Regularity conjecture for zero-dimensional subschemes in toric varieties

Let XX be the projective variety with Cox ring SS, let UXU\subset X be the open set used to define the Cox-ring setup, and let I=f1,,fnSI=\left\langle f_1,\ldots,f_n\right\rangle\subset S be a homogeneous ideal such that VX(I)V_X(I) is a zero-dimensional subscheme of UU. Write αi=deg(fi)Pic(X)\alpha_i=\deg(f_i)\in\operatorname{Pic}(X) for the basepoint-free degrees of the generators, and let Cl(X)+\operatorname{Cl}(X)_+ denote the positive degree semigroup. For a degree α\alpha, let Reg(I)\operatorname{Reg}(I) be the set of degrees for which the equivalent interpolation and Hilbert-function conditions in the definition of regularity hold. Let ζj\zeta_j be the points associated with the setup, and say that a point is a basepoint of Sα0S_{\alpha_0} when every section of degree α0\alpha_0 vanishes there.

Regularity conjecture. Then

α0+α1++αnReg(I)\alpha_0+\alpha_1+\cdots+\alpha_n\in\operatorname{Reg}(I)

for every α0Cl(X)+\alpha_0\in\operatorname{Cl}(X)_+ such that no ζj\zeta_j is a basepoint of Sα0S_{\alpha_0}.

This conjecture predicts a uniform degree in which the Cox-ring interpolation conditions become regular for zero-dimensional schemes, extending the familiar principle that sufficiently large basepoint-free degrees should recover the ideal component and the expected Hilbert function. The paper presents it as motivated by experimental evidence; no resolution is supplied in the given source.

Sources & referencesView supporting material

Primary source

Simon Telen, “Numerical Root Finding via Cox Rings”, arXiv:1903.12002 (2020).

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