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One orbit step toward a transported tuple, rank at least two

Proved
OAI.FreeGroupFactorMain.Generation.orbit_step_ge_two

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 M=L(F(Fin n⊔{∗}))M=L(\mathbb F(\mathrm{Fin}\,n\sqcup\{*\}))M=L(F(Finn⊔{∗})), and let the orbit be the set of tuples (Φ(λ(j)))j∈Fin n(\Phi(\lambda(j)))_{j\in\mathrm{Fin}\,n}(Φ(λ(j)))j∈Finn​ as Φ\PhiΦ runs over the τ\tauτ-preserving ∗*∗-automorphisms of MMM. For a tuple VVV of unitaries, VgV_gVg​ is the evaluation of the word ggg and poly(V,p)=∑gp(g)Vg\mathrm{poly}(V,p)=\sum_g p(g)V_gpoly(V,p)=∑g​p(g)Vg​. Tuples carry the metric dist(V,U)=max⁡j∥Vj−Uj∥\mathrm{dist}(V,U)=\max_j\|V_j-U_j\|dist(V,U)=maxj​∥Vj​−Uj​∥ in the operator norm. Let xjx_jxj​ be the free generators of Fn\mathbb F_nFn​. Let n≥2n\ge2n≥2, let Φ\PhiΦ be a τ\tauτ-preserving ∗*∗-automorphism of MMM, and θ>0\theta>0θ>0. There are VVV in the orbit with dist(V,(Φ(λ(j)))j)<θ\mathrm{dist}\bigl(V,(\Phi(\lambda(j)))_j\bigr)<\thetadist(V,(Φ(λ(j)))j​)<θ and a word w∈Fnw\in\mathbb F_nw∈Fn​ such that

∥Vxj−Φ(λ(j))∥2<θ(j∈Fin n),∥Vw−Φ(λ(∗))∥2<θ.\|V_{x_j}-\Phi(\lambda(j))\|_2<\theta\quad(j\in\mathrm{Fin}\,n),\qquad \|V_w-\Phi(\lambda(*))\|_2<\theta.∥Vxj​​−Φ(λ(j))∥2​<θ(j∈Finn),∥Vw​−Φ(λ(∗))∥2​<θ.

Thus, after a small move in the orbit, a word in the nnn-tuple approximates the transported extra generator.

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

namespace FreeGroupFactorMain

noncomputable section

open Set Metric Filter
open scoped NNReal Topology BigOperators ContDiff

attribute [local instance] Classical.propDecidable

open UnitaryEvaluation

namespace Generation

Formal statement
theorem orbit_step_ge_two (n : ℕ) (hn : 2 ≤ n)
    (Φ : Ambient n ≃⋆ₐ[ℂ] Ambient n)
    (ht : ∀ T, canonicalTrace (Φ T) = canonicalTrace T) {θ : ℝ} (hθ : 0 < θ) :
    ∃ V ∈ orbit n, dist V (mapTuple Φ (initial n)) < θ ∧
      ∃ w : FreeGroup (Fin n), ∀ j : Option (Fin n),
        twoNorm ((wordTuple V (fun j => j.elim w FreeGroup.of) j : Ambient n) -
          (mapTuple Φ generatorUnitary j : Ambient n)) < θ := by
  sorry

end Generation
end
end FreeGroupFactorMain
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.Generation.orbit_step_ge_two`, https://github.com/openai/math/blob/adc7f1241b42e322a6451854ab7e4b4c146bf78a/lean/OAI/Analysis/FreeGroupFactor/FiniteRank.lean#L57

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