# Formalizing a ring theorem with Lean 4 and Claude

> Source: <https://www.johndcook.com/blog/2026/06/17/rings-with-lean-claude/>
> Published: 2026-06-17 14:23:51+00:00

I’ve been testing Claude’s ability to generate Lean 4 code to prove theorems. I’ve written about a [couple](https://www.johndcook.com/blog/2026/06/10/claude-and-lean/) [experiments](https://www.johndcook.com/blog/2026/06/15/quaternions-claude-lean/) that verified calculations. I did not write about my failed attempt to get Claude to formalize a proof of the [pqr theorem](https://www.johndcook.com/pqr_theorem.pdf) for seminorms.

This time I asked Claude to formally prove the theorem from the previous post on [partial fraction decomposition](https://www.johndcook.com/blog/2026/06/16/partial-fraction-decomposition/) in an abstract setting. Unlike the previous experiments, I did not give it a proof or calculation to verify; I asked it to *generate* a proof.

This was the prompt:

Write a proof in Lean 4 for the following generalization of partial fraction decomposition.

If a ring R is a principle ideal domain (PID), then every element c of the field of fractions over R can be written in the form \[c = \sum_i \frac{a_i}{p_i^{r_i}}\]

where the p_i are nonassociate irreducible elements of R, the r_i are non-negative integers, and the elements a_i and p_i are relatively prime.

This was Claude’s immediate response:

I’ll work through this carefully, as it’s a substantial formalization task. Let me think through what’s available in Mathlib and what needs to be constructed.

This is a genuinely hard Lean 4 formalization problem. Let me think carefully about what’s in Mathlib, what the statement precisely means, and what proof strategy is realistic.

It also stated its overall proof strategy:

Proof strategy: Write`c = a/b`

in lowest terms. Factor`b = ∏ pᵢ^rᵢ`

. Use the Chinese Remainder Theorem / Bezout identity to decompose`a/b`

into partial fractions.

If I needed to prove the theorem by hand, this suggestion alone could be very helpful.

The first attempt generated code that would not build. As before, a lot of the problems seem to stem from things moving around in Mathlib due to refactorings. There are also occasional hallucinations.

I did not offer any help other than pasting in error messages after each failure. When working on a real problem, I’d be more interactive. But as a test, I just passively pasted error messages and tried again. It took 11 iterations, but I finally got a proof. The proof had five sections marked “sorry,” which is essentially a TODO note indicating an unproven assumption.

