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Jean-Baptiste Teyssier

Publications and source records attributed to Jean-Baptiste Teyssier.

At least 19 recordsLinked to original sources
arXiv2026

Semi-continuity for conductor divisors of étale sheaves

In this article, we prove a semi-continuity property for both conductor divisors and logarithmic conductor divisors for étale sheaves on higher relative dimensions in a geometric situation. It generalizes a semi-continuity result for conductors of étale sheaves on relative curves to higher relative dimensions, and it can be considered as a higher dimensional $\ell$-adic analogy of André's result on the semi-continuity of Poincaré-Katz ranks of meromorphic connections on smooth relative curves.

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arXiv2026

Characteristic cycle and wild Lefschetz theorems

By relying on a new approach to Lefschetz type questions based on Beilinson's singular support and Saito's characteristic cycle, we prove an instance of the wild Lefschetz theorem envisioned by Deligne. Our main tool are new finiteness results for the characteristic cycles of perverse sheaves.

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arXiv2026

The derived moduli of perverse sheaves

We construct higher derived Artin stacks parametrizing constructible sheaves on complex algebraic varieties and compact real analytic varieties. Furthermore, we show that every perversity function gives rise to an open substack of perverse sheaves, which is a 1-Artin stack locally of finite presentation that generalizes usual character stacks. As a sample application of the derived structure, we construct new examples of cohomological Hall algebras associated to punctured Riemann surfaces.

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arXiv2026

Exodromy beyond conicality

We show that compact subanalytic stratified spaces and algebraic stratifications of real varieties have finite exit-path $\infty$-categories, refining classical theorems of Lefschetz-Whitehead, Lojasiewicz, and Hironaka on the finiteness of the underlying homotopy types of these spaces. These stratifications are typically not conical; hence we cannot rely on the currently available exodromy equivalence between constructible sheaves on a stratified space, which requires conicality as a fundamental hypothesis. Building on ideas of Clausen and Jansen, we study the class of exodromic stratified spaces, for which the conclusion of the exodromy theorem holds. We prove two new fundamental properties of this class of stratified spaces: coarsenings of exodromic stratifications are exodromic, and every morphism between exodromic stratified spaces induces a functor between the associated exit path $\infty$-categories. As a consequence, we produce many new examples of exodromic stratified spaces, including: coarsenings of conical stratifications, locally finite subanalytic stratifications of real analytic spaces, and algebraic stratifications of real varieties. Our proofs are at the generality of stratified $\infty$-topoi, hence apply to even more general situations such as stratified topological stacks. In a subsequent paper, we use the previously mentioned finiteness results to construct derived moduli stacks of constructible and perverse sheaves.

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arXiv2026

Topological exodromy with coefficients

The exodromy equivalence relates the derived $\infty$-category of constructible sheaves on a stratified space (X,P) with the $\infty$-category of representations of the exit-paths $\infty$-category of (X,P). Originally envisioned by MacPherson, it has been rigorously developed by Treumann and later improved by Lurie. This paper provides a new proof of the strongest version of this equivalence. This allows us to remove several limitations from Lurie's treatement; for instance we prove that the exodromy equivalence is functorial in arbitrary morphism of stratified spaces. We also remove all noetherianity assumptions, consider more general coefficients (e.g. compactly assembled or stable presentable $\infty$-categories), and we allow stratified spaces that have locally weakly contractible strata, rather than being locally of singular shape.

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arXiv2025

Bounding ramification with coherent sheaves

Given a coherent sheaf E on a scheme of finite type X over a perfect field, we introduce a category of complexes of étale sheaves on X with logarithmic conductors bounded by E and study its compatibilities with finite push-forward.

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arXiv2025

Standard $t$-structures

We provide a general construction of induced $t$-structures, that generalizes standard $t$-structures for $\infty$-categories of sheaves. More precisely, given a presentable $\infty$-category $\mathcal{X}$ and a presentable stable $\infty$-category $\mathcal{E}$ equipped with an accessible $t$-structure $τ= (\mathcal{E}_{\geq 0}, \mathcal{E}_{\leq 0})$, we show that $\mathcal{X} \otimes \mathcal{E}$ is equipped with a canonical $t$-structure whose coconnective part is given in $\mathcal{X} \otimes \mathcal{E}_{\leq 0}$. When $\mathcal{X}$ is an $\infty$-topos, we give a more explicit description of the connective part as well.

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arXiv2025

The derived moduli of Stokes data

The goal of this paper is to show that Stokes data coming from flat bundles form a locally geometric derived stack locally of finite presentation. This generalizes existing geometricity results on Stokes data in four different directions: our result applies in any dimension, $\infty$-categorical coefficients are allowed, derived structures on moduli spaces are considered and more general spaces than those arising from flat bundles are permitted.

