Log K3 surfaces with irreducible boundary
We determine the automorphism group of an open log K3 surface with irreducible boundary.
math.AG↗
arXiv subjects
János Kollár
Publications and source records attributed to János Kollár.
Automorphisms of unstable ${\mathbb P}^1$-bundles
Let $P\to X$ be a ${\mathbb P}^1$-bundle over a variety $X$. The aim of this note is to understand all connected, algebraic groups $$ \operatorname{Aut}^\circ(P)\subset G\subset \operatorname{Bir}( X\times {\mathbb P}^1). $$ We get a quite complete answer if $\operatorname{Aut}^\circ(X)$ is a maximal, connected, algebraic subgroup of $\operatorname{Bir}(X)$, and $P$ is sufficiently unstable. This gives examples of connected, algebraic subgroups of $\operatorname{Bir}({\mathbb P}^4)$ that are not contained in any maximal one. Version 2: references updated.
math.AG↗
KSB stability is automatic in codimension 3
KSB stability holds at codimension 1 points trivially, and it is quite well understood at codimension 2 points, since we have a complete classification of 2-dimensional slc singularities. We show that it is automatic in codimension 3.
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Flat pushforwards of Chern classes and the smoothability of cycles below the middle dimension
We prove in this paper the smoothability of cycles modulo rational equivalence in the Whitney range, that is, when the dimension is strictly smaller than the codimension. We introduce and study the class of cycles obtained as ``flat pushforwards of Chern classes" (or equivalently, flat pushforwards of products of divisors) and prove that they are smoothable in the Whitney range. Our main result is that all cycles (of any dimension) on a smooth projective variety are flat pushforwards of Chern classes. In the case of abelian varieties, one can even restrict to smooth pushforwards of Chern classes.
math.AG↗
Comments on Fano varieties with torsion in $H^3(X, {\mathbb Z})$ by J. C. Ottem and J. V. Rennemo
Ottem and Rennemo constructed a Fano 4-fold with torsion in $H_2(X, \mathbb Z)$. We simplify the computation of $H_2(X, \mathbb Z)$ and also exhibit 2 lines $L', L''\subset X$ such that $L'- L''$ generates the torsion.
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KSB smoothings of surface pairs
We describe KSB smoothings of log canonical surface pairs $(S, D)$, where $D$ is a reduced curve. In sharp contrast with the $D=\emptyset$ case, cyclic quotient pairs always have KSB smoothings, usually forming many irreducible components. v.2: References added, small changes.
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Real analytic functions and monomial curves
We show that a function is real analytic at the origin iff it is arc-analytic, has a subanalytic graph, and its restriction to every monomial curve is analytic. This complements recent results of Kucharz and Kurdyka.
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Symmetric Powers
Given a faithful finite-dimensional representation $V$ of a finite group $G$ over any field $\mathbb{F}$, we show that any irreducible ${\mathbb{F}}G$-module $W$ appears, as a submodule or a quotient, in $\mathrm{Sym}^m(V)$ for some integer $1 \leq m \leq |G|$ (that may depend on $W$).
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Automorphisms and twisted forms of quotient singularities; notes on a paper of Bresciani--Vistoli
A recent paper of Bresciani and Vistoli studies fields of moduli using residual gerbes. One of their applications is to twisted forms of quotient singularities. We discuss a direct approach to automorphisms and twisted forms, which also works in many wildly ramified cases.
math.AG↗
Automorphisms and twisted forms of rings of invariants
Let $G\subset GL_n(k)$ be a finite subgroup and $k[x_1,\dots, x_n]^G\subset k[x_1,\dots, x_n]$ its ring of invariants. We show that, in many cases, the automorphism group of $k[x_1,\dots, x_n]^G$ is $k^\times$. Version 2: Incorporates parts of arXiv:2210.16265.
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On the local étale fundamental group of KLT threefold singularities
Let $S$ be KLT threefold singularity over an algebraically closed field of positive characteristic $p>5$. We prove that its local étale fundamental group is tame and finite. Further, we show that every finite unipotent torsor over a big open of $S$ is realized as the restriction of a finite unipotent torsor over $S$.
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Stable maps of curves and algebraic equivalence of 1-cycles
We show that algebraic equivalence of images of stable maps of curves lifts to deformation equivalence of the stable maps. The main applications concern $A_1(X)$, the group of 1-cycles modulo algebraic equivalence, for smooth, separably rationally connected varieties. If $K/k$ is an algebraic extension, then the kernel of $A_1(X_k)\to A_1(X_K)$ is at most ${\mathbb Z}/2{\mathbb Z}$. If $k$ is finite, then the image equals the subgroup of Galois invariant cycles. This paper replaces Sections~2--3 of 2211.15915.v.1 and Sections~2--3 of 2211.15911.v.1. The other Sections are retained in the revised versions of these papers.
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Families of stable 3-folds in positive characteristic
We show that flat families of stable 3-folds do not lead to proper moduli spaces in any characteristic $p>0$. As a byproduct, we obtain log canonical 4-fold pairs, whose log canonical centers are not weakly normal.
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Du Bois property of log centers
In this note we generalize the results of [KK10] and [KK20] by showing that if a closed subset V of X is "close enough" to being a union of log canonical centers, then it is Du Bois.
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A Fulton-Hansen theorem for almost homogeneous spaces
We prove a generalization of the Fulton-Hansen connectedness theorem, where ${\mathbb P}^n$ is replaced by a normal variety on which an algebraic group acts with a dense orbit.
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The log canonical threshold and rational singularities
We show that if $f$ is a nonzero, noninvertible function on a smooth complex variety $X$ and $J_f$ is the Jacobian ideal of $f$, then ${\rm lct}(f,J_f^2)>1$ if and only if the hypersurface defined by $f$ has rational singularities. Moreover, if it does not have rational singularities, then ${\rm lct}(f,J_f^2)={\rm lct}(f)$. We give two proofs, one relying on arc spaces and one that goes through the inequality $\widetildeα(f)\geq{\rm lct}(f,J_f^2)$, where $\widetildeα(f)$ is the minimal exponent of $f$. In the case of a polynomial over $\overline{\mathbf{Q}}$, we also prove an analogue of this latter inequality, with $\widetildeα(f)$ replaced by the motivic oscillation index ${\rm moi}(f)$. We also show a part of Igusa's strong monodromy conjecture, for poles larger than $-{\rm lct}(f,J_f^2)$. We end with a discussion of lct-maximal ideals: these are ideals $I$ with the property that ${\rm lct}(I)<{\rm lct}(J)$ for every $J$ with $I\subsetneq J$.
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Bounded meromorphic functions on the complex 2-disc
We describe bounded, holomorphic functions on the complex 2-disc, that admit meromorphic extension to a larger 2-disc. This solves a conjecture of Bickel, Knese, Pascoe and Sola. The key technical ingredient is an old theorem of Zariski about integrally closed ideals in 2-dimensional regular rings. v2: Remark 15 is added.
math.CV↗
Deformations of varieties of general type
We prove that small deformations of a projective variety of general type are also projective varieties of general type, with the same plurigenera. Version 2: small changes in first half. Improved version of the second half is now a separate preprint (Seshadri's criterion and openness of projectivity). Version 3: Footnote corrects citation on p.4.
math.AG↗