arXiv subjects

Christopher Lazda

Publications and source records attributed to Christopher Lazda.

22 records · Page 2Linked to original sources

arXiv2015

Rigid cohomology over Laurent series field I: First definitions and basic properties

This is the first in a series of papers in which we construct and study a new $p$-adic cohomology theory for varieties over Laurent series fields $k(\!(t)\!)$ in characteristic $p$. This will be a version of rigid cohomology, taking values in the bounded Robba ring $\mathcal{E}_K^\dagger$, and in this paper, we give the basic definitions and constructions. The cohomology theory we define can be viewed as a relative version of Berthelot's rigid cohomology, and is constructed by compactifying $k(\!(t)\!)$-varieties as schemes over $k[\![ t]\!]$ rather than over $k(\!(t)\!)$. We reprove the foundational results necessary in our new context to show that the theory is well defined and functorial, and we also introduce a category of `twisted' coefficients. In latter papers we will show some basic structural properties of this theory, as well as discussing some arithmetic applications including the weight monodromy conjecture and independence of $\ell$ results for equicharacteristic local fields.

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arXiv2015

Rigid cohomology over Laurent series fields II: Finiteness and Poincaré duality for smooth curves

In this paper we prove that the $\mathcal{E}^\dagger_K$-valued cohomology, introduced in [9] is finite dimensional for smooth curves over Laurent series fields $k((t))$ in positive characteristic, and forms an $\mathcal{E}^\dagger_K$-lattice inside `classical' $\mathcal{E}_K$-valued rigid cohomology. We do so by proving a suitable version of the p-adic local monodromy theory over $\mathcal{E}^\dagger_K$, and then using an étale pushforward for smooth curves to reduce to the case of $\mathbb{A}^1$. We then introduce $\mathcal{E}^\dagger_K$-valued cohomology with compact supports, and again prove that for smooth curves, this is finite dimensional and forms an $\mathcal{E}^\dagger_K$-lattice in $\mathcal{E}_K$-valued cohomology with compact supports. Finally, we prove Poincaré duality for smooth curves, but with restrictions on the coefficients.

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arXiv2015

Rigid cohomology over Laurent series fields III: Absolute coefficients and arithmetic applications

In this paper we investigate the arithmetic aspects of the theory of $\mathcal{E}_K^\dagger$-valued rigid cohomology introduced and studied in [11,12]. In particular we show that these cohomology groups have compatible connections and Frobenius structures, and therefore are naturally $(φ,\nabla)$-modules over $\mathcal{E}_K^\dagger$ whenever they are finite dimensional. We also introduce a category of `absolute' coefficients for the theory; the same results are true for cohomology groups with coefficients. We moreover prove a $p$-adic version of the weight monodromy conjecture for smooth (not necessarily proper) curves, and use a construction of Marmora to prove a version of $\ell$-independence for smooth curves over $k(\!(t)\!)$ that includes the case $\ell=p$. This states that after tensoring with $\mathcal{R}_K$, our $p$-adic cohomology groups agree with the $\ell$-adic Galois representations $H^i_{\mathrm{ét}}(X_{k(\!(t)\!)^\mathrm{sep}},\mathbb{Q}_\ell)$ for $\ell\neq p$.

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arXiv2013

A $p$-adic Bertini theorem for unipotent local systems

In this short note we prove a version of Bertini's theorem for unipotent rigid fundamental groups, stating that for every smooth, projective, geometrically connected variety $X$ over an infinite perfect field $k$ of characteristic $p>0$, there exists a smooth, projective, geometrically connected curve $C\subset X$ such that the induced map on rigid fundamental groups is surjective.

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