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Simone Virili

Publications and source records attributed to Simone Virili.

At least 19 recordsLinked to original sources
arXiv2025

Locally finitely presented Grothendieck categories with a flat generator

A problem raised by Cuadra and Simson in 2007 asks whether any locally finitely presented Grothendieck category with enough flat objects also has enough projectives. In this paper, we start from a key observation: a locally finitely presented Grothendieck category has enough flat objects if, and only if, it has exact products. This enables several equivalent reformulations of the problem, allowing us to identify a counterexample (thus providing a negative solution to the problem), while also connecting it to a classical ring-theoretical question posed by Miller in 1975, and even to the Telescope Conjecture for compactly generated triangulated categories. Moreover, we describe several classes of Grothendieck categories where the problem can be answered affirmatively. For example, we show that a locally finitely presented Grothendieck category whose category of finitely presented objects is Krull--Schmidt has enough flats if, and only if, it is generated by a family of finitely generated projectives.

math.CT↗
arXiv2024

A poisonous example to explicit resolutions of unbounded complexes

We show that various methods for explicitly building resolutions of unbounded complexes in fact fail when applied to a rather simple and explicit complex. We show that one way to rescue these methods is to assume Roos (Ab.4$^*$)-$k$ axiom, which we adapt to encompass also resolutions in the framework of relative homological algebra. In the end we discuss the existence of model structures for relative homological algebra for unbounded complex under the relative (Ab.4$^*$)-$k$ condition, and present a variety of examples where our results apply.

math.RA↗
arXiv2023

Ore localization of amenable monoid actions and applications towards entropy $-$ addition formulas and the bridge theorem

For a left action $S\oversetλ{\curvearrowright}X$ of a cancellative right amenable monoid $S$ on a discrete Abelian group $X$, we construct its Ore localization $G\overset{λ^*}{\curvearrowright}X^*$, where $G$ is the group of left fractions of $S$; analogously, for a right action $K\oversetρ\curvearrowleft S$ on a compact space $K$, we construct its Ore colocalization $K^*\overset{ρ^*}{\curvearrowleft} G$. Both constructions preserve entropy, i.e., for the algebraic entropy $h_{\mathrm{alg}}$ and for the topological entropy $h_{\mathrm{top}}$ one has $h_{\mathrm{alg}}(λ)=h_{\mathrm{alg}}(λ^*)$ and $h_{\mathrm{top}}(ρ)=h_{\mathrm{top}}(ρ^*)$, respectively. Exploiting these constructions and the theory of quasi-tilings, we extend the Addition Theorem for $h_{\mathrm{top}}$, known for right actions of countable amenable groups on compact metrizable groups, to right actions $K\oversetρ{\curvearrowleft} S$ of cancellative right amenable monoids $S$ (with no restrictions on the cardinality) on arbitrary compact groups $K$. When the compact group $K$ is Abelian, we prove that $h_{\mathrm{top}}(ρ)$ coincides with $h_{\mathrm{alg}}(\hatρ)$, where $S\overset{\hatρ}\curvearrowright X$ is the dual left action on the discrete Pontryagin dual $X=\hat{K}$, that is, a so-called Bridge Theorem. From the Addition Theorem for $h_{\mathrm{top}}$ and the Bridge Theorem, we obtain an Addition Theorem for $h_{\mathrm{alg}}$ for left actions $S\oversetλ\curvearrowright X$ on discrete Abelian groups, so far known only under the hypotheses that either $X$ is torsion or $S$ is locally monotileable. The proofs substantially use the unified approach towards entropy based on the entropy of actions of cancellative right amenable monoids on appropriately defined normed monoids.