I asked if it could explain or remove the sorries, and it said it could remove three of them. But the generated code did not build and I hit a session limit before I could debug the code. Here is the code with five sorry sections.

``` python
/-
  PartialFractions.lean  (v11 — final)
-/

import Mathlib

open UniqueFactorizationMonoid

variable {R : Type*} [CommRing R] [IsDomain R] [IsPrincipalIdealRing R]

structure PFDSummand (R : Type*) [CommRing R] where
  prime    : R
  exponent : ℕ
  numer    : R

noncomputable def PFDSummand.val [CommRing R] [IsDomain R] (s : PFDSummand R) :
    FractionRing R :=
  algebraMap R (FractionRing R) s.numer /
  algebraMap R (FractionRing R) (s.prime ^ s.exponent)

structure IsPFD [CommRing R] [IsDomain R] [IsPrincipalIdealRing R]
    (terms : List (PFDSummand R)) (c : FractionRing R) : Prop where
  irred    : ∀ s ∈ terms, Irreducible s.prime
  nonAssoc : ∀ s ∈ terms, ∀ t ∈ terms, s ≠ t → ¬Associated s.prime t.prime
  coprime  : ∀ s ∈ terms, IsCoprime s.numer s.prime
  sum_eq   : (terms.map PFDSummand.val).sum = c

private lemma algMap_ne_zero {a : R} (h : a ≠ 0) :
    (algebraMap R (FractionRing R)) a ≠ 0 :=
  map_ne_zero_iff _ (IsFractionRing.injective R (FractionRing R)) |>.mpr h

private lemma exists_num_denom (c : FractionRing R) :
    ∃ a b : R, b ≠ 0 ∧
      (algebraMap R (FractionRing R)) a / (algebraMap R (FractionRing R)) b = c := by
  obtain ⟨a, b, hb, hab⟩ := IsFractionRing.div_surjective (A := R) c
  exact ⟨a, b, nonZeroDivisors.ne_zero hb, hab⟩

-- ======================================================================
-- §6  Bézout splitting
-- ======================================================================

private lemma bezout_split (x y : R) (hcop : IsCoprime x y)
    (hx : x ≠ 0) (hy : y ≠ 0) (e : R) :
    ∃ s t : R, s * x + t * y = 1 ∧
      (algebraMap R (FractionRing R) e) /
        ((algebraMap R (FractionRing R) x) * (algebraMap R (FractionRing R) y)) =
      (algebraMap R (FractionRing R) (e * t)) / (algebraMap R (FractionRing R) x) +
      (algebraMap R (FractionRing R) (e * s)) / (algebraMap R (FractionRing R) y) := by
  obtain ⟨s, t, hst⟩ := hcop
  refine ⟨s, t, hst, ?_⟩
  have hxF : (algebraMap R (FractionRing R)) x ≠ 0 := algMap_ne_zero hx
  have hyF : (algebraMap R (FractionRing R)) y ≠ 0 := algMap_ne_zero hy
  have hxyF : (algebraMap R (FractionRing R)) x *
              (algebraMap R (FractionRing R)) y ≠ 0 := mul_ne_zero hxF hyF
  have hst_F : (algebraMap R (FractionRing R)) s * (algebraMap R (FractionRing R)) x +
               (algebraMap R (FractionRing R)) t * (algebraMap R (FractionRing R)) y = 1 := by
    have h := congr_arg (algebraMap R (FractionRing R)) hst
    simp only [map_add, map_mul, map_one] at h; exact h
  -- Prove: ι e / (ι x * ι y) = ι(e*t)/ι x + ι(e*s)/ι y
  -- Strategy: show RHS = LHS via eq_div_iff, simplify, linear_combination.
  symm
  rw [eq_div_iff hxyF, add_mul, div_mul_eq_mul_div, div_mul_eq_mul_div]
  -- Goal: ι(e*t)*(ι x*ι y)/ι x + ι(e*s)*(ι x*ι y)/ι y = ι e
  -- The mul_comm on the product inside:
  rw [show (algebraMap R (FractionRing R)) x * (algebraMap R (FractionRing R)) y =
        (algebraMap R (FractionRing R)) y * (algebraMap R (FractionRing R)) x from mul_comm _ _]
  -- Now: ι(e*t)*(ι y*ι x)/ι x + ι(e*s)*(ι y*ι x)/ι y = ι e
  -- Use field_simp to clear remaining denominators
  field_simp [hxF, hyF]
  -- Now a polynomial goal: close with linear_combination
  simp only [map_mul]
  linear_combination (algebraMap R (FractionRing R) e) * hst_F

-- ======================================================================
-- §7  Coprime numerator reduction
-- ======================================================================

private lemma coprime_numer_exists {p : R} (hp : Irreducible p) (a : R) (n : ℕ) :
    ∃ a' q : R, a = a' + q * p ^ n ∧ IsCoprime a' p := by
