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The regulator is independent of the generators and of the deleted column, and positive

Proved
DirichletUnitTheorem.regulator_well_defined

by Lucas · Oct 9, 2026 · Mathlib 0df444a (Lean v4.33.1)

algebraic-number-theorynumber-theory

Let KKK be a number field, r=r1+r2−1r=r_1+r_2-1r=r1​+r2​−1, and let u1,…,uru_1,\dots,u_ru1​,…,ur​ and v1,…,vrv_1,\dots,v_rv1​,…,vr​ be two systems of generators of OK×\mathcal O_K^\timesOK×​ modulo roots of unity. Form the r×(r+1)r\times(r+1)r×(r+1) matrices (Njlog⁡∣σj(ui)∣)\big(N_j\log\lvert\sigma_j(u_i)\rvert\big)(Nj​log∣σj​(ui​)∣) and (Njlog⁡∣σj(vi)∣)\big(N_j\log\lvert\sigma_j(v_i)\rvert\big)(Nj​log∣σj​(vi​)∣), columns indexed by the infinite places. Delete any one column from the first and any one column from the second. Then the two square submatrices have determinants of the same absolute value RRR, and R>0R>0R>0.

This common value RRR is the regulator of KKK; the statement says it does not depend on the choice of generators nor on the deleted column, and is a positive real number.

Formalization Note Deleting the column of place w1w_1w1​ leaves a square matrix whose rows and columns are both indexed by the places ≠w1\neq w_1=w1​, the rows being matched to u1,…,uru_1,\dots,u_ru1​,…,ur​ through an arbitrary bijection e1e_1e1​; reordering rows only changes the sign of the determinant.

Preamble
import Mathlib
open NumberField
Formal statement
namespace DirichletUnitTheorem

theorem regulator_well_defined (K : Type*) [Field K] [NumberField K]
    [DecidableEq (InfinitePlace K)]
    (u v : Fin (InfinitePlace.nrRealPlaces K + InfinitePlace.nrComplexPlaces K - 1) → (𝓞 K)ˣ)
    (hu : ∀ x : (𝓞 K)ˣ, ∃ ζ : (𝓞 K)ˣ, IsOfFinOrder ζ ∧ ∃ n : Fin _ → ℤ, x = ζ * ∏ i, u i ^ n i)
    (hv : ∀ x : (𝓞 K)ˣ, ∃ ζ : (𝓞 K)ˣ, IsOfFinOrder ζ ∧ ∃ n : Fin _ → ℤ, x = ζ * ∏ i, v i ^ n i)
    (w₁ w₂ : InfinitePlace K)
    (e₁ : {w // w ≠ w₁} ≃ Fin (InfinitePlace.nrRealPlaces K + InfinitePlace.nrComplexPlaces K - 1))
    (e₂ : {w // w ≠ w₂} ≃ Fin (InfinitePlace.nrRealPlaces K + InfinitePlace.nrComplexPlaces K - 1)) :
    |(Matrix.of fun (i j : {w // w ≠ w₁}) =>
        ((j : InfinitePlace K).mult : ℝ) * Real.log ((j : InfinitePlace K) ((u (e₁ i) : 𝓞 K) : K))).det| =
      |(Matrix.of fun (i j : {w // w ≠ w₂}) =>
        ((j : InfinitePlace K).mult : ℝ) * Real.log ((j : InfinitePlace K) ((v (e₂ i) : 𝓞 K) : K))).det| ∧
    0 < |(Matrix.of fun (i j : {w // w ≠ w₁}) =>
        ((j : InfinitePlace K).mult : ℝ) * Real.log ((j : InfinitePlace K) ((u (e₁ i) : 𝓞 K) : K))).det| := by sorry

end DirichletUnitTheorem
Source
Wikipedia, "Dirichlet's unit theorem" (article supplied by the account owner as a PDF), section "The regulator" ("the absolute value R of the determinant of the submatrix formed by deleting one column is independent of the column ... it does not depend on the choice of generators ui"; lead: "The regulator is a positive real number").
Read-back

What the Lean code literally says, in plain math · Aristotle (Harmonic) — same agent as the drafter; non-blind

Non-blind read-back — not independent testimony. This read-back was written by the same agent (Aristotle, by Harmonic) that drafted this Lean statement, at the explicit request of the account owner. Its author knew the intended meaning while writing it, so it must not be mistaken for a blind audit by an independent reviewer. Please compare it against the Lean code itself.

Throughout, KKK is a number field (a field of characteristic zero that is finite-dimensional over Q\mathbb QQ), OK\mathcal O_KOK​ is its ring of integers and OK×\mathcal O_K^\timesOK×​ the unit group. r1r_1r1​ denotes the number of real infinite places of KKK and r2r_2r2​ the number of complex infinite places (each complex place corresponds to a pair of complex-conjugate embeddings K→CK\to\mathbb CK→C). For an infinite place www of KKK, ∣x∣w\lvert x\rvert_w∣x∣w​ denotes the absolute value ∣σ(x)∣\lvert\sigma(x)\rvert∣σ(x)∣ for an embedding σ\sigmaσ inducing www, and NwN_wNw​ is 111 if www is real and 222 if www is complex. log⁡\loglog is the real logarithm (Mathlib's convention gives log⁡0=0\log 0 = 0log0=0, but units have ∣u∣w>0\lvert u\rvert_w>0∣u∣w​>0). Let r=r1+r2−1r=r_1+r_2-1r=r1​+r2​−1 (truncated subtraction). Statement. Let KKK be a number field (with a chosen decidable equality on its infinite places, needed to form determinants; the value does not depend on it). Let u,v:{0,…,r−1}→OK×u,v:\{0,\dots,r-1\}\to\mathcal O_K^\timesu,v:{0,…,r−1}→OK×​ be families of units such that every unit xxx can be written as x=ζ∏iuinix=\zeta\prod_i u_i^{n_i}x=ζ∏i​uini​​ for some unit ζ\zetaζ of finite order and some integers nin_ini​, and likewise for vvv (only existence of such a representation is assumed, not uniqueness). Let w1,w2w_1,w_2w1​,w2​ be infinite places, and let e1e_1e1​ (resp. e2e_2e2​) be any bijection from the set of infinite places different from w1w_1w1​ (resp. w2w_2w2​) onto {0,…,r−1}\{0,\dots,r-1\}{0,…,r−1}. Define the square matrix M1M_1M1​ with rows and columns indexed by the places ≠w1\neq w_1=w1​ by (M1)i,j=Njlog⁡∣ue1(i)∣j(M_1)_{i,j}=N_j\log\lvert u_{e_1(i)}\rvert_j(M1​)i,j​=Nj​log∣ue1​(i)​∣j​, and M2M_2M2​ analogously with w2,e2,vw_2,e_2,vw2​,e2​,v. Then

∣det⁡M1∣=∣det⁡M2∣and0<∣det⁡M1∣.\lvert\det M_1\rvert=\lvert\det M_2\rvert \quad\text{and}\quad 0<\lvert\det M_1\rvert.∣detM1​∣=∣detM2​∣and0<∣detM1​∣.
Human review
  • Endorsed by Shuze Chen · Oct 10, 2026

    Confirmed by the moderator at approval.

  • Endorsed by Lucas · Oct 10, 2026

    Confirmed by the mission captain (proposal self-audit).

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