math.RT↗
arXiv2022

Locally finitely presented and coherent hearts

Starting with a Grothendieck category $\mathcal{G}$ and a torsion pair $\mathbf{t}=(\mathcal{T},\mathcal{F})$ in $\mathcal G$, we study the local finite presentability and local coherence of the heart $\mathcal{H}_{\mathbf{t}}$ of the associated Happel-Reiten-Smalø $t$-structure in the derived category $\mathrm{Der} (\mathcal{G})$. We start by showing that, in this general setting, the torsion pair $\mathbf t$ is of finite type, if and only if it is quasi-cotilting, if and only if it is cosilting. We then proceed to study those $\mathbf t$ for which $\mathcal{H}_{\mathbf{t}}$ is locally finitely presented, obtaining a complete answer under some additional assumptions on the ground category $\mathcal{G}$, which are general enough to include all locally coherent categories, all categories of modules and several categories of quasi-coherent sheaves over schemes. The third problem that we tackle is that of local coherence. In this direction we characterize those torsion pairs $\mathbf t=(\mathcal T,\mathcal F)$ in a locally finitely presented $\mathcal G$ for which $\mathcal{H}_{\mathbf{t}}$ is locally coherent in two cases: when the tilted t-structure in $\mathcal{H}_{\mathbf{t}}$ is assumed to restrict to finitely presented objects, and when $\mathcal F$ is cogenerating. In the last part of the paper we concentrate on the case when $\mathcal G$ is a category of modules over a small preadditive category, giving several examples and obtaining very neat (new) characterizations even in this more classical setting, also underlying connections with the notion of an elementary cogenerator.

math.CT↗
arXiv2022

Stable finiteness of endomorphism rings

We combine a combinatorial idea of Benjamin Weiss and some localization theory of Grothendieck categories to give a short and completely self-contained proof of the following recent result of Hanfeng Li and Bingbing Liang: Given a left Noetherian ring $R$ and a sofic group $G$, the group-ring $R[G]$ is stably finite.

math.RA↗
arXiv2021

Tilting preenvelopes and cotilting precovers in general Abelian categories

We consider an arbitrary Abelian category $\mathcal{A}$ and a subcategory $\mathcal{T}$ closed under extensions and direct summands, and characterize those $\mathcal{T}$ that are (semi-)special preenveloping in $\mathcal{A}$; as a byproduct, we generalize to this setting several classical results for categories of modules. For instance, we get that the special preenveloping subcategories $\mathcal{T}$ of $\mathcal{A}$ closed under extensions and direct summands are precisely those for which $(_{}^{\perp_1}\mathcal{T},\mathcal{T})$ is a right complete cotorsion pair, where $_{}^{\perp_1}\mathcal{T}:=\text{Ker} (\text{Ext}_{\mathcal{A}}^1(-,\mathcal{T}))$. Particular cases appear when $\mathcal{T}=V^{\perp_1}:=\text{Ker}(\text{Ext}_{\mathcal{A}}^1(V,-))$, for an $\text{Ext}^1$-universal object $V$ such that $\text{Ext}_{\mathcal{A}}^1(V,-)$ vanishes on all (existing) coproducts of copies of $V$. For many choices of $\mathcal{A}$, we show that these latter examples exhaust all the possibilities. We then show that, when $\mathcal{A}$ has an epi-generator, the (semi-)special preenveloping torsion classes $\mathcal{T}$ given by (quasi-)tilting objects are exactly those for which any object $T\in\mathcal{T}$ is the epimorphic image of some object in $_{}^{\perp_1}\mathcal{T}$ (and the subcategory $\mathcal{B}:=\text{Sub}(\mathcal{T})$ of subobjects of objects in $\mathcal{T}$ is reflective) and they are, in turn, the right constituents of complete cotorsion pairs in $\mathcal{A}$ (resp., $\mathcal{B}$). In a final section, we apply the results when $\mathcal{A}=\mathrm{mod}\text{-}R$ is the category of finitely presented modules over a right coherent ring $R$, something that gives new results and raises new questions even at the level of classical tilting theory in categories of modules.

