Betti-number conjecture for lattice polygons and toric surfaces

Let ΔR2\Delta\subseteq\mathbb{R}^2 be a lattice polygon whose interior polygon Δ(1)\Delta^{(1)} is two-dimensional and contains g4g\geq 4 lattice points. Assume that Δ(1)≇Υ\Delta^{(1)}\not\cong\Upsilon. Denote the graded Betti table of XΔ(1)Pg1X_{\Delta^{(1)}}\subseteq\mathbb{P}^{g-1} by bb_\ell as in the source. The toric Betti-table conjecture. Then

min{bg0}={lw(Δ(1))+1if Δ(1)(d3)Σ for some d5,lw(Δ(1))+1if Δ(1)2Υ,lw(Δ(1))+2in all other cases.\min\{\ell\mid b_{g-\ell}\neq 0\}=\begin{cases} \operatorname{lw}(\Delta^{(1)})+1 & \text{if }\Delta^{(1)}\cong(d-3)\Sigma\text{ for some }d\geq 5,\\ \operatorname{lw}(\Delta^{(1)})+1 & \text{if }\Delta^{(1)}\cong2\Upsilon,\\ \operatorname{lw}(\Delta^{(1)})+2 & \text{in all other cases.} \end{cases}

The conjecture predicts the first relevant nonzero Betti number of the toric surface from the lattice width of its interior polygon, with two explicitly identified exceptional families. The supplied material gives no resolution status.

Sources & referencesView supporting material

Primary source

Wouter Castryck, Filip Cools, Jeroen Demeyer and Alexander Lemmens, “Canonical syzygies of smooth curves on toric surfaces”, arXiv:1609.02360 (2019).

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