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Flow extension with inactive free generators

Proved
OAI.FreeGroupFactorMain.FiniteActive.extension_at_time

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 ι\iotaι be finite, κ\kappaκ arbitrary, and let M=L(F(ι⊔κ⊔{∗}))M=L(\mathbb F(\iota\sqcup\kappa\sqcup\{*\}))M=L(F(ι⊔κ⊔{∗})) with c=λ(∗)c=\lambda(*)c=λ(∗). Let l:Z→Cl:\mathbb Z\to\mathbb Cl:Z→C be finitely supported with S=∑kl(k)ckS=\sum_k l(k)c^kS=∑k​l(k)ck self-adjoint, let hih_ihi​ (i∈ιi\in\iotai∈ι) be finitely supported real functions on F(ι)\mathbb F(\iota)F(ι), and let X:R→U(M)ιX:\mathbb R\to U(M)^\iotaX:R→U(M)ι solve X˙i=i(∑ghi(g)XgSXg∗)Xi\dot X_i=i\bigl(\sum_g h_i(g)X_gSX_g^*\bigr)X_iX˙i​=i(∑g​hi​(g)Xg​SXg∗​)Xi​ with Xi(0)=λ(i)X_i(0)=\lambda(i)Xi​(0)=λ(i), where XgX_gXg​ is the evaluation of the word ggg in the tuple XXX. Let T(t)T(t)T(t) be the tuple consisting of Xi(t)X_i(t)Xi​(t) on ι\iotaι and λ(k)\lambda(k)λ(k) on κ\kappaκ. For every ttt there is a τ\tauτ-preserving ultraweakly continuous ∗*∗-automorphism Φ\PhiΦ of MMM such that Φ(λ(g))\Phi(\lambda(g))Φ(λ(g)) is the evaluation of ggg in (T(t),c)(T(t),c)(T(t),c), for every word ggg.

Together with moment preservation, this extends the flow automorphisms of the paper to infinitely many inactive generators.

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 uι uκ

open Set Metric Filter
open scoped NNReal Topology BigOperators ContDiff ENNReal

attribute [local instance] Classical.propDecidable

namespace FreeGroupFactorMain.FiniteActive

section
open UnitaryEvaluation
open scoped ENNReal

end
open UnitaryEvaluation

Formal statement
theorem extension_at_time {ι : Type uι} {κ : Type uκ} [Fintype ι] [DecidableEq ι] [DecidableEq κ]
    (h : ι → FreeGroup ι →₀ ℝ) (l : ℤ →₀ ℂ)
    (hS : IsSelfAdjoint (laurentValue (generatorUnitary (none : Option (ι ⊕ κ))) l))
    (X : ℝ → ι → Ambient ι κ) (hU : ∀ t i, X t i ∈ unitary (Ambient ι κ))
    (hX₀ : X 0 = fun i => groupUnitary (FreeGroup.of (some (Sum.inl i))))
    (hX : ∀ t, HasDerivAt X (PolynomialFlow.field
      (laurentValue (generatorUnitary (none : Option (ι ⊕ κ))) l) h (X t)) t) (t : ℝ) :
    ∃ Φ : Ambient ι κ ≃⋆ₐ[ℂ] Ambient ι κ,
        (∀ T, canonicalTrace (Φ T) = canonicalTrace T) ∧
        @Continuous _ _ (groupUltraweakTopology (FreeGroup (Option (ι ⊕ κ))))
          (groupUltraweakTopology (FreeGroup (Option (ι ⊕ κ)))) Φ ∧
        ∀ g, Φ (groupUnitary g) = wordHom
          (extendUnitary (fun j => ⟨tuple (X t) j,tuple_unitary (X t) (hU t) j⟩)
            (generatorUnitary none)) g := by
  sorry

end FreeGroupFactorMain.FiniteActive
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.FiniteActive.extension_at_time`, https://github.com/openai/math/blob/adc7f1241b42e322a6451854ab7e4b4c146bf78a/lean/OAI/Analysis/FreeGroupFactor/FiniteActive.lean#L96

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