Failure of local extension of semipositive metrics in a smooth projective family
Abstract
We construct a smooth projective family of rational surfaces and an effective line bundle on its total space that give a negative answer to Păun's metric-extension question. The restricted bundle on the central fibre carries a semipositively curved singular Hermitian metric with analytic and minimal singularities, but this metric admits no semipositive extension to any neighbourhood of the whole fibre, even up to singularity type. The construction uses four-point blow-ups of the plane, with three centres becoming collinear on the central fibre. This change produces a nonzero integrable adjoint section on the central fibre, whereas the corresponding adjoint systems vanish on nearby noncentral fibres. If a metric extension existed, an L2 extension theorem would extend that section, contradicting this vanishing.
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BibTeX
@misc{qin2026failure,
title = {{Failure of local extension of semipositive metrics in a smooth projective family}},
author = {Xiangsen Qin},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2609.36410},
eprint = {2609.36410},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.36410},
note = {CC BY-SA 4.0},
}
RIS
TY - UNPB
TI - Failure of local extension of semipositive metrics in a smooth projective family
AU - Xiangsen Qin
PY - 2026
DA - 2026/10/27
PB - arXiv
DO - 10.48550/arxiv.2609.36410
UR - https://arxiv.org/abs/2609.36410
AB - We construct a smooth projective family of rational surfaces and an effective line bundle on its total space that give a negative answer to Păun's metric-extension question. The restricted bundle on the central fibre carries a semipositively curved singular Hermitian metric with analytic and minimal singularities, but this metric admits no semipositive extension to any neighbourhood of the whole fibre, even up to singularity type. The construction uses four-point blow-ups of the plane, with three centres becoming collinear on the central fibre. This change produces a nonzero integrable adjoint section on the central fibre, whereas the corresponding adjoint systems vanish on nearby noncentral fibres. If a metric extension existed, an L^2 extension theorem would extend that section, contradicting this vanishing.
ER -