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arXiv2025

Estimates for Betti numbers and relative Hermite-Minkowski theorem for perverse sheaves

We prove estimates for the Betti numbers of constructible sheaves in characteristic p>0 depending only on their rank, stratification and wild ramification. In particular, given a smooth proper variety of dimension n over an algebraically closed field and a divisor D of X, for every $0\leq i \leq n$, there is a polynomial $P_i$ of degree $\max \{i,2n-i\}$ such that the i-th Betti number of any rank r local system L on X-D is smaller than $P_i(lc_D(L))\cdot r$ where $lc_D(L)$ is the highest logarithmic conductor of L at the generic points of D. As application, we show that the Betti numbers of the inverse and higher direct images of a local system are controlled by the rank and the highest logarithmic conductor. We also reprove Deligne's finiteness for simple $\ell$-adic local systems with bounded rank and ramification on a smooth variety over a finite field and extend it in two different directions. In particular, perverse sheaves over arbitrary singular schemes are allowed and the bounds we obtain are uniform in algebraic families and do not depend on $\ell$.

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arXiv2023

Cohomological boundedness for flat bundles on surfaces and applications

This paper explores the cohomological consequences of the existence of moduli spaces for flat bundles with bounded rank and irregularity at infinity and gives unconditional proofs. Namely, we prove the existence of a universal bound for the dimension of De Rham cohomology of flat bundles with bounded rank and irregularity on surfaces. In any dimension, we prove a Lefschetz recognition principle stating the existence of hyperplane sections distinguishing flat bundles with bounded rank and irregularity after restriction. We obtain in any dimension a universal bound for the degrees of the turning loci of flat bundles with bounded rank and irregularity. Along the way, we introduce a new operation on the group of b-divisors on a smooth surface (the partial discrepancy) and prove a closed formula for the characteristic cycles of flat bundles on surfaces in terms of the partial discrepancy of the irregularity b-divisor attached to any flat bundle by Kedlaya.

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arXiv2022

The homotopy-invariance of constructible sheaves

The purpose of this paper is to explain why the functor that sends a stratified topological space $S$ to the $\infty$-category of constructible (hyper)sheaves on $S$ with coefficients in a large class of presentable $\infty$categories is homotopy-invariant. To do this, we first establish a number of results in the unstratified setting, i.e., the setting of locally constant (hyper)sheaves. For example, if $X$ is a locally weakly contractible topological space and $\mathcal{E}$ is a presentable $\infty$-category, then we give a concrete formula for the constant hypersheaf functor $\mathcal{E}\to \mathrm{Sh}^{\mathrm{hyp}}(X;\mathcal{E})$. This formula lets us show that the constant hypersheaf functor is a right adjoint, and is fully faithful if $X$ is also weakly contractible. It also lets us prove a general monodromy equivalence and categorical Künneth formula for locally constant hypersheaves.

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arXiv2022

Characteristic cycle and wild ramification for nearby cycles of étale sheaves

In this article, we give a bound for the wild ramification of the monodromy action on the nearby cycles complex of a locally constant étale sheaf on the generic fiber of a smooth scheme over an equal characteristic trait in terms of Abbes and Saito's logarithmic ramification filtration. This provides a positive answer to the main conjecture in Isabel Leal's article "On the ramification of étale cohomology groups" for smooth morphisms in equal characteristic. We also study the ramification along vertical divisors of étale sheaves on relative curves and abelian schemes over a trait.

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arXiv2019

Moduli of Stokes torsors and singularities of differential equations

Let M be a meromorphic connection with poles along a smooth divisor D in a smooth algebraic variety. Let Sol M be the solution complex of M. We prove that the good formal decomposition locus of M coincides with the locus where the restrictions to D of Sol M and Sol End M are local systems. By contrast to the very different natures of these loci (the first one is defined via algebra, the second one is defined via analysis), the proof of their coincidence is geometric. It relies on the moduli of Stokes torsors.

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arXiv2017

Skeletons and moduli of Stokes torsors

We prove an analogue for Stokes torsors of Deligne's skeleton conjecture and deduce from it the representability of the functor of relative Stokes torsors by an affine scheme of finite type over C. This provides, in characteristic 0, a local analogue of the existence of a coarse moduli for skeletons with bounded ramification, due to Deligne. As an application, we use the geometry of this moduli to derive quite strong finiteness results for integrable systems of differential equations in several variables which did not have any analogue in one variable.

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arXiv2015

Nearby slopes and boundedness for $\ell$-adic sheaves in positive characteristic

The goal of this paper is to motivate a boundedness conjecture on nearby slopes of $\ell$-adic sheaves in positive characteristic, and to prove it for smooth curves. For a constructible $\ell$-adic sheaf, we prove the finiteness of the set of nearby slopes associated to a given morphism. For the constant sheaf, we prove the vanishing of nearby slopes in case of generalized semi-stable reduction.

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arXiv2015

La construction d'Abbes et Saito pour les connexions méromorphes: aspect formel en dimension 1

By using a blow-up construction, the nearby-cycle functor and l-adic Fourier transform, Abbes and Saito are able to define a geometric measure of wild ramification of l-adic sheaves on the generic point of any complete discrete valuation ring of equal characteristic p with perfect residue field, where p is different from l. In this paper, we adapt their construction to differential modules over the field of formal Laurent series with coefficients in any characteristic zero field K. For such a module M, we prove a formula relating Abbes and Saito's construction to the differential forms occuring in the Levelt-Turrittin decomposition of M. If K is algebraically closed, one recovers a version of Laurent's micro-characteristic cycles.

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