⁰ K)⁻¹ = 0
  · rw [hJ0, inv_zero']; exact zero_le _
  intro x hx
  -- In particular, we'll show all `x ∈ J⁻¹` are integral.
  suffices x ∈ integralClosure A K by
    rwa [IsIntegrallyClosed.integralClosure_eq_bot, Algebra.mem_bot, Set.mem_range,
      ← mem_one_iff] at this
  -- For that, we'll find a subalgebra that is f.g. as a module and contains `x`.
  -- `A` is a noetherian ring, so we just need to find a subalgebra between `{x}` and `I⁻¹`.
  rw [mem_integralClosure_iff_mem_fg]
  have x_mul_mem : ∀ b ∈ (I⁻¹ : FractionalIdeal A⁰ K), x * b ∈ (I⁻¹ : FractionalIdeal A⁰ K) := by
    intro b hb
    rw [mem_inv_iff (coeIdeal_ne_zero.mpr hI0)]
    dsimp only at hx
    rw [val_eq_coe, mem_coe, mem_inv_iff hJ0] at hx
    

Proof state:
R : Type u_1
A : Type u_2
K : Type u_3
inst✝⁵ : CommRing R
inst✝⁴ : CommRing A
inst✝³ : Field K
inst✝² : IsDomain A
inst✝¹ : Algebra A K
inst✝ : IsFractionRing A K
h : IsDedekindDomain A
I : Ideal A
hI0 : I ≠ ⊥
hJ0 : ¬↑I * (↑I)⁻¹ = 0
x : K
hx : ∀ y ∈ ↑I * (↑I)⁻¹, x * y ∈ 1
b : K
hb : b ∈ (↑I)⁻¹
⊢ ∀ y ∈ ↑I, x * b * y ∈ 1

Next tactic:
simp only [mul_assoc, mul_comm b] at hx ⊢
/-- Nonzero integral ideals in a Dedekind domain are invertible.

We will use this to show that nonzero fractional ideals are invertible,
and finally conclude that fractional ideals in a Dedekind domain form a group with zero.
-/
theorem coe_ideal_mul_inv [h : IsDedekindDomain A] (I : Ideal A) (hI0 : I ≠ ⊥) :
    I * (I : FractionalIdeal A⁰ K)⁻¹ = 1 := by
  -- We'll show `1 ≤ J⁻¹ = (I * I⁻¹)⁻¹ ≤ 1`.
  apply mul_inv_cancel_of_le_one hI0
  by_cases hJ0 : I * (I : FractionalIdeal A⁰ K)⁻¹ = 0
  · rw [hJ0, inv_zero']; exact zero_le _
  intro x hx
  -- In particular, we'll show all `x ∈ J⁻¹` are integral.
  suffices x ∈ integralClosure A K by
    rwa [IsIntegrallyClosed.integralClosure_eq_bot, Algebra.mem_bot, Set.mem_range,
      ← mem_one_iff] at this
  -- For that, we'll find a subalgebra that is f.g. as a module and contains `x`.
  -- `A` is a noetherian ring, so we just need to find a subalgebra between `{x}` and `I⁻¹`.
  rw [mem_integralClosure_iff_mem_fg]
  have x_mul_mem : ∀ b ∈ (I⁻¹ : FractionalIdeal A⁰ K), x * b ∈ (I⁻¹ : FractionalIdeal A⁰ K) := by
    intro b hb
    rw [mem_inv_iff (coeIdeal_ne_zero.mpr hI0)]
    dsimp only at hx
    rw [val_eq_coe, mem_coe, mem_inv_iff hJ0] at hx
    simp only [mul_assoc, mul_comm b] at hx ⊢
    

Proof state:
R : Type u_1
A : Type u_2
K : Type u_3
inst✝⁵ : CommRing R
inst✝⁴ : CommRing A
inst✝³ : Field K
inst✝² : IsDomain A
inst✝¹ : Algebra A K
inst✝ : IsFractionRing A K
h : IsDedekindDomain A
I : Ideal A
hI0 : I ≠ ⊥
hJ0 : ¬↑I * (↑I)⁻¹ = 0
x : K
hx : ∀ y ∈ ↑I * (↑I)⁻¹, x * y ∈ 1
b : K
hb : b ∈ (↑I)⁻¹
⊢ ∀ y ∈ ↑I, x * (y * b) ∈ 1

Next tactic:
intro y hy
/-- Nonzero integral ideals in a Dedekind domain are invertible.

