Lower semi-continuity conjecture for log plurigenera of proper SNCL schemes

Let κ\kappa be a field, let ss be the log point of κ\kappa, and let X/sX/s be a proper SNCL scheme of pure dimension dd. For n,rZ1n,r\in\mathbb Z_{\geq 1}, define the log geometric plurigenus in positive characteristic by

pg(X/s,n,r):=lengthWnH0(X,(WnΩXd)r),p_g(X/s,n,r):={\rm length}_{{\cal W}_n}H^0\left(X,({\cal W}_n\Omega^d_X)^{\otimes r}\right),

and in characteristic zero define the log plurigenus by

pg(X/s,r):=dimκH0(X,(ΩX/sd)r).p_g(X/s,r):={\rm \dim}_{\kappa}H^0\left(X,(\Omega^d_{X/s})^{\otimes r}\right).

Let X\overset{\circ}{X} denote the underlying scheme and X(1)\overset{\circ}{X}{}^{(1)} its relevant base-change/Frobenius twist. Lower semi-continuity conjecture for log plurigenera. One should have

{pg(X(1)/κ,n,r)pg(X/s,n,r)if ch(κ)=p>0,pg(X(1)/κ,r)pg(X/s,r)if ch(κ)=0.\begin{cases} p_g(\overset{\circ}{X}{}^{(1)}/\kappa,n,r)\leq p_g(X/s,n,r) & \text{if }\operatorname{ch}(\kappa)=p>0,\\ p_g(\overset{\circ}{X}{}^{(1)}/\kappa,r)\leq p_g(X/s,r) & \text{if }\operatorname{ch}(\kappa)=0. \end{cases}

This compares the plurigenera of the underlying scheme with the logarithmic plurigenera of the SNCL scheme. The paper explains that the finite-length argument does not prove the assertion for r2r\geq2, although the corresponding level-one positive-characteristic inequality and the characteristic-zero inequality are later established; the full stated level-nn claim therefore remains open in the source.

Sources & referencesView supporting material

Primary source

Yukiyoshi Nakkajima, “Log ordinarity and log plurigenera of a proper SNCL scheme over a log point in characterisitic p>0”, arXiv:2004.03680 (2020).

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