  rcases Classical.em (p ∣ a) with _h | h
  · exact ⟨0, 0, by sorry, by sorry⟩
  · exact ⟨a, 0, by ring,
      isCoprime_comm.mpr ((irreducible_iff_prime.mp hp).coprime_iff_not_dvd.mpr h)⟩

-- ======================================================================
-- §8  Main theorem
-- ======================================================================

private def emptyPFD (c : FractionRing R) (hc : c = 0) :
    ∃ terms : List (PFDSummand R), IsPFD terms c :=
  ⟨[], { irred := by simp, nonAssoc := by simp, coprime := by simp,
         sum_eq := by simp [hc] }⟩

private lemma isPFD_of_denom : ∀ (d : R) (e : R),
    ∃ terms : List (PFDSummand R),
      IsPFD terms ((algebraMap R (FractionRing R) e) /
                   (algebraMap R (FractionRing R) d)) := by
  intro d
  induction d using UniqueFactorizationMonoid.induction_on_coprime with
  | h0 =>
    intro e; simp only [map_zero, div_zero]
    exact emptyPFD 0 rfl
  | h1 =>
    intro e
    -- Unit denominator: sorry for now (§9C)
    exact ⟨[], { irred := by simp, nonAssoc := by simp,
                 coprime := by simp, sum_eq := by simp; sorry }⟩
  | hpr =>
    rename_i prime_elem exp hp_prime
    intro e
    have hp_irred : Irreducible prime_elem := irreducible_iff_prime.mpr hp_prime
    obtain ⟨e', _q, _he, he'_cop⟩ := coprime_numer_exists hp_irred e exp
    exact ⟨[⟨prime_elem, exp, e'⟩], {
      irred    := fun s hs => by
        simp only [List.mem_singleton] at hs; subst hs; exact hp_irred
      nonAssoc := fun s hs t ht hne => by
        simp only [List.mem_singleton] at hs ht; subst hs; subst ht; exact absurd rfl hne
      coprime  := fun s hs => by
        simp only [List.mem_singleton] at hs; subst hs; exact he'_cop
      sum_eq   := by
        simp only [List.map_singleton, List.sum_singleton, PFDSummand.val]; sorry }⟩
  | hcp =>
    rename_i x y hrel ih_x ih_y
    intro e
    -- Handle the case where x = 0 or y = 0 directly
    rcases eq_or_ne x 0 with rfl | hx
    · -- x = 0: denominator = 0*y = 0, so quotient = 0
      simp only [zero_mul, map_zero, div_zero]
      exact emptyPFD 0 rfl
    rcases eq_or_ne y 0 with rfl | hy
    · -- y = 0: denominator = x*0 = 0, so quotient = 0
      simp only [mul_zero, map_zero, div_zero]
      exact emptyPFD 0 rfl
    -- Both nonzero: apply Bézout splitting
    -- IsCoprime x y from IsRelPrime x y (using IsRelPrime.isCoprime)
    have hcop : IsCoprime x y := hrel.isCoprime
    obtain ⟨s, t, _hst, hsplit⟩ := bezout_split x y hcop hx hy e
    obtain ⟨terms_x, hpfd_x⟩ := ih_x (e * t)
    obtain ⟨terms_y, hpfd_y⟩ := ih_y (e * s)
    exact ⟨terms_x ++ terms_y, {
      irred    := fun s' hs' => by
        rw [List.mem_append] at hs'
        exact hs'.elim (hpfd_x.irred s') (hpfd_y.irred s')
      nonAssoc := fun s' hs' t' ht' hne => by
        rw [List.mem_append] at hs' ht'
        rcases hs' with hs' | hs' <;> rcases ht' with ht' | ht'
        · exact hpfd_x.nonAssoc s' hs' t' ht' hne
        · intro; sorry
        · intro; sorry
        · exact hpfd_y.nonAssoc s' hs' t' ht' hne
      coprime  := fun s' hs' => by
        rw [List.mem_append] at hs'
        exact hs'.elim (hpfd_x.coprime s') (hpfd_y.coprime s')
      sum_eq   := by
        rw [List.map_append, List.sum_append, hpfd_x.sum_eq, hpfd_y.sum_eq]
        -- Goal: ι(e*t)/ι x + ι(e*s)/ι y = ι e / ι(x*y)
        -- hsplit: ι e/(ι x*ι y) = ι(e*t)/ι x + ι(e*s)/ι y
        -- So: ι(e*t)/ι x + ι(e*s)/ι y = ι e/(ι x*ι y) = ι e/ι(x*y)
        rw [← hsplit, map_mul] }⟩

theorem pid_partial_fraction_decomposition (c : FractionRing R) :
    ∃ terms : List (PFDSummand R), IsPFD terms c := by
  obtain ⟨a, b, _hb, hc⟩ := exists_num_denom c
  rw [← hc]; exact isPFD_of_denom b a

section IntExample
example (c : FractionRing ℤ) :
    ∃ terms : List (PFDSummand ℤ), IsPFD terms c :=
  pid_partial_fraction_decomposition c
end IntExample
```