Projective normality and quadratic generation for polarized Hilbert squares of K3 surfaces

Let (S,L)(S,L) be a polarized K3K3 surface of genus gg with Pic(S)=ZL\operatorname{Pic}(S)=\mathbb{Z}\cdot L. Consider the embedding defined by the complete linear system L22δ|L_2-2\delta|:

S[2]P(H0(S[2],L22δ)).S^{[2]}\hookrightarrow \mathbb{P}\left(H^0(S^{[2]},L_2-2\delta)^\vee\right).

Projective-normality and quadratic-generation conjecture. If g8g\geq8 (equivalently, q(L22δ)6q(L_2-2\delta)\geq6), then this embedding is projectively normal; and if g10g\geq10 (equivalently, q(L22δ)10q(L_2-2\delta)\geq10), then the homogeneous ideal of S[2]S^{[2]} is generated by quadrics.

These expectations concern the projective geometry of Hilbert squares in arbitrary genus and are motivated by the authors' results in genera 77 and 88, together with computer experiments. The source presents them as reasonable expectations rather than established results, so they remain open on the supplied evidence.

Sources & referencesView supporting material

Primary source

Ángel David Ríos Ortiz, Andrés Rojas and Jieao Song, “Projective models for Hilbert squares of K3 surfaces”, arXiv:2510.02065 (2025).

Additional references

4 papers in this index state this conjecture (2010–2025). The statement above is taken from the most recent of them; the others are arXiv:2309.05301, arXiv:1506.00603, arXiv:1011.3236.

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