A counterexample to the odd-dimensional rank bound for abelian p-group actions

Jihao Liu, Yanze Wang

The smooth Fermat quintic Calabi–Yau threefold has a faithful action of (μ₅)⁵/Δ, an elementary abelian 5-group of rank four. Since 4>15/4, this refutes Moraga’s proposed odd-dimensional extension of a rank bound.

Sources for this record

What was already known

Kollár and Zhuang proved the bound under Euler-characteristic conditions that do not cover Calabi–Yau threefolds; the odd-dimensional extension was conjectural.

AI involvement AI-led

Liu and Wang credit ChatGPT 5.5 Pro with the proof sketch and Danus with verification and writeup, followed by their own checking and polishing.

The authors attribute the counterexample proof to ChatGPT and Danus before human verification.

Importance Notable

A concrete threefold and group action disprove that proposed extension.

Limitations

The example does not challenge Kollár and Zhuang’s proved theorem. The authors report human verification, but the paper does not divide the checking steps between them.

Technical verification Formal (human)

Liu and Wang expressly report checking the completed elementary proof; no proof-assistant check is documented.

Independent checking

Reported human verification of the completed proof after Danus

Checker
Jihao Liu and Yanze Wang, paper authors
Scope
The Fermat-quintic faithful rank-four action and violation of Moraga's odd-dimensional bound
Basis
Reported by the linked checker
Evidence source

Human understanding Few

No named human explanation of the AI-written central argument is documented; Few is the evidence default.

Correction and publication history

2026-10-18 · publication

This refutes the conjectured odd-dimensional extension, not the proved Euler-characteristic case.

Read this published revision