We will use this to show that nonzero fractional ideals are invertible,
and finally conclude that fractional ideals in a Dedekind domain form a group with zero.
-/
theorem coe_ideal_mul_inv [h : IsDedekindDomain A] (I : Ideal A) (hI0 : I ≠ ⊥) :
    I * (I : FractionalIdeal A⁰ K)⁻¹ = 1 := by
  -- We'll show `1 ≤ J⁻¹ = (I * I⁻¹)⁻¹ ≤ 1`.
  apply mul_inv_cancel_of_le_one hI0
  by_cases hJ0 : I * (I : FractionalIdeal A⁰ K)⁻¹ = 0
  · rw [hJ0, inv_zero']; exact zero_le _
  intro x hx
  -- In particular, we'll show all `x ∈ J⁻¹` are integral.
  suffices x ∈ integralClosure A K by
    rwa [IsIntegrallyClosed.integralClosure_eq_bot, Algebra.mem_bot, Set.mem_range,
      ← mem_one_iff] at this
  -- For that, we'll find a subalgebra that is f.g. as a module and contains `x`.
  -- `A` is a noetherian ring, so we just need to find a subalgebra between `{x}` and `I⁻¹`.
  rw [mem_integralClosure_iff_mem_fg]
  have x_mul_mem : ∀ b ∈ (I⁻¹ : FractionalIdeal A⁰ K), x * b ∈ (I⁻¹ : FractionalIdeal A⁰ K) := by
    intro b hb
    rw [mem_inv_iff (coeIdeal_ne_zero.mpr hI0)]
    dsimp only at hx
    rw [val_eq_coe, mem_coe, mem_inv_iff hJ0] at hx
    simp only [mul_assoc, mul_comm b] at hx ⊢
    intro y hy
    

Proof state:
R : Type u_1
A : Type u_2
K : Type u_3
inst✝⁵ : CommRing R
inst✝⁴ : CommRing A
inst✝³ : Field K
inst✝² : IsDomain A
inst✝¹ : Algebra A K
inst✝ : IsFractionRing A K
h : IsDedekindDomain A
I : Ideal A
hI0 : I ≠ ⊥
hJ0 : ¬↑I * (↑I)⁻¹ = 0
x : K
hx : ∀ y ∈ ↑I * (↑I)⁻¹, x * y ∈ 1
b : K
hb : b ∈ (↑I)⁻¹
y : K
hy : y ∈ ↑I
⊢ x * (y * b) ∈ 1

Next tactic:
exact hx _ (mul_mem_mul hy hb)
/-- Nonzero integral ideals in a Dedekind domain are invertible.

We will use this to show that nonzero fractional ideals are invertible,
and finally conclude that fractional ideals in a Dedekind domain form a group with zero.
-/
theorem coe_ideal_mul_inv [h : IsDedekindDomain A] (I : Ideal A) (hI0 : I ≠ ⊥) :
    I * (I : FractionalIdeal A⁰ K)⁻¹ = 1 := by
  -- We'll show `1 ≤ J⁻¹ = (I * I⁻¹)⁻¹ ≤ 1`.
  apply mul_inv_cancel_of_le_one hI0
  by_cases hJ0 : I * (I : FractionalIdeal A⁰ K)⁻¹ = 0
  · rw [hJ0, inv_zero']; exact zero_le _
  intro x hx
  -- In particular, we'll show all `x ∈ J⁻¹` are integral.
  suffices x ∈ integralClosure A K by
    rwa [IsIntegrallyClosed.integralClosure_eq_bot, Algebra.mem_bot, Set.mem_range,
      ← mem_one_iff] at this
  -- For that, we'll find a subalgebra that is f.g. as a module and contains `x`.
  -- `A` is a noetherian ring, so we just need to find a subalgebra between `{x}` and `I⁻¹`.
  rw [mem_integralClosure_iff_mem_fg]
  have x_mul_mem : ∀ b ∈ (I⁻¹ : FractionalIdeal A⁰ K), x * b ∈ (I⁻¹ : FractionalIdeal A⁰ K) := by
    intro b hb
    rw [mem_inv_iff (coeIdeal_ne_zero.mpr hI0)]
    dsimp only at hx
    rw [val_eq_coe, mem_coe, mem_inv_iff hJ0] at hx
    simp only [mul_assoc, mul_comm b] at hx ⊢
    intro y hy
    exact hx _ (mul_mem_mul hy hb)
  -- It turns out the subalgebra consisting of all `p(x)` for `p : A[X]` works.
  

