arXiv subjects

Shiji Lyu

Publications and source records attributed to Shiji Lyu.

9 recordsLinked to original sources

arXiv2026

Regular rings over valuation rings

Bertin (1972) defined regularity for coherent local rings, and Knaf (2004) studied the property for a local ring $A$ essentially finitely presented over a valuation ring $V$. We discuss several properties of this notion of regularity for such $A$, obtaining results parallel to results for regularity of Noetherian local rings. We include classical and modern topics: openness of loci, perfectoid big Cohen--Macaulay algebras, and cotangent complexes. We also give an application to Noetherian rings, showing a version of Kodaira's vanishing theorem in large enough residue characteristics.

math.AC↗

arXiv2025

$(S_2)$-ifications, semi-Nagata rings, and the lifting problem

This is a two-part article. In the first part, we study an alternative notion to Nagata rings. A Nagata ring is a Noetherian ring $R$ such that every finite $R$-algebra that is an integral domain has finite normalization. We replace the normalization by an $(S_2)$-ification, study new phenomena, and prove parallel results. In particular, we show a Nagata domain has a finite $(S_2)$-ification. In the second part, we study the local lifting problem. We show that for a semilocal Noetherian ring $R$ that is $I$-adically complete for an ideal $I$, if $R/I$ has $(S_k)$ (resp. Cohen--Macaulay, Gorenstein, lci) formal fibers, so does $R$. As a consequence, we show if $R/I$ is a quotient of a Cohen--Macaulay ring, so is $R$. We also discuss difficulties in lifting geometrically $(R_k)$ formal fibers.

math.AC↗

arXiv2025

Uniform bounds in excellent $\mathbf{F}_p$-algebras and applications to semi-continuity

We study two important numerical invariants, Hilbert--Kunz multiplicity and $F$-signature, on the spectrum of a Noetherian $\mathbf{F}_p$-algebra $R$ that is not necessarily $F$-finite. When $R$ is excellent, we show that the limits defining the invariants are uniform. As a consequence, we show that the $F$-signature is lower semi-continuous, and the Hilbert--Kunz multiplicity is upper semi-continuous provided $R$ is locally equidimensional. Uniform convergence is achieved via a uniform version of Cohen--Gabber theorem. We prove the results under weaker conditions than excellence.

math.AC↗

arXiv2025

The Japanese and universally Japanese properties for valuation rings and Pr\"ufer domains

We discuss the Japanese and universally Japanese properties for valuation rings and Pr\"ufer domains. These properties, regarding finiteness of integral closure, have been studied extensively for Noetherian rings, but very rarely, if ever, for non-Noetherian rings. Among other results, we show that for valuation rings and Pr\"ufer domains, the Japanese and universally Japanese properties are equivalent. This result can be seen as a counterpart to Nagata's classical result for Noetherian rings. This result also tells us many non-Noetherian rings, including all absolutely integrally closed valuation rings and Pr\"ufer domains, are universally Japanese.

math.AC↗

arXiv2023

Formal lifting of dualizing complexes and consequences

We show that for a Noetherian ring $A$ that is $I$-adically complete for an ideal $I$, if $A/I$ admits a dualizing complex, so does $A$. This gives an alternative proof of the fact that a Noetherian complete local ring admits a dualizing complex. We discuss several consequences of this result. We also consider a generalization of the notion of dualizing complexes to infinite-dimensional rings and prove the results in this generality. In addition, we give an alternative proof of the fact that every excellent Henselian local ring admits a dualizing complex, using ultrapower.

math.AC↗

arXiv2022

The gamma-construction and permanence properties of the (relative) $F$-rational signature

We study some permanence properties of the relative $F$-rational signature defined and studied by Smirnov--Tucker. We show that this invariant is compatible with the gamma-construction, and then derive other main results from the $F$-finite case established by Smirnov--Tucker. We also obtain limited results about the $F$-rational signature defined and studied by Hochster--Yao. We explore some features of the gamma-construction along the way, which may be of independent interest.

math.AC↗

arXiv2022

On some properties of birational derived splinters

A Noetherian reduced ring $A$ is called a birational derived splinter if for all proper birational maps $X\to\operatorname{Spec}(A)$, the canonical map $A\to Rf_*\mathcal{O}_X$ splits. In equal characteristic zero this property characterizes rational singularities, but much less can be said in positive or mixed characteristics. In this paper, we prove some fundamental properties of this notion, including the behavior under localization, taking a pure subring, taking direct limit, and along an étale extension. In particular, direct limit of rational singularities in characteristic zero has rational singularities. Then, we study residue extensions (in arbitrary characteristic), and openness and regular extensions in positive characteristic, parallel to Datta-Tucker and the author's previous works on splinters.

math.AG↗

arXiv2022

The relative minimal model program for excellent algebraic spaces and analytic spaces in equal characteristic zero

We establish the relative minimal model program with scaling for locally projective morphisms of quasi-excellent algebraic spaces admitting dualizing complexes, quasi-excellent formal schemes admitting dualizing complexes, semianalytic germs of complex analytic spaces, rigid analytic spaces, Berkovich spaces, and adic spaces locally of weakly finite type over a field, all in equal characteristic zero. To do so, we prove finite generation of relative adjoint rings associated to projective morphisms of such spaces using the strategy of Cascini and Lazi\'c and the generalization of the Kawamata-Viehweg vanishing theorem to the scheme setting recently established by the second author. To prove these results uniformly, we prove GAGA theorems for Grothendieck duality and dualizing complexes to reduce to the algebraic case. In addition, we apply our methods to establish the relative minimal model program with scaling for spaces of the form above in dimensions $\le 3$ in positive and mixed characteristic, and to show that one can run the relative minimal model program with scaling for complex analytic spaces without shrinking the base at each step.

math.AG↗

arXiv2022

Permanence properties of splinters via ultrapower

We show that the splinter property ascends along regular residue field extensions, and along arbitrary regular maps in equal characteristic. We also study the splinter property of non-Noetherian rings, especially those related to ultrapowers, to the extent necessary for our main results.

math.AC↗