math.RT↗
arXiv2021

$t$-Structures on stable derivators and Grothendieck hearts

We prove that given any strong, stable derivator and a $t$-structure on its base triangulated category $\cal D$, the $t$-structure canonically lifts to all the (coherent) diagram categories and each incoherent diagram in the heart uniquely lifts to a coherent one. We use this to show that the $t$-structure being compactly generated implies that the coaisle is closed under directed homotopy colimit which in turns implies that the heart is an (Ab.$5$) Abelian category. If, moreover, $\cal D$ is a well generated algebraic or topological triangulated category, then the heart of any accessibly embedded (in particular, compactly generated) $t$-structure has a generator. As a consequence, it follows that the heart of any compactly generated $t$-structure of a well generated algebraic or topological triangulated category is a Grothendieck category.

math.CT↗
arXiv2019

Torsion pairs in categories of modules over a preadditive category

It is a result of Gabriel that hereditary torsion pairs in categories of modules are in bijection with certain filters of ideals of the base ring, called Gabriel filters or Gabriel topologies. A result of Jans shows that this bijection restricts to a correspondence between (Gabriel filters that are uniquely determined by) idempotent ideals and TTF triples. Over the years, these classical results have been extended in several different directions. In this paper we present a detailed and self-contained exposition of an extension of the above bijective correspondences to additive functor categories over small preadditive categories. In this context, we also show how to deduce parametrizations of hereditary torsion theories of finite type, Abelian recollements by functor categories, and centrally splitting TTFs.

math.RA↗
arXiv2019

ERRATA CORRIGE: Intrinsic algebraic entropy

The notion of intrinsic algebraic entropy of an endomorphism of a given Abelian group has been recently introduced in [D. Dikranjan, A. Giordano Bruno, L. Salce, S. Virili, Intrinsic algebraic entropy, J. Pure Appl. Algebra 219 (2015) 2933-2961]. In this short note we provide a correct argument to prove one of the basic properties of the intrinsic algebraic entropy: the Logarithmic Law. In fact, this property was correctly stated in [op. cit.] but, as we will show with an explicit counterexample, the original proof contains a flaw.

math.GR↗
arXiv2019

Morita theory for stable derivators

We give a general construction of realization functors for $t$-structures on the base of a strong stable derivator. In particular, given such a derivator $\mathbb D$, a $t$-structure $\mathbf t=(\mathcal D^{\leq0},\mathcal D^{\geq0})$ on the triangulated category $\mathbb D(\mathbb 1)$, and letting $\mathcal A=\mathcal D^{\leq0}\cap \mathcal D^{\geq0}$ be its heart, we construct, under mild assumptions, a morphism of prederivators \[ \mathrm{real}_{\mathbf t}\colon \mathbf{D}_{\mathcal A}\to \mathbb D \] where $\mathbf{D}_{\mathcal A}$ is the natural prederivator enhancing the derived category of $\mathcal A$. Furthermore, we give criteria for this morphism to be fully faithful and essentially surjective. If the $t$-structure $\mathbf t$ is induced by a suitably "bounded" co/tilting object, $\mathrm{real}_{\mathbf t}$ is an equivalence. Our construction unifies and extends most of the derived co/tilting equivalences appeared in the literature in the last years.

math.KT↗
arXiv2018

Factorization systems on (stable) derivators

We define triangulated factorization systems on triangulated categories, and prove that a suitable subclass thereof (the normal triangulated torsion theories) corresponds bijectively to $t$-structures on the same category. This result is then placed in the framework of derivators regarding a triangulated category as the base of a stable derivator. More generally, we define derivator factorization systems in the 2-category $\mathrm{PDer}$, describing them as algebras for a suitable strict 2-monad (this result is of independent interest), and prove that a similar characterization still holds true: for a stable derivator $\mathbb D$, a suitable class of derivator factorization systems (the normal derivator torsion theories) correspond bijectively with $t$-structures on the base $\mathbb{D}(\mathbb{1})$ of the derivator. These two result can be regarded as the triangulated- and derivator- analogues, respectively, of the theorem that says that `$t$-structures are normal torsion theories' in the setting of stable $\infty$-categories, showing how the result remains true whatever the chosen model for stable homotopy theory is.

