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A simultaneous right-module average of a finite family of completely bounded maps

Proved
OAI.BoundedHochschild.KadisonRingrose.exists_finite_right_module_average

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

bounded-hochschild-cohomologyoai-math

Throughout, MMM is a unital C*-algebra with a partial order making it star-ordered, XXX is a complex Banach space, and e:X∗→Me:X^*\to Me:X∗→M is an isometric conjugate-linear isomorphism, so MMM is a von Neumann algebra with predual XXX (the Lean hypothesis e). A family (ai)(a_i)(ai​) converges in the predual sense to bbb (PredualTendsto) when e−1(ai)(z)→e−1(b)(z)e^{-1}(a_i)(z)\to e^{-1}(b)(z)e−1(ai​)(z)→e−1(b)(z) for every z∈Xz\in Xz∈X. The hypothesis NoFiniteCentralSummand M says that a central projection ppp whose corner pMppMppMp is finite (every isometry of the corner is unitary) must vanish. Let MMM be nontrivial with no finite central summand, and let T0,…,Tn:M→MT_0,\dots,T_n:M\to MT0​,…,Tn​:M→M be bounded linear maps with complete bounds CiC_iCi​. Then there are bounded maps SiS_iSi​ with ∥Si∥≤Ci\|S_i\|\le C_i∥Si​∥≤Ci​ and Si(ab)=Si(a)bS_i(ab)=S_i(a)bSi​(ab)=Si​(a)b for all a,ba,ba,b, such that the family (Si)(S_i)(Si​) lies in the weak closure of the simultaneous averages (a↦∑jTi(amj)mj∗)i\bigl(a\mapsto\sum_jT_i(am_j)m_j^*\bigr)_i(a↦∑j​Ti​(amj​)mj∗​)i​ over finite (mj)(m_j)(mj​) with ∑jmjmj∗=1\sum_jm_jm_j^*=1∑j​mj​mj∗​=1 (weakFamilyAverageOrbit).

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. Only the keyword lemma, where used upstream, becomes theorem, and a leading include ... in or omit ... in moves to the preamble. It is a step of the Lean proof of the vanishing of higher bounded Hochschild cohomology.

Preamble
import Mathlib
import Definitions.Def_KadisonRingroseSupport2

namespace OAI

noncomputable section

open Topology

namespace BoundedHochschild.KadisonRingrose

open HochschildComparison

universe u
variable {M X : Type u} [CStarAlgebra M] [PartialOrder M] [StarOrderedRing M]
  [NormedAddCommGroup X] [NormedSpace ℂ X] [CompleteSpace X]

Formal statement
theorem exists_finite_right_module_average [Nontrivial M]
    (e : StrongDual ℂ X ≃ₗᵢ⋆[ℂ] M) (hno : NoFiniteCentralSummand M)
    {n : ℕ} (T : Fin (n+1) → M →L[ℂ] M) (C : Fin (n+1) → ℝ)
    (hT : ∀ i, CompletelyBoundedEstimate (T i) (C i)) :
    ∃ S : Fin (n+1) → M →L[ℂ] M,
      (∀ i, ‖S i‖ ≤ C i ∧ ∀ a b, S i (a*b) = S i a * b) ∧
      (fun i => weakOperatorImage e (S i)) ∈ weakFamilyAverageOrbit e T := by
  sorry

end BoundedHochschild.KadisonRingrose
end
end OAI
Source
OpenAI, Vanishing of higher bounded Hochschild cohomology, September 23, 2026, https://github.com/openai/math (paper: preprints/Vanishing-of-higher-bounded-Hochschild-cohomology-September-23-2026/paper.pdf; Apache License 2.0); Lean declaration `OAI.BoundedHochschild.KadisonRingrose.exists_finite_right_module_average`, https://github.com/openai/math/blob/adc7f1241b42e322a6451854ab7e4b4c146bf78a/lean/OAI/Analysis/BoundedHochschild/FiniteModuleAverages.lean#L30

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