arXiv subjects

Geoff Vooys

Publications and source records attributed to Geoff Vooys.

10 recordsLinked to original sources

arXiv2026

A Deep Dive Into the Tangent Category of Schemes

In this largely expository paper we provide a deep and explicit exploration and exposition of the tangent structure on the category of schemes $\mathbf{Sch}_{/S}$ whose tangent functor $T(X) = T_{X/S}$ is the relative tangent scheme of Grothendieck described in \emph{\'El\'ements de G\'eom\'etrie Alg\'ebrique} 4. In particular we provide explicit descriptions of the ways that the bifibration of quasicoherent sheaves and bifbration of quesicoherent sheaves of algebras over schemes may be built from the ways in which the bifibrations of modules and commutative algebras over commutative rings interact. We also show the ways in which these interactions give rise to an explicit description of the standard tangent structure on the category of schemes in terms of sheaves of K\"ahler differentials, properties of the relative spectrum functor, and more. Finally, we show that quasi-coherent sheaves can be reconstructed from their category of differential bundles by showing that for quasi-separated schemes $X$ and $Y$, there is an isomorphism $X \cong Y$ if and only if there is an equivalence of categories $\mathbf{DBun}(X) \simeq \mathbf{DBun}(Y)$.

math.AG↗

arXiv2025

Important Classes of Morphisms and the Relative Cotangent Sequence in Tangent Categories

In this paper we provide a deep and systematic study of what it means to be an immersion, a submersion, a local diffeomorphism, and unramified in a tangent category. We also give a systematic study of the ways in which these classes of morphisms interact, their properties, and give very explicit and concrete characterizations of how each class appears in algebraic geometry, differential geometry, algebra, and in Cartesian differential categories. Additionally, we discuss the notion of being carrable with respect to the tangent bundle projection, then use this to define the notion of horizontal descent in a tangent category, which we then use as a key tool to study the aforementioned classes of morphisms. In particular, we use this to define a de Rham relative cotangent complex in an arbitrary tangent category.

math.CT↗

arXiv2024

Categories of Pseudocones and Equivariant Descent

In this monograph we provide an in-depth and systematic study of pseudolimits of pseudofunctors $F:\mathscr{C}^{op} \to \mathfrak{Cat}$ in the $2$-category of categories where $\mathscr{C}$ is a $1$-category and use this to give an explicit and careful study of the category theory used in representation theory, equivariant algebraic geometry, and equivariant algebraic topology and give a unifying language to study equivariant sheaves, equivariant perverse sheaves, and their equivariant derived categories. We show how to use the pseudocone construction $\mathsf{Bicat}(\mathscr{C}^{op},\mathfrak{Cat})(\operatorname{cnst}(1),F)$ in order to derive categorical and homological properties of the pseudolimit of $F$. We explicitly show when the pseudolimit of $F$ is complete, cocomplete, enriched in models of a Lawvere theory, (braided) monoidal, regular, triangulated, admits $t$-structures, and more. We use these various structural results to give a new category-theoretic proof and construction of the equivariant standard and pervese $t$-structures and equivariant six functor formalism for the equivariant derived category $D_G^b(X)$ in both the geometric and topological cases as well as for $D_G^b(X;\overline{\mathbb{Q}}_{\ell})$ in the geometric case. We also show in what sense precise sense we can view the equivariant derived category in terms of localizations. After restricting to the case of group resolution categories, we show the existence of a natural isomorphism $\Theta:\alpha_X^{\ast} \Rightarrow \pi_2^{\ast}$ which satisfies a pseudofunctorial version of the cocycle condition $d_1^{\ast}\Theta = d_2^{\ast}\Theta \circ d_0^{\ast}\Theta$. We also use the pseudocone formalism to give an in-depth analysis of change of groups functors. We use the pseudocone formalism and $\Theta$ to develop a notion of equivariant trace with an eye towards the representation theory of $p$-adic groups.

math.AG↗

arXiv2024

On the Equivariant Derived Category of Perverse Sheaves

In this paper we extend Beilinson's realization formalism for triangulated categories and filtered triangulated categories to a pseudofunctorial and pseudonatural setting. As a consequence we prove an equivariant version of Beilinson's Theorem: for any algebraic group $G$ over an algebraically closed field $K$ and for any $G$-variety $X$, there is an equivalence of categories $D_G^b(X; \overline{\mathbb{Q}}_{\ell}) \simeq D_G^b(\mathbf{Perv}(X;\overline{\mathbb{Q}}_{\ell}))$ where $\ell$ is an integer prime coprime to the characteristic of $K$. We also show that the equivariant analogues of the other non-$D$-module aspects of Beilinson's Theorem hold in the equivariant case.

math.AG↗

arXiv2023

Measurable Functions and Topolgical Algebra

In this paper we show that if $(X,\mathcal{A})$ is a measurable space and if $Y$ is a topological model of a Lawvere theory $\mathcal{T}$ equipped with $\mathcal{B}$ the Borel $\sigma$-algebra on $Y$, then the set of $\mathcal{B}$-measurable functions from $X$ to $Y$, $\operatorname{Meas}(X,Y)$, is a set-theoretic model of $\mathcal{T}$. As a corollary we give short proofs of the facts that the set of real-valued measurable functions on a measurable space $X$ is a ring and the set of complex-valued measurable functions from $X$ to $\mathbb{C}$ is a ring.

math.FA↗

arXiv2023

Pseudolimits for Tangent Categories with Applications to Equivariant Algebraic and Differential Geometry