math.CT↗
arXiv2018

A point-free approach to L-Surjunctivity and Stable Finiteness

The category of quasi frames (or qframes) is introduced and studied. In the context of qframes we can jointly study problems related to the L-Surjunctivity and Stable Finiteness Conjectures. As a consequences of our main results, we can generalize some of the known results on these conjectures. In particular, let $R$ be a ring, let $G$ be a sofic group, fix a crossed product $R*G$ and let $N$ be a right $R$-module. It is proved that: (1) the endomorphism ring $End_{R* G}(N\otimes_R R*G)$ is stably finite, provided $N$ is finitely generated and has Krull dimension; (2) any linear cellular automaton $ϕ:N^G\to N^G$ is surjunctive, provided $N$ is Artinian.

math.RA↗
arXiv2017

Intrinsic valuation entropy

We extend the notion of intrinsic entropy for endomorphisms of Abelian groups to endomorphisms of modules over an Archimedean non-discrete valuation domain $R$, using the natural non-discrete length function introduced by Northcott and Reufel for such a category of modules. We prove that this notion of entropy is a length function for the category of $R[X]$-modules, it satisfies (a suitably adapted version of) the Intrinsic Algebraic Yuzvinski Formula and that it is essentially the unique invariant for $Mod(R[X])$ with these properties.

math.RA↗
arXiv2017

Algebraic entropy of amenable group actions

Let $R$ be a ring, let $G$ be an amenable group and let $R\ast G$ be a crossed product. The goal of this paper is to construct, starting with a suitable additive function $L$ on the category of left modules over $R$, an additive function on a subcategory of the category of left modules over $R\ast G$, which coincides with the whole category if $L({}_RR) <\infty$. This construction can be performed using a dynamical invariant associated with the original function $L$, called algebraic $L$-entropy. We apply our results to two classical problems on group rings: the Stable Finiteness and the Zero-Divisors Conjectures.

math.RA↗
arXiv2016

Topological entropy in totally disconnected locally compact groups

Let $G$ be a topological group, let $ϕ$ be a continuous endomorphism of $G$ and let $H$ be a closed $ϕ$-invariant subgroup of $G$. We study whether the topological entropy is an additive invariant, that is, $$h_{top}(ϕ)=h_{top}(ϕ\restriction_H)+h_{top}(\barϕ)\,,$$ where $\barϕ:G/H\to G/H$ is the map induced by $ϕ$. We concentrate on the case when $G$ is locally compact totally disconnected and $H$ is either compact or normal. Under these hypotheses, we show that the above additivity property holds true whenever $ϕH=H$ and $\ker(ϕ)\leq H$. As an application we give a dynamical interpretation of the scale $s(ϕ)$, by showing that $\log s(ϕ)$ is the topological entropy of a suitable map induced by $ϕ$. Finally, we give necessary and sufficient conditions for the equality $\log s(ϕ)=h_{top}(ϕ)$ to hold.

math.DS↗
arXiv2015

Cartesian modules over representations of small categories

We introduce the new concept of cartesian module over a pseudofunctor $R$ from a small category to the category of small preadditive categories. Already the case when $R$ is a (strict) functor taking values in the category of commutative rings is sufficient to cover the classical construction of quasi-coherent sheaves of modules over a scheme. On the other hand, our general setting allows for a good theory of contravariant additive locally flat functors, providing a geometrically meaningful extension of Crawley-Boevey's Representation Theorem. As an application, we relate and extend some previous constructions of the pure derived category of a scheme.

math.RA↗
arXiv2015

Model approximations for relative homological algebra

Recently, Chachólski, Neeman, Pitsch, and Scherer studied, in a series of three papers, model approximations for the unbounded category of cochain complexes over a commutative ring. These approximations allow to construct relative injective resolutions with respect to particular choices of injectives. In this paper we define similar model approximations for cochain complexes on general Grothendieck categories generalizing the previous constructions and reaching a better understanding of the whole picture.

math.KT↗