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Quentin Guignard

Publications and source records attributed to Quentin Guignard.

5 recordsLinked to original sources
arXiv2021

Compatible systems of $\ell$-adic sheaves

We introduce a notion of compatibility for families $(\mathcal{F}_{\ell})_{\ell}$ of bounded constructible $\ell$-adic complexes of étale sheaves on schemes. For schemes of finite type over a field, this notion is preserved by the usual six functors. We prove that the compatibility of a family is preserved by the nearby cycles functor and by the linearized $\varepsilon$-factors introduced recently by the author. We establish independence of $\ell$ for the characteristic cycles and characteristic $\varepsilon$-cycles of compatible families.

math.AG↗
arXiv2019

Geometric local $\varepsilon$-factors in higher dimensions

We use former results on geometric local $\varepsilon$-factors over curves in order to prove a factorization result for the determinant of the cohomology of an $\ell$-adic sheaf over an arbitrary proper scheme over a perfect field of positive characteristic $p$ distinct from $\ell$. The local contributions are constructed by iterating vanishing cycle functors as well as certain "refined Artin conductors", the latter being exact additive functors which can be considered as linearized versions of Artin conductors and local $\varepsilon$-factors. We provide several applications of our higher dimensional product formula, such as twist formulas for global $\varepsilon$-factors.

math.AG↗
arXiv2019

Geometric local epsilon factors

Inspired by the work of Laumon on $\varepsilon$-factors and by Deligne's $1974$ letter to Serre, we give an explicit cohomological definition of $\varepsilon$-factors for $\ell$-adic Galois representations over henselian discrete valuation fields of positive equicharacteristic $p \neq \ell$, with (not necessarily finite) perfect residue fields. These geometric local $\varepsilon$-factors are completely characterized by an explicit list of purely local properties, such as an induction formula and the compatibility with geometric class field theory in rank $1$, and satisfy a product formula for $\ell$-adic sheaves on a curve over a perfect field of characteristic $p$.

math.AG↗
arXiv2019

A new proof of Raynaud-Gruson's flattening theorem

We give a new proof of Raynaud-Gruson's theorem regarding flattening by blow-ups. The proof is direct, by working directly on the inverse limit of admissible blow-ups, which is a valuative space similar to the classical Zariski-Riemann space. These valuative spaces are defined in a broader context and always have an analogous flattening property: this yields a generalization of Raynaud-Gruson's theorem.

math.AG↗