An example of a very non-movable effective divisor

Jihao Liu

Liu reports effective divisors D_n on one fixed smooth projective surface with D_n²=4 and h⁰(mD_n)=1 for every 1≤m<n²/4+1, for arbitrarily large odd n. Thus no uniform movable multiple depending only on that surface exists.

Sources for this record

What was already known

Ciliberto and coauthors asked in 2017 whether such a uniform multiple exists for effective divisors with one section and positive self-intersection.

AI involvement AI-led

Jihao Liu says ChatGPT 5.5 Pro found the example. Rethlas identified an error in its first section-vanishing proof and supplied the retained repair; Liu directed and released the work.

The initial example came from GPT-5.5 Pro with no mathematical hint, and the essential repair was also model-generated through Rethlas; Liu directed and released the work.

Importance Notable

Answers that algebraic-geometry question negatively with an explicit family.

Limitations

The first model proof of section vanishing was wrong and a later model run supplied the repair. The paper does not document a completed rigorous check of the corrected final argument; help checking an earlier blueprint does not establish that scope.

Technical verification Reported

A model-based verifier detected the original error and Rethlas repaired it, but the paper does not document a completed rigorous human or machine check of the corrected section-vanishing argument. Assistance checking an earlier blueprint does not establish final-proof coverage.

Human understanding Few

The disclosure describes an AI-origin example and AI correction, without a named human explaining the corrected section-vanishing mechanism. Few is the evidence default, not a claim that Liu cannot understand it.

Correction and publication history

2026-10-18 · publication

If correct, answers that algebraic-geometry question negatively with an explicit family.

Read this published revision