A Counterexample to Talagrand's Operator Cotype Problem

Xinglong Wu

For sufficiently large n=2ᵏ, Wu’s Walsh–Hadamard norm and identity map Uₙ:Xₙ→ℓ∞ⁿ make C₂ʳ(Uₙ)/max(C₂ᵍ(Uₙ),‖Uₙ‖(2,1)) grow at least as c√log(n+1). Thus the proposed universal operator-cotype bound fails already at q=2.

Sources for this record

What was already known

Talagrand proposed universal control by Gaussian cotype and the (q,1)-summing norm; Kwapień’s related decomposition problem was also open.

AI involvement AI-centered

Xinglong Wu says ChatGPT (GPT-5.6) discovered the counterexample during discussions. Wu independently checked all arguments and explains the central estimates.

The central counterexample is explicitly model-discovered in human discussions; autonomous development is not documented.

Importance Major

Provides an unbounded counterexample and refutes the related universal decomposition bound.

Limitations

The result concerns operator cotype at q=2; the Kwapień consequence follows from the paper’s argument. No separate machine certificate or third-party complete review is documented.

Technical verification Formal (human)

The named author explicitly reports independently verifying all arguments; this is a source-reported completed human check.

Independent checking

independent human verification of all paper arguments

Checker
Xinglong Wu
Scope
Theorem 1.1 operator-cotype counterexample and its arguments
Basis
Reported by the linked checker
Evidence source

Human understanding Full

Wu lays out every central estimate in §2 and the decomposition implication in §1.

Correction and publication history

2026-10-18 · publication

Provides an unbounded counterexample and refutes the related universal decomposition bound.

Read this published revision