Proof state:
case neg
R : Type u_1
A : Type u_2
K : Type u_3
inst✝⁵ : CommRing R
inst✝⁴ : CommRing A
inst✝³ : Field K
inst✝² : IsDomain A
inst✝¹ : Algebra A K
inst✝ : IsFractionRing A K
h : IsDedekindDomain A
I : Ideal A
hI0 : I ≠ ⊥
hJ0 : ¬↑I * (↑I)⁻¹ = 0
x : K
hx : x ∈ (fun a => ↑a) (↑I * (↑I)⁻¹)⁻¹
x_mul_mem : ∀ b ∈ (↑I)⁻¹, x * b ∈ (↑I)⁻¹
⊢ ∃ M, Submodule.FG (Subalgebra.toSubmodule M) ∧ x ∈ M

Next tactic:
refine ⟨AlgHom.range (Polynomial.aeval x : A[X] →ₐ[A] K),
    isNoetherian_submodule.mp (isNoetherian (I : FractionalIdeal A⁰ K)⁻¹) _ fun y hy => ?_,
    ⟨Polynomial.X, Polynomial.aeval_X x⟩⟩
/-- Nonzero integral ideals in a Dedekind domain are invertible.

We will use this to show that nonzero fractional ideals are invertible,
and finally conclude that fractional ideals in a Dedekind domain form a group with zero.
-/
theorem coe_ideal_mul_inv [h : IsDedekindDomain A] (I : Ideal A) (hI0 : I ≠ ⊥) :
    I * (I : FractionalIdeal A⁰ K)⁻¹ = 1 := by
  -- We'll show `1 ≤ J⁻¹ = (I * I⁻¹)⁻¹ ≤ 1`.
  apply mul_inv_cancel_of_le_one hI0
  by_cases hJ0 : I * (I : FractionalIdeal A⁰ K)⁻¹ = 0
  · rw [hJ0, inv_zero']; exact zero_le _
  intro x hx
  -- In particular, we'll show all `x ∈ J⁻¹` are integral.
  suffices x ∈ integralClosure A K by
    rwa [IsIntegrallyClosed.integralClosure_eq_bot, Algebra.mem_bot, Set.mem_range,
      ← mem_one_iff] at this
  -- For that, we'll find a subalgebra that is f.g. as a module and contains `x`.
  -- `A` is a noetherian ring, so we just need to find a subalgebra between `{x}` and `I⁻¹`.
  rw [mem_integralClosure_iff_mem_fg]
  have x_mul_mem : ∀ b ∈ (I⁻¹ : FractionalIdeal A⁰ K), x * b ∈ (I⁻¹ : FractionalIdeal A⁰ K) := by
    intro b hb
    rw [mem_inv_iff (coeIdeal_ne_zero.mpr hI0)]
    dsimp only at hx
    rw [val_eq_coe, mem_coe, mem_inv_iff hJ0] at hx
    simp only [mul_assoc, mul_comm b] at hx ⊢
    intro y hy
    exact hx _ (mul_mem_mul hy hb)
  -- It turns out the subalgebra consisting of all `p(x)` for `p : A[X]` works.
  refine ⟨AlgHom.range (Polynomial.aeval x : A[X] →ₐ[A] K),
    isNoetherian_submodule.mp (isNoetherian (I : FractionalIdeal A⁰ K)⁻¹) _ fun y hy => ?_,
    ⟨Polynomial.X, Polynomial.aeval_X x⟩⟩
  

Proof state:
case neg
R : Type u_1
A : Type u_2
K : Type u_3
inst✝⁵ : CommRing R
inst✝⁴ : CommRing A
inst✝³ : Field K
inst✝² : IsDomain A
inst✝¹ : Algebra A K
inst✝ : IsFractionRing A K
h : IsDedekindDomain A
I : Ideal A
hI0 : I ≠ ⊥
hJ0 : ¬↑I * (↑I)⁻¹ = 0
x : K
hx : x ∈ (fun a => ↑a) (↑I * (↑I)⁻¹)⁻¹
x_mul_mem : ∀ b ∈ (↑I)⁻¹, x * b ∈ (↑I)⁻¹
y : K
hy : y ∈ Subalgebra.toSubmodule (AlgHom.range (Polynomial.aeval x))
⊢ y ∈ ↑(↑I)⁻¹

Next tactic:
obtain ⟨p, rfl⟩ := (AlgHom.mem_range _).mp hy
/-- Nonzero integral ideals in a Dedekind domain are invertible.