In this paper we show that if $\mathscr{C}$ is a category and if $F\colon\mathscr{C}^{\operatorname{op}} \to \mathfrak{Cat}$ is a pseudofunctor such that for each object $X$ of $\mathscr{C}$ the category $F(X)$ is a tangent category and for each morphism $f$ of $\mathscr{C}$ the functor $F(f)$ is part of a strong tangent morphism $(F(f),{}_{f}{\alpha})$ and that furthermore the natural transformations ${}_{f}{\alpha}$ vary pseudonaturally in $\mathscr{C}^{\operatorname{op}}$, then there is a tangent structure on the pseudolimit $\mathbf{PC}(F)$ which is induced by the tangent structures on the categories $F(X)$ together with how they vary through the functors $F(f)$. We use this observation to show that the forgetful $2$-functor $\operatorname{Forget}:\mathfrak{Tan} \to \mathfrak{Cat}$ creates and preserves pseudolimits indexed by $1$-categories. As an application, this allows us to describe how equivariant descent interacts with the tangent structures on the category of smooth (real) manifolds and on various categories of (algebraic) varieties over a field.

math.CT↗

arXiv2023

Tangent Ind-Categories

In this paper we show that if $\mathscr{C}$ is a tangent category then the Ind-category $\operatorname{Ind}(\mathscr{C})$ is a tangent category as well with a tangent structure which locally looks like the tangent structure on $\mathscr{C}$. Afterwards we give a pseudolimit description of $\operatorname{Ind}(\mathscr{C})_{/X}$ when $\mathscr{C}$ admits finite products, show that the $\operatorname{Ind}$-tangent category of a representable tangent category remains representable (in the sense that it has a microlinear object), and we characterize the differential bundles in $\operatorname{Ind}(\mathscr{C})$ when $\mathscr{C}$ is a Cartesian differential category. Finally we compute the $\operatorname{Ind}$-tangent category for the categories $\mathbf{CAlg}_{A}$ of commutative $A$-algebras, $\mathbf{Sch}_{/S}$ of schemes over a base scheme $S$, $A$-$\mathbf{Poly}$ (the Cartesian differential category of $A$-valued polynomials), and $\mathbb{R}$-$\mathbf{Smooth}$ (the Cartesian differential category of Euclidean spaces). In particular, during the computation of $\operatorname{Ind}(\mathbf{Sch}_{/S})$ we give a definition of what it means to have a formal tangent scheme over a base scheme $S$.

math.CT↗

arXiv2023

Equivariant Functors and Sheaves

In this thesis we study two main topics which culminate in a proof that four distinct definitions of the equivariant derived category of a smooth algebraic group $G$ acting on a variety $X$ are in fact equivalent. In the first part of this thesis we introduce and study equivariant categories on a quasi-projective variety $X$. These are a generalization of the equivariant derived category of Lusztig and are indexed by certain pseudofunctors that take values in the 2-category of categories. This 2-categorical generalization allow us to prove rigorously and carefully when such categories are additive, monoidal, triangulated, admit $t$-structures, among and more. We also define equivariant functors and natural transformations before using these to prove how to lift adjoints to the equivariant setting. We also give a careful foundation of how to manipulate $t$-structures on these equivariant categories for future use and with an eye towards future applications. In the final part of this thesis we prove a four-way equivalence between the different formulations of the equivariant derived category of $\ell$-adic sheaves on a quasi-projective variety $X$. We show that the equivariant derived category of Lusztig is equivalent to the equivariant derived category of Bernstein-Lunts and the simplicial equivariant derived category. We then show that these equivariant derived categories are equivalent to the derived $\ell$-adic category of Behrend on the algebraic stack $[G \backslash X]$. We also provide an isomorphism of the simplicial equivariant derived category on the variety $X$ with the simplicial equivariant derived category on the simplicial presentation of $[G \backslash X]$, as well as prove explicit equivalences between the categories of equivariant $\ell$-adic sheaves, local systems, and perverse sheaves with the classical incarnations of such categories of equivariant sheaves.

math.AG↗

arXiv2020

Serre-Hazewinkel Local Class Field Theory and a Geometric Proof of the Local Langlands Correspondence for $\operatorname{GL}(1)$

In this expository paper we provide a geometric proof of the local Langlands Correspondence for the groups $\operatorname{GL}_{1}$ defined over $p$-adic fields $K$. We do this by redeveloping the theory of proalgebraic groups and use this to derive local class field theory in the style of Serre and Hazewinkel. In particular, we show that the local class field theory of Serre and Hazewinkel is valid for both equal characteristic and mixed characteristic ultrametric local fields. Finally, we use this to prove an equivalence of the categories of smooth representations of $K^{\ast}$ with continuous representations of $W_K^{\text{Ab}}$ in order to deduce the Local Langlands Correspondence for $\operatorname{GL}_{1,K}$.

math.NT↗

arXiv2019

The Greenberg Functor is Site Cocontinuous

In this paper we show that it is possible to define a topology on the category of formal schemes over a ring of $p$-adic integers such that the left adjoint of the Greenberg Transform is a site cocontinuous functor when we equip the category of schemes over the residue field with the étale topology. We show furthermore that this topology allows us to give an isomorphism between the corresponding fundamental groups, and use this isomorphism to show that it is possible to geometrize the quasicharacters of a $p$-adic torus by a local system on a formal scheme over the ring of $p$-adic integers.

math.AG↗