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Absorbing any extra generator from the distinguished one

Proved
OAI.FreeGroupFactorMain.InfiniteGeneration.one_step_extra_of_none

by wurtle · Nov 4, 2026 · Mathlib 0df444a (Lean v4.33.1)

free-group-factorsoai-math

Throughout, L(G)L(G)L(G) is the group von Neumann algebra of a discrete group GGG acting on ℓ2(G)\ell^2(G)ℓ2(G), with canonical trace τ(x)=⟨xδe,δe⟩\tau(x)=\langle x\delta_e,\delta_e\rangleτ(x)=⟨xδe​,δe​⟩, left-regular unitaries λ(g)\lambda(g)λ(g), and ∥x∥2=∥xδe∥ℓ2\|x\|_2=\|x\delta_e\|_{\ell^2}∥x∥2​=∥xδe​∥ℓ2​. Let Index=Fin 3⊔N⊔{∗}\mathrm{Index}=\mathrm{Fin}\,3\sqcup\mathbb N\sqcup\{*\}Index=Fin3⊔N⊔{∗} and M=L(F(Index))M=L(\mathbb F(\mathrm{Index}))M=L(F(Index)). Let Ai=λ(i)A_i=\lambda(i)Ai​=λ(i) for i∈Fin 3i\in\mathrm{Fin}\,3i∈Fin3 be the initial tuple, and for q∈N⊔{∗}q\in\mathbb N\sqcup\{*\}q∈N⊔{∗} let Eq=λ(q)E_q=\lambda(q)Eq​=λ(q) be the extra generators. The orbit is the set of tuples (Φ(Ai))i(\Phi(A_i))_i(Φ(Ai​))i​ for τ\tauτ-preserving ∗*∗-automorphisms Φ\PhiΦ of MMM. Assume that for every ε>0\varepsilon>0ε>0 there are a τ\tauτ-preserving automorphism α\alphaα fixing each EkE_kEk​, k∈Nk\in\mathbb Nk∈N, and a word w∈F3w\in\mathbb F_3w∈F3​ with ∥α(Ai)−Ai∥<ε\|\alpha(A_i)-A_i\|<\varepsilon∥α(Ai​)−Ai​∥<ε and ∥α(Aw)−E∗∥2<ε\|\alpha(A_w)-E_*\|_2<\varepsilon∥α(Aw​)−E∗​∥2​<ε. Then for every qqq and ε>0\varepsilon>0ε>0 there are a τ\tauτ-preserving automorphism α\alphaα fixing each ErE_rEr​, r≠qr\ne qr=q, and a word w∈F3w\in\mathbb F_3w∈F3​ with

∥α(Ai)−Ai∥<ε,∥α(Aw)−Eq∥2<ε.\|\alpha(A_i)-A_i\|<\varepsilon,\qquad\|\alpha(A_w)-E_q\|_2<\varepsilon.∥α(Ai​)−Ai​∥<ε,∥α(Aw​)−Eq​∥2​<ε.

This reduces the absorption of an arbitrary extra generator to the distinguished one.

Formalization Note The statement is verbatim from OpenAI's Lean development, with the listed Definitions imported in place of the upstream modules; the definitions are supporting code for this statement.

Preamble
import Mathlib
import Definitions.Def_FreeGroupFactorProof18

namespace OAI

noncomputable section

universe uA uG uι

open Set Metric Filter
open scoped NNReal Topology BigOperators ContDiff ENNReal

attribute [local instance] Classical.propDecidable

namespace FreeGroupFactorMain.Generation

end FreeGroupFactorMain.Generation

namespace FreeGroupFactorMain.InfiniteGeneration
open Generation

Formal statement
theorem one_step_extra_of_none
    (hone : ∀ ε : ℝ, 0 < ε → ∃ α : M ≃⋆ₐ[ℂ] M,
      (∀ T, canonicalTrace (α T) = canonicalTrace T) ∧
      (∀ k : ℕ, α (extra (some k) : M) = (extra (some k) : M)) ∧
      ∃ w : FreeGroup (Fin 3),
        (∀ i, ‖α (initial i : M)-(initial i : M)‖ < ε) ∧
        twoNorm (α (word initial w)-(extra none : M)) < ε)
    (q : Option ℕ) {ε : ℝ} (hε : 0 < ε) :
    ∃ α : M ≃⋆ₐ[ℂ] M, (∀ T, canonicalTrace (α T) = canonicalTrace T) ∧
      (∀ r : Option ℕ, r ≠ q → α (extra r : M) = (extra r : M)) ∧
      ∃ w : FreeGroup (Fin 3),
        (∀ i, ‖α (initial i : M)-(initial i : M)‖ < ε) ∧
        twoNorm (α (word initial w)-(extra q : M)) < ε := by
  sorry

end FreeGroupFactorMain.InfiniteGeneration
end
end OAI
Source
OpenAI, An isomorphism of the free group factors, September 23, 2026, https://github.com/openai/math (paper: preprints/An-isomorphism-of-the-free-group-factors-September-23-2026/An-isomorphism-of-the-free-group-factors-September-23-2026.pdf; Lean: lean/OAI/Analysis/FreeGroupFactor and lean/OAI/Analysis/InterpolatedFactors, Apache License 2.0); Lean declaration `OAI.FreeGroupFactorMain.InfiniteGeneration.one_step_extra_of_none`, https://github.com/openai/math/blob/adc7f1241b42e322a6451854ab7e4b4c146bf78a/lean/OAI/Analysis/FreeGroupFactor/InfiniteGeneration.lean#L128

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