We will use this to show that nonzero fractional ideals are invertible,
and finally conclude that fractional ideals in a Dedekind domain form a group with zero.
-/
theorem coe_ideal_mul_inv [h : IsDedekindDomain A] (I : Ideal A) (hI0 : I ≠ ⊥) :
    I * (I : FractionalIdeal A⁰ K)⁻¹ = 1 := by
  -- We'll show `1 ≤ J⁻¹ = (I * I⁻¹)⁻¹ ≤ 1`.
  apply mul_inv_cancel_of_le_one hI0
  by_cases hJ0 : I * (I : FractionalIdeal A⁰ K)⁻¹ = 0
  · rw [hJ0, inv_zero']; exact zero_le _
  intro x hx
  -- In particular, we'll show all `x ∈ J⁻¹` are integral.
  suffices x ∈ integralClosure A K by
    rwa [IsIntegrallyClosed.integralClosure_eq_bot, Algebra.mem_bot, Set.mem_range,
      ← mem_one_iff] at this
  -- For that, we'll find a subalgebra that is f.g. as a module and contains `x`.
  -- `A` is a noetherian ring, so we just need to find a subalgebra between `{x}` and `I⁻¹`.
  rw [mem_integralClosure_iff_mem_fg]
  have x_mul_mem : ∀ b ∈ (I⁻¹ : FractionalIdeal A⁰ K), x * b ∈ (I⁻¹ : FractionalIdeal A⁰ K) := by
    intro b hb
    rw [mem_inv_iff (coeIdeal_ne_zero.mpr hI0)]
    dsimp only at hx
    rw [val_eq_coe, mem_coe, mem_inv_iff hJ0] at hx
    simp only [mul_assoc, mul_comm b] at hx ⊢
    intro y hy
    exact hx _ (mul_mem_mul hy hb)
  -- It turns out the subalgebra consisting of all `p(x)` for `p : A[X]` works.
  refine ⟨AlgHom.range (Polynomial.aeval x : A[X] →ₐ[A] K),
    isNoetherian_submodule.mp (isNoetherian (I : FractionalIdeal A⁰ K)⁻¹) _ fun y hy => ?_,
    ⟨Polynomial.X, Polynomial.aeval_X x⟩⟩
  obtain ⟨p, rfl⟩ := (AlgHom.mem_range _).mp hy
  

Proof state:
case neg.intro
R : Type u_1
A : Type u_2
K : Type u_3
inst✝⁵ : CommRing R
inst✝⁴ : CommRing A
inst✝³ : Field K
inst✝² : IsDomain A
inst✝¹ : Algebra A K
inst✝ : IsFractionRing A K
h : IsDedekindDomain A
I : Ideal A
hI0 : I ≠ ⊥
hJ0 : ¬↑I * (↑I)⁻¹ = 0
x : K
hx : x ∈ (fun a => ↑a) (↑I * (↑I)⁻¹)⁻¹
x_mul_mem : ∀ b ∈ (↑I)⁻¹, x * b ∈ (↑I)⁻¹
p : A[X]
hy : (Polynomial.aeval x) p ∈ Subalgebra.toSubmodule (AlgHom.range (Polynomial.aeval x))
⊢ (Polynomial.aeval x) p ∈ ↑(↑I)⁻¹

Next tactic:
rw [Polynomial.aeval_eq_sum_range]
/-- Nonzero integral ideals in a Dedekind domain are invertible.

