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Densities of Picard Rank Jumps for K3 Surfaces Over Global Fields
Abstract
Let X be an algebraic K3 surface defined over a global field K of characteristic $0$ or p ≥ 3, assumed ordinary in the case where K is a global function field. We provide an exact formula for the density of primes p of K at which the geometric Neron-Severi group of the reduction Xp has a given rank in terms of the Galois representation attached to X. As an extension, we prove that there are infinitely many reductions of X with an element of the geometric Neron-Severi group representing a fixed square class under the intersection pairing and prove results about the density of such places. In particular, we show that if X corresponds to a sufficiently generic moduli point, a density zero set of places of K satisfy this condition.
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BibTeX
@misc{huang2026densities,
title = {{Densities of Picard Rank Jumps for K3 Surfaces Over Global Fields}},
author = {Robin Huang},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.06888},
eprint = {2610.06888},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.06888},
}
RIS
TY - UNPB
TI - Densities of Picard Rank Jumps for K3 Surfaces Over Global Fields
AU - Robin Huang
PY - 2026
DA - 2026/10/01
PB - arXiv
DO - 10.48550/arxiv.2610.06888
UR - https://arxiv.org/abs/2610.06888
AB - Let X be an algebraic K3 surface defined over a global field K of characteristic $0$ or p ≥ 3, assumed ordinary in the case where K is a global function field. We provide an exact formula for the density of primes p of K at which the geometric Neron-Severi group of the reduction X_p has a given rank in terms of the Galois representation attached to X. As an extension, we prove that there are infinitely many reductions of X with an element of the geometric Neron-Severi group representing a fixed square class under the intersection pairing and prove results about the density of such places. In particular, we show that if X corresponds to a sufficiently generic moduli point, a density zero set of places of K satisfy this condition.
ER -