arXiv subjects
Publications and source records attributed to Jefferson Baudin.
We prove a Witt vector version of the usual Grauert-Riemenschneider vanishing theorem over fields of positive characteristic, solving a question raised by Blickle, Esnault, Chatzistamatiaou and Rülling. We then deduce some rationality consequences for $F$-rational singularities.
We solve certain questions related to the geometry and singularities of quasi-$F$-split varieties with trivial canonical bundle. First, we prove that regular quasi-$F^{\infty}$-split varieties are not geometrically uniruled (this generalizes and significantly simplifies the earlier results of Patakfalvi and Zdanowicz) and have geometrically canonical singularities. Second, we show that there exist quasi-$F$-split surfaces with trivial canonical bundle which are not quasi-$F^{\infty}$-split, answering negatively a question raised by Kawakami, Takamatsu, Tanaka, Witaszek, Yobuko and Yoshikawa. Third, we show that normal quasi-$F$-split varieties with trivial canonical bundle are geometrically normal (this extends a result of Kawakami, Takamatsu and Yoshikawa), and finally we prove that quasi-$F^e$-pure normal varieties $X$ such that $mp^eK_X$ is Cartier for $m$ coprime to $p$ are log canonical, under a resolution of singularities hypothesis.
We obtain effective characterizations of complex semi-abelian varieties in arbitrary dimension in terms of logarithmic irregularity and the first two logarithmic plurigenera, among varieties of maximal Albanese dimension, and among varieties whose compactification has large irregularity (these hypotheses are sharp). To the authors' knowledge, this is the first result in this direction for quasi-projective varieties in arbitrary dimension.
We prove that for $n \leq 4$ and $p > 5$, quasi--Gorenstein $F$--pure and $\mathbb{Q}_p$--rational $n$--fold singularities are canonical. This is analogous to the usual fact that rational Gorenstein singularities are canonical. The proof is based on a careful analysis of the dual complex of a dlt modification of a log canonical singularity. The result for $n = 4$ is contingent upon the existence of log resolutions.
We simplify and improve the main fundamental theorems of positive characteristic generic vanishing theory. As a quick corollary of the theory, we prove that a normal variety $X$ of maximal Albanese dimension satisfies $H^0(X, ω_X) \neq 0$ and that if $\mathrm{Alb}(X)$ is ordinary, then $S^0(X, ω_X) \neq 0$.
We show that the Frobenius--stable version of the Grauert--Riemenschneider vanishing theorem fails for threefolds in any positive characteristic, and for terminal 3-folds in characteristic $p \in \{2, 3, 5\}$. To prove this, we introduce the notion of $\mathbb{F}_p$-rationality for singularities in positive characteristic and we show that klt singularities in dimension at most 4 are $\mathbb{F}_p$-rational. We apply this to prove a Frobenius--stable version of the Kawamata--Viehweg vanishing theorem on $K$-trivial 3-folds.
Extending Enriques' characterization to algebraically closed fields of characteristic $p \geq 7$, we show that every smooth projective surface $X$ with $h^1(X, \mathcal{O}_X) = 2$ and $p_1(X) = p_2(X) = 1$ is birational to an Abelian surface. This characterization fails if $p \leq 5$, and we give a sharp alternative.
We prove that the Albanese morphism of any normal proper variety $X$ in positive characteristic satisfying $S^0(X, ω_X) \neq 0$ and $P_2(X) = 1$ is surjective with connected fibers, adn that $\mathrm{Alb}(X)$ is ordinary. We obtain from a variant of the above a purely positive characteristic proof of Chen and Hacon's effective birational characterization of complex abelian varieties.
We show that a smooth proper weakly ordinary variety $X$ of maximal Albanese dimension satisfies $χ(X, ω_X) \geq 0$. We also show that if $X$ is not of general type, then $χ(X, ω_X) = 0$ and the Albanese image of $X$ is fibered by abelian varieties. The proof uses the positive characteristic generic vanishing theory developed by Hacon-Patakfalvi, as well as our recent Witt vector version of Grauert-Riemenschneider vanishing.
We show that in positive characteristic, the Albanese morphism of normal proper varieties $X$ with $κ_S(X, ω_X) = 0$ is separable, surjective, has connected fibers, and the generic fiber $F$ also satisfies $κ(F, ω_F) = 0$. As a corollary, we deduce a new case of Iitaka's subadditivity conjecture for fibrations over abelian varieties, when the generic fiber has non-nilpotent Hasse-Witt matrix.
Given a Cohen-Macaulay scheme of klt type $X$ and a resolution $π\colon Y\to X$, we show that $R^1π_*ω_Y=0$. We deduce that if $\mathrm{dim}(X)=3$, then $X$ satisfies Grauert-Riemenschneider vanishing and therefore has rational singularities. We also obtain that in arbitrary dimension, if $X$ is of finite type over a perfect field of characteristic $p>0$, then $X$ has $\mathbb{Q}_p$-rational singularities.
We show that on any Noetherian $F$-finite $\mathbb{F}_p$-scheme, there is an anti-equivalence of categories between Cartier crystals and étale perverse $\mathbb{F}_p$-sheaves, commuting with derived proper pushforwards. We use this duality to construct an upper shriek functor for Cartier crystals, and give new proofs of Kashiwara's equivalence and the finite length of Cartier crystals. Finally, we deduce a generic vanishing statement for perverse $\overline{\mathbb{F}}_p$-sheaves on abelian varieties of characteristic $p > 0$, reminiscent of the characteristic zero and $l$-adic statements.
We generalize the results in [Bau23] to obtain a duality between $W_n$-Cartier crystals and perverse $\mathbb{Z}/p^n\mathbb{Z}$-sheaves.