arXiv subjects

Katharina Hübner

Publications and source records attributed to Katharina Hübner.

10 recordsLinked to original sources

arXiv2026

Tame fundamental groups of rigid spaces

We introduce the tame étale fundamental group $π_1^t(X/K)$ of a rigid space X over a non-archimedean field K. We show that if X is qcqs and K has topologically finitely generated tame Galois group (e.g. algebraically closed or a local field), then $π_1^t(X/K)$ is topologically finitely generated. If X is moreover the rigid generic fibre of a strictly semistable formal scheme such that the smooth locus of its special fibre admits a projective snc compactification, then $π_1^t(X/K)$ is topologically finitely presented. The proofs rely on techniques of logarithmic geometry (extended beyond its usual scope of finitely generated monoids), in particular on an analogous finiteness statement for the tame log étale fundamental group, and on the 'vertical compactification' of a map of adic spaces.

math.AG↗

arXiv2025

Tame proper base change for discretely ringed adic spaces

We consider a proper morphism $X \to S$ and a locally closed immersion $S' \to S$ of discretely ringed adic spaces and prove proper base change for the tame topology in this setting. More precisely, we show that for an abelian $p$-torsion sheaf ($p = char^+(S)$) on the tame site of $X$ that the base change homomorphism for the derived pushforward along $X \to S$ with the pullback along $S' \to S$ is an isomorphism.

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arXiv2024

Logarithmic geometry beyond fs

We develop the foundations of logarithmic structures beyond the standard finiteness conditions. The motivation is the study of semistable models over general valuation rings. The key new notion is that of a morphism of finite presentation up to saturation (sfp), which is one that is qcqs and which is locally isomorphic to the saturated base change of a finitely presented morphism between fs log schemes. As in the case of schemes, sfp maps can (locally on the base) be approximated by maps between fs log schemes of finite type over $\mathbb{Z}$. Based on sfp maps, we define smooth, étale, and Kummer étale maps. Importantly, the maps of schemes underlying such maps are no longer of finite type in general, though surprisingly they are if the base is the spectrum of a valuation ring with algebraically closed field of fractions. These foundations allow us to extend beyond the fs case the theory of the Kummer étale site and of the Kummer étale fundamental group.

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arXiv2024

Logarithmic differentials on discretely ringed adic spaces

On a smooth discretely ringed adic space $\mathcal{X}$ over a field $k$ we define a subsheaf $Ω_{\mathcal{X}}^+$ of the sheaf of differentials $Ω_{\mathcal{X}}$. It is defined in a similar way as the subsheaf $\mathcal{O}^+_{\mathcal{X}}$ of $\mathcal{O}_{\mathcal{X}}$ using Kähler seminorms on $Ω_{\mathcal{X}}$. We give a description of $Ω^+_{\mathcal{X}}$ in terms of logarithmic differentials. If $\mathcal{X}$ is of the form $\mathrm{Spa}(X,\bar{X})$ for a scheme $\bar{X}$ and an open subscheme $X$ such that the corresponding log structure on $\bar{X}$ is smooth, we show that $Ω^+_{\mathcal{X}}(\mathcal{X})$ is isomorphic to the logarithmic differentials of $(X,\bar{X})$.

math.AG↗

arXiv2024

Adic curves: stable reduction, skeletons and metric structure

We study the structure of adic curves over an affinoid field of arbitrary rank. In particular, quite analogously to Berkovich geometry we classify points on curves, prove a semistable reduction theorem in the version of Ducros' triangulations, define associated curve skeletons and prove that they are deformational retracts in a suitable sense. An important new technical tool is an appropriate compactification of ordered groups that we call the ranger compactification. Intervals of rangers are then used to define metric structures and construct deformational retractions.

math.AG↗

arXiv2024

Adic spaces

These lecture notes are based on the second course in a series of lectures at the Spring school "Non-archimedean geometry and Eigenvarieties" in March 2023 in Heidelberg. The objective of the first three courses was to give an introduction to the theory of adic spaces. Building up on the theory of Huber pairs presented in John Bergdall's lecture we explain the construction of adic spaces. We study some important classes of adic spaces such as rigid analytic spaces and formal schemes and show the connections between them. In the course of the lecture we will illustrate the respective concepts with the fundamental examples of the open and closed disc and the affine line.

math.NT↗

arXiv2021

The adic tame site

For every adic space $Z$ we construct a site $Z_t$, the tame site of $Z$. For a scheme $X$ over a base scheme $S$ we obtain a tame site by associating with $X/S$ an adic space $\textit{Spa}(X,S)$ and considering the tame site $\textit{Spa}(X,S)_t$. We examine the connection of the cohomology of the tame site with étale cohomology and compare its fundamental group with the conventional tame fundamental group. Finally, assuming resolution of singularities, for a regular scheme $X$ over a base scheme $S$ of characteristic $p > 0$ we prove a cohomological purity theorem for the constant sheaf $\mathbb{Z}/p\mathbb{Z}$ on $\textit{Spa}(X,S)_t$. As a corollary we obtain homotopy invariance for the tame cohomology groups of $\textit{Spa}(X,S)$.

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arXiv2020

The tame site of a scheme

Étale cohomology with non-invertible coefficients has some unpleasant properties, e.g., it is not A^1-homotopy invariant and for constructible coefficients the expected finiteness properties do not hold. In this paper we introduce the `tame site' which is slightly coarser than the étale site. Tame cohomology coincides with étale cohomology for invertible coefficients but is better behaved in the general case. The fundamental group of the tame site is the (curve-)tame fundamental group of Wiesend and Kerz/Schmidt. The higher tame homotopy groups hopefully have a better behaviour than the higher étale homotopy groups, which vanish for affine schemes in positive characteristic by a result of Achinger.

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arXiv2020

Tame and strongly étale cohomology of curves

For a curve $C$ over a perfect field $k$ of characteristic $p > 0$ we study the tame cohomology of $X = \textit{Spa}(C,k)$ introduced in arXiv:1801.04776. We prove that the tame cohomology groups of $X$ with $p$-torsion coefficients satisfy cohomological purity (which is not true in full generality for the étale cohomology). Using purity we show Poincaré duality for the tame cohomology of $X$ with $p$-torsion coefficients.

math.AG↗

arXiv2018

Aspherical neighborhoods on arithmetic surfaces: the local case

On arithmetic surfaces over local rings of integers we examine whether any geometric point has a basis of étale neighborhoods whose $\mathfrak{c}$-completed étale homotopy types are of type $K(π,1)$ for a given full class~$\mathfrak{c}$ of finite groups.

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