Twelve common flex lines in a general pencil of cubics

Jihao Liu, Yanze Wang

Liu and Wang prove that a general pencil of plane cubics over C has exactly twelve common flex lines. They identify all twelve units of the degree-nine flex-line curve’s genus defect as ordinary nodes, each representing a line that is a flex of two distinct smooth members.

Sources for this record

What was already known

Ciliberto, Miranda and Roé had computed the degree, genus and absence of hyperflexes and asked whether the defect counts common flex lines; those invariants did not rule out other singularities.

AI involvement AI-led

Liu and Wang credit ChatGPT 5.5 Pro and Danus with obtaining the main result. They subsequently verified and polished the proof and explain its geometry.

The authors directly credit generative AI with obtaining the main theorem, while reserving later verification and exposition for humans.

Importance Notable

Settles the precise common-flex-line question for general cubic pencils.

Limitations

The theorem concerns general complex cubic pencils. The authors report human verification, but do not separate ChatGPT’s and Danus’s individual lemmas or identify an external proof review.

Technical verification Formal (human)

The authors explicitly report human verification of the AI-generated main result and give the full argument for the twelve-line theorem. This is a source-reported author-side check; no Lean proof or external audit was identified.

Formal checking

Post-generation human verification and polishing of the full proof, including generic nodality and identification of nodes with common flex lines

Checker
Jihao Liu and Yanze Wang
Scope
A general complex cubic pencil has exactly twelve common flex lines
Basis
Reported by the linked checker
Evidence source

Checking a formal statement does not establish that it matches the original problem.

Human understanding Full

The authors explain the twisted-cubic secant mechanism and why all twelve units of genus defect are ordinary common-flex nodes.

Correction and publication history