We will use this to show that nonzero fractional ideals are invertible,
and finally conclude that fractional ideals in a Dedekind domain form a group with zero.
-/
theorem coe_ideal_mul_inv [h : IsDedekindDomain A] (I : Ideal A) (hI0 : I ≠ ⊥) :
    I * (I : FractionalIdeal A⁰ K)⁻¹ = 1 := by
  -- We'll show `1 ≤ J⁻¹ = (I * I⁻¹)⁻¹ ≤ 1`.
  apply mul_inv_cancel_of_le_one hI0
  by_cases hJ0 : I * (I : FractionalIdeal A⁰ K)⁻¹ = 0
  · rw [hJ0, inv_zero']; exact zero_le _
  intro x hx
  -- In particular, we'll show all `x ∈ J⁻¹` are integral.
  suffices x ∈ integralClosure A K by
    rwa [IsIntegrallyClosed.integralClosure_eq_bot, Algebra.mem_bot, Set.mem_range,
      ← mem_one_iff] at this
  -- For that, we'll find a subalgebra that is f.g. as a module and contains `x`.
  -- `A` is a noetherian ring, so we just need to find a subalgebra between `{x}` and `I⁻¹`.
  rw [mem_integralClosure_iff_mem_fg]
  have x_mul_mem : ∀ b ∈ (I⁻¹ : FractionalIdeal A⁰ K), x * b ∈ (I⁻¹ : FractionalIdeal A⁰ K) := by
    intro b hb
    rw [mem_inv_iff (coeIdeal_ne_zero.mpr hI0)]
    dsimp only at hx
    rw [val_eq_coe, mem_coe, mem_inv_iff hJ0] at hx
    simp only [mul_assoc, mul_comm b] at hx ⊢
    intro y hy
    exact hx _ (mul_mem_mul hy hb)
  -- It turns out the subalgebra consisting of all `p(x)` for `p : A[X]` works.
  refine ⟨AlgHom.range (Polynomial.aeval x : A[X] →ₐ[A] K),
    isNoetherian_submodule.mp (isNoetherian (I : FractionalIdeal A⁰ K)⁻¹) _ fun y hy => ?_,
    ⟨Polynomial.X, Polynomial.aeval_X x⟩⟩
  obtain ⟨p, rfl⟩ := (AlgHom.mem_range _).mp hy
  rw [Polynomial.aeval_eq_sum_range]
  

Proof state:
case neg.intro
R : Type u_1
A : Type u_2
K : Type u_3
inst✝⁵ : CommRing R
inst✝⁴ : CommRing A
inst✝³ : Field K
inst✝² : IsDomain A
inst✝¹ : Algebra A K
inst✝ : IsFractionRing A K
h : IsDedekindDomain A
I : Ideal A
hI0 : I ≠ ⊥
hJ0 : ¬↑I * (↑I)⁻¹ = 0
x : K
hx : x ∈ (fun a => ↑a) (↑I * (↑I)⁻¹)⁻¹
x_mul_mem : ∀ b ∈ (↑I)⁻¹, x * b ∈ (↑I)⁻¹
p : A[X]
hy : (Polynomial.aeval x) p ∈ Subalgebra.toSubmodule (AlgHom.range (Polynomial.aeval x))
⊢ (Finset.sum (Finset.range (Polynomial.natDegree p + 1)) fun i => Polynomial.coeff p i • x ^ i) ∈ ↑(↑I)⁻¹

Next tactic:
refine Submodule.sum_mem _ fun i hi => Submodule.smul_mem _ _ ?_
/-- Nonzero integral ideals in a Dedekind domain are invertible.

