English

Unexpected hypersurfaces of type $(d+k,d)$

Algebraic Geometry 2025-11-17 v1

Abstract

Unexpected hypersurfaces arise when vanishing in points of a set ZZ and higher-order vanishing along a general linear subspace fails to impose the expected number of independent conditions on forms of a fixed degree. The phenomenon was first observed for planar curves by Cook, Harbourne, Migliore and Nagel. This paper shows a syzygy-based construction of, possibly unexpected, hypersurfaces of degree d+kd+k in Pn\mathbb{P}^n, vanishing along a codimension two general linear subspace with multiplicity dd; thus generalizing the work of Trok and the previous work of the last two authors. Our framework unifies the classical planar cases with higher-dimensional examples, including Trok's construction. We give a sufficient criterion for unexpectedness (via the splitting behaviour the syzygy bundles of the powers of the Jacobian ideal, associated with the hyperplane arrangement dual to ZZ) and provide explicit examples in P3\mathbb{P}^3 and P4\mathbb{P}^4.

Keywords

Cite

@article{arxiv.2511.10772,
  title  = {Unexpected hypersurfaces of type $(d+k,d)$},
  author = {Marek Janasz and Grzegorz Malara and Halszka Tutaj-Gasińska},
  journal= {arXiv preprint arXiv:2511.10772},
  year   = {2025}
}

Comments

24 pages

R2 v1 2026-07-01T07:36:37.205Z