We will use this to show that nonzero fractional ideals are invertible,
and finally conclude that fractional ideals in a Dedekind domain form a group with zero.
-/
theorem coe_ideal_mul_inv [h : IsDedekindDomain A] (I : Ideal A) (hI0 : I ≠ ⊥) :
    I * (I : FractionalIdeal A⁰ K)⁻¹ = 1 := by
  -- We'll show `1 ≤ J⁻¹ = (I * I⁻¹)⁻¹ ≤ 1`.
  apply mul_inv_cancel_of_le_one hI0
  by_cases hJ0 : I * (I : FractionalIdeal A⁰ K)⁻¹ = 0
  · rw [hJ0, inv_zero']; exact zero_le _
  intro x hx
  -- In particular, we'll show all `x ∈ J⁻¹` are integral.
  suffices x ∈ integralClosure A K by
    rwa [IsIntegrallyClosed.integralClosure_eq_bot, Algebra.mem_bot, Set.mem_range,
      ← mem_one_iff] at this
  -- For that, we'll find a subalgebra that is f.g. as a module and contains `x`.
  -- `A` is a noetherian ring, so we just need to find a subalgebra between `{x}` and `I⁻¹`.
  rw [mem_integralClosure_iff_mem_fg]
  have x_mul_mem : ∀ b ∈ (I⁻¹ : FractionalIdeal A⁰ K), x * b ∈ (I⁻¹ : FractionalIdeal A⁰ K) := by
    intro b hb
    rw [mem_inv_iff (coeIdeal_ne_zero.mpr hI0)]
    dsimp only at hx
    rw [val_eq_coe, mem_coe, mem_inv_iff hJ0] at hx
    simp only [mul_assoc, mul_comm b] at hx ⊢
    intro y hy
    exact hx _ (mul_mem_mul hy hb)
  -- It turns out the subalgebra consisting of all `p(x)` for `p : A[X]` works.
  refine ⟨AlgHom.range (Polynomial.aeval x : A[X] →ₐ[A] K),
    isNoetherian_submodule.mp (isNoetherian (I : FractionalIdeal A⁰ K)⁻¹) _ fun y hy => ?_,
    ⟨Polynomial.X, Polynomial.aeval_X x⟩⟩
  obtain ⟨p, rfl⟩ := (AlgHom.mem_range _).mp hy
  rw [Polynomial.aeval_eq_sum_range]
  refine Submodule.sum_mem _ fun i hi => Submodule.smul_mem _ _ ?_
  

Proof state:
case neg.intro
R : Type u_1
A : Type u_2
K : Type u_3
inst✝⁵ : CommRing R
inst✝⁴ : CommRing A
inst✝³ : Field K
inst✝² : IsDomain A
inst✝¹ : Algebra A K
inst✝ : IsFractionRing A K
h : IsDedekindDomain A
I : Ideal A
hI0 : I ≠ ⊥
hJ0 : ¬↑I * (↑I)⁻¹ = 0
x : K
hx : x ∈ (fun a => ↑a) (↑I * (↑I)⁻¹)⁻¹
x_mul_mem : ∀ b ∈ (↑I)⁻¹, x * b ∈ (↑I)⁻¹
p : A[X]
hy : (Polynomial.aeval x) p ∈ Subalgebra.toSubmodule (AlgHom.range (Polynomial.aeval x))
i : ℕ
hi : i ∈ Finset.range (Polynomial.natDegree p + 1)
⊢ x ^ i ∈ ↑(↑I)⁻¹

Next tactic:
clear hi
/-- Nonzero integral ideals in a Dedekind domain are invertible.

We will use this to show that nonzero fractional ideals are invertible,
and finally conclude that fractional ideals in a Dedekind domain form a group with zero.
-/
theorem coe_ideal_mul_inv [h : IsDedekindDomain A] (I : Ideal A) (hI0 : I ≠ ⊥) :
    I * (I : FractionalIdeal A⁰ K)⁻¹ = 1 := by
  -- We'll show `1 ≤ J⁻¹ = (I * I⁻¹)⁻¹ ≤ 1`.
  apply mul_inv_cancel_of_le_one hI0
  by_cases hJ0 : I * (I : FractionalIdeal A⁰ K)⁻¹ = 0
  · rw [hJ0, inv_zero']; exact zero_le _
  intro x hx
  -- In particular, we'll show all `x ∈ J⁻¹` are integral.
  suffices x ∈ integralClosure A K by
    rwa [IsIntegrallyClosed.integralClosure_eq_bot, Algebra.mem_bot, Set.mem_range,
      ← mem_one_iff] at this
  -- For that, we'll find a subalgebra that is f.g. as a module and contains `x`.
  -- `A` is a noetherian ring, so we just need to find a subalgebra between `{x}` and `I⁻¹`.
  rw [mem_integralClosure_iff_mem_fg]
  have x_mul_mem : ∀ b ∈ (I⁻¹ : FractionalIdeal A⁰ K), x * b ∈ (I⁻¹ : FractionalIdeal A⁰ K) := by
    intro b hb
    rw [mem_inv_iff (coeIdeal_ne_zero.mpr hI0)]
    dsimp only at hx
    rw [val_eq_coe, mem_coe, mem_inv_iff hJ0] at hx
    simp only [mul_assoc, mul_comm b] at hx ⊢
    intro y hy
    exact hx _ (mul_mem_mul hy hb)
  -- It turns out the subalgebra consisting of all `p(x)` for `p : A[X]` works.
  refine ⟨AlgHom.range (Polynomial.aeval x : A[X] →ₐ[A] K),
    isNoetherian_submodule.mp (isNoetherian (I : FractionalIdeal A⁰ K)⁻¹) _ fun y hy => ?_,
    ⟨Polynomial.X, Polynomial.aeval_X x⟩⟩
  obtain ⟨p, rfl⟩ := (AlgHom.mem_range _).mp hy
  rw [Polynomial.aeval_eq_sum_range]
  refine Submodule.sum_mem _ fun i hi => Submodule.smul_mem _ _ ?_
  clear hi
  

Proof state:
case neg.intro
R : Type u_1
A : Type u_2
K : Type u_3
inst✝⁵ : CommRing R
inst✝⁴ : CommRing A
inst✝³ : Field K
inst✝² : IsDomain A
inst✝¹ : Algebra A K
inst✝ : IsFractionRing A K
h : IsDedekindDomain A
I : Ideal A
hI0 : I ≠ ⊥
hJ0 : ¬↑I * (↑I)⁻¹ = 0
x : K
hx : x ∈ (fun a => ↑a) (↑I * (↑I)⁻¹)⁻¹
x_mul_mem : ∀ b ∈ (↑I)⁻¹, x * b ∈ (↑I)⁻¹
p : A[X]
hy : (Polynomial.aeval x) p ∈ Subalgebra.toSubmodule (AlgHom.range (Polynomial.aeval x))
i : ℕ
⊢ x ^ i ∈ ↑(↑I)⁻¹

Next tactic:
induction' i with i ih
/-- Nonzero integral ideals in a Dedekind domain are invertible.

We will use this to show that nonzero fractional ideals are invertible,
and finally conclude that fractional ideals in a Dedekind domain form a group with zero.
-/
theorem coe_ideal_mul_inv [h : IsDedekindDomain A] (I : Ideal A) (hI0 : I ≠ ⊥) :
    I * (I : FractionalIdeal A⁰ K)⁻¹ = 1 := by
  -- We'll show `1 ≤ J⁻¹ = (I * I⁻¹)⁻¹ ≤ 1`.
  apply mul_inv_cancel_of_le_one hI0
  by_cases hJ0 : I * (I : FractionalIdeal A⁰ K)⁻¹ = 0
  · rw [hJ0, inv_zero']; exact zero_le _
  intro x hx
  -- In particular, we'll show all `x ∈ J⁻¹` are integral.
  suffices x ∈ integralClosure A K by
    rwa [IsIntegrallyClosed.integralClosure_eq_bot, Algebra.mem_bot, Set.mem_range,
      ← mem_one_iff] at this
  -- For that, we'll find a subalgebra that is f.g. as a module and contains `x`.
  -- `A` is a noetherian ring, so we just need to find a subalgebra between `{x}` and `I⁻¹`.
  rw [mem_integralClosure_iff_mem_fg]
  have x_mul_mem : ∀ b ∈ (I⁻¹ : FractionalIdeal A⁰ K), x * b ∈ (I⁻¹ : FractionalIdeal A⁰ K) := by
    intro b hb
    rw [mem_inv_iff (coeIdeal_ne_zero.mpr hI0)]
    dsimp only at hx
    rw [val_eq_coe, mem_coe, mem_inv_iff hJ0] at hx
    simp only [mul_assoc, mul_comm b] at hx ⊢
    intro y hy
    exact hx _ (mul_mem_mul hy hb)
  -- It turns out the subalgebra consisting of all `p(x)` for `p : A[X]` works.
  refine ⟨AlgHom.range (Polynomial.aeval x : A[X] →ₐ[A] K),
    isNoetherian_submodule.mp (isNoetherian (I : FractionalIdeal A⁰ K)⁻¹) _ fun y hy => ?_,
    ⟨Polynomial.X, Polynomial.aeval_X x⟩⟩
  obtain ⟨p, rfl⟩ := (AlgHom.mem_range _).mp hy
  rw [Polynomial.aeval_eq_sum_range]
  refine Submodule.sum_mem _ fun i hi => Submodule.smul_mem _ _ ?_
  clear hi
  induction' i with i ih
  · 

Proof state:
case neg.intro.zero
R : Type u_1
A : Type u_2
K : Type u_3
inst✝⁵ : CommRing R
inst✝⁴ : CommRing A
inst✝³ : Field K
inst✝² : IsDomain A
inst✝¹ : Algebra A K
inst✝ : IsFractionRing A K
h : IsDedekindDomain A
I : Ideal A
hI0 : I ≠ ⊥
hJ0 : ¬↑I * (↑I)⁻¹ = 0
x : K
hx : x ∈ (fun a => ↑a) (↑I * (↑I)⁻¹)⁻¹
x_mul_mem : ∀ b ∈ (↑I)⁻¹, x * b ∈ (↑I)⁻¹
p : A[X]
hy : (Polynomial.aeval x) p ∈ Subalgebra.toSubmodule (AlgHom.range (Polynomial.aeval x))
⊢ x ^ Nat.zero ∈ ↑(↑I)⁻¹

Next tactic:
rw [pow_zero]
/-- Nonzero integral ideals in a Dedekind domain are invertible.

We will use this to show that nonzero fractional ideals are invertible,
and finally conclude that fractional ideals in a Dedekind domain form a group with zero.
-/
theorem coe_ideal_mul_inv [h : IsDedekindDomain A] (I : Ideal A) (hI0 : I ≠ ⊥) :
    I * (I : FractionalIdeal A⁰ K)⁻¹ = 1 := by
  -- We'll show `1 ≤ J⁻¹ = (I * I⁻¹)⁻¹ ≤ 1`.
  apply mul_inv_cancel_of_le_one hI0
  by_cases hJ0 : I * (I : FractionalIdeal A⁰ K)⁻¹ = 0
  · rw [hJ0, inv_zero']; exact zero_le _
  intro x hx
  -- In particular, we'll show all `x ∈ J⁻¹` are integral.
  suffices x ∈ integralClosure A K by
    rwa [IsIntegrallyClosed.integralClosure_eq_bot, Algebra.mem_bot, Set.mem_range,
      ← mem_one_iff] at this
  -- For that, we'll find a subalgebra that is f.g. as a module and contains `x`.
  -- `A` is a noetherian ring, so we just need to find a subalgebra between `{x}` and `I⁻¹`.
  rw [mem_integralClosure_iff_mem_fg]
  have x_mul_mem : ∀ b ∈ (I⁻¹ : FractionalIdeal A⁰ K), x * b ∈ (I⁻¹ : FractionalIdeal A⁰ K) := by
    intro b hb
    rw [mem_inv_iff (coeIdeal_ne_zero.mpr hI0)]
    dsimp only at hx
    rw [val_eq_coe, mem_coe, mem_inv_iff hJ0] at hx
    simp only [mul_assoc, mul_comm b] at hx ⊢
    intro y hy
    exact hx _ (mul_mem_mul hy hb)
  -- It turns out the subalgebra consisting of all `p(x)` for `p : A[X]` works.
  refine ⟨AlgHom.range (Polynomial.aeval x : A[X] →ₐ[A] K),
    isNoetherian_submodule.mp (isNoetherian (I : FractionalIdeal A⁰ K)⁻¹) _ fun y hy => ?_,
    ⟨Polynomial.X, Polynomial.aeval_X x⟩⟩
  obtain ⟨p, rfl⟩ := (AlgHom.mem_range _).mp hy
  rw [Polynomial.aeval_eq_sum_range]
  refine Submodule.sum_mem _ fun i hi => Submodule.smul_mem _ _ ?_
  clear hi
  induction' i with i ih
  · rw [pow_zero]; 

Proof state:
case neg.intro.zero
R : Type u_1
A : Type u_2
K : Type u_3
inst✝⁵ : CommRing R
inst✝⁴ : CommRing A
inst✝³ : Field K
inst✝² : IsDomain A
inst✝¹ : Algebra A K
inst✝ : IsFractionRing A K
h : IsDedekindDomain A
I : Ideal A
hI0 : I ≠ ⊥
hJ0 : ¬↑I * (↑I)⁻¹ = 0
x : K
hx : x ∈ (fun a => ↑a) (↑I * (↑I)⁻¹)⁻¹
x_mul_mem : ∀ b ∈ (↑I)⁻¹, x * b ∈ (↑I)⁻¹
p : A[X]
hy : (Polynomial.aeval x) p ∈ Subalgebra.toSubmodule (AlgHom.range (Polynomial.aeval x))
⊢ 1 ∈ ↑(↑I)⁻¹

Next tactic:
exact one_mem_inv_coe_ideal hI0