Brown Representability Test Problems in Locally Grothendieck Categories

Open questions concerning Brown representability have recently led Rosicky and Muro to formulate two test problems for Grothendieck categories. We prove that these test problems have a negative solution in the locally Grothendieck category C = è_{k 3 à0} Mod-K^k\mathcal C = \bigcup_{\kappa \geq \aleph_0} {\mbox{\rm{Mod}--}K^{\kappa}} where K is a field.     

The Exchange Property in Grothendieck Categories: Applications

We introduce the concept of an object with the (finite) exchange property in an arbitrary Grothendieck category, and we present the basic properties of such an object. Applications are given for categories of graded modules and for categories of comodules over a coalgebra. Among other results, it is proved that an arbitrary coalgebra 𝒞 over a field has the finite exchange property.     

Morita Duality for Grothendieck Categories with Applications to Coalgebras

Finiteness conditions of reflexive objects of a Morita duality of Grothendieck category is studied. It is observed that the relation between coproduct and product is a fundamental fact. We show that for coalgebras with a duality, the class of reflexive objects coincides with the class of quasi-finite comodules.     

Decomposition of the Grothendieck Group of Stable Categories of Finite Groups

Let G be a finite group with a normal Sylow p-subgroup H such that the corresponding quotient is Abelian. We prove that the Grothendieck group of the stable category of G (over an algebraically closed field of characteristic p) contains a cyclic direct summand of order . Bibliography: 6 titles.     

Curves in Grothendieck Categories

Noncommutative projective geometry studies noncommutative graded rings by replacing the variety by a suitable Grothendieck category. One way of studying the resulting category is to examine the full subcategories which behave like curves on a commutative variety. Smith and Zhang initiated such a study by considering the subcategory generated by a particular type of module they called a pure curve module in good position. This paper generalizes their construction by allowing more general modules. The resulting category is shown to be categorically equivalent to a quotient of the category of graded modules over a graded ring. In the course of defining the category equivalence, several dimensions, including projective, injective and Krull dimensions, are calculated. In particular, this extension allows examination of the category created from a line module over more general AS-regular rings than those considered by Smith and Zhang. For instance, suppose that C is a generic line module over R _d , Stafford's Sklyanin-like algebra. Let C denote the category C generates. Then C is equivalent to the category of graded k[x, y]/(x ² – y ²) modules under the Z × Z/2Z-grading where deg(x) = (–1, 0) and deg(y) = (–1,1).     

Higher Dualizability and Singly-Generated Grothendieck Categories

Applied Categorical Structures - Let k be a field. We show that locally presentable, k-linear categories $${mathcal {C}}$$ dualizable in the sense that the identity functor can be recovered as...     

Concretely Locally Presentable and Locally Generated Categories

We give concrete versions of the characterizations of locally -presentable (resp. -generated) categories as categories of models of (resp. weakly) -ary limit-theories and of (resp. weakly) -ary essentially algebraic theories. By concrete we mean that starting with a category of -structures, the theories obtained are extensions of the original ones, and the equivalences of categories are concrete isomorphisms. -presentable and -generated objects are also investigated from this viewpoint.     

Grothendieck Representations of Categories and Canonical Noncommutative Topologies

From a representation of a category in terms of a class of Grothendieck categories, a canonical topologisation of the objects follows. The noncommutative topology recently introduced in noncommutative geometry is an example of this. Basic results of sheaf theory are derived in this setting.     

Covariant Isotropy of Grothendieck Toposes and Extensive Categories

We provide an explicit characterization of the covariant isotropy group of any Grothendieck topos, i.e. the group of (extended) inner automorphisms of any sheaf over a small site. In order to do so, we first extend previous techniques for computing covariant isotropy from locally finitely presentable categories to locally presentable categories. As a consequence, we also obtain an explicit characterization of the centre of a Grothendieck topos, i.e. the automorphism group of its identity functor. We conclude by providing a more categorical approach to show that these characterizations also extend to any extensive category.

     

Structure Theorems for Pure Semisimple Grothendieck Locally PI-Categories

Let Q be a nondegenerate quadratic form over the finite field F _q and O _n (F _q ) be the associated orthogonal group. Let O _n (F _q ) act linearly on the polynomial ring F _q [x ₁, …, x _n ]. We find the invariant subring of O ₄(F _q ) with explicit generators.     

Grothendieck环及其应用

本文以Ｇｒｏｔｈｅｎｄｉｅｃｋ环为工具，利用相关环刻划了环的Ｇｒｏｔｈｅｎｄｉｅｃｋ群，得到了环的连通性、稳定自由性等性质的重要特征．最后给出了它们在群环的应用．     

Descent in Locally Presentable Categories

Let \(\mathbb {V}=(VV, \otimes , I)\) be a symmetric monoidal category such that \(\mathcal {V}\) is locally presentable and that all functors \(V\otimes - : \mathcal {V} \rightarrow \mathcal {V}\) for \(V \in \mathcal {V}\) preserve reflexive coequalizers and directed colimits. It is proved that any pure morphism of commutative ??-monoids is an effective descent morphism with respect to the indexed category given by commutative ??-monoids and modules over them. As a by-product, we prove that pure morphisms in a locally presentable category are effective for codescent.     

The Ziegler Spectrum of a Locally Coherent Grothendieck Category

The general theory of locally coherent Grothendieck categoriesis presented. To each locally coherent Grothendieck categoryC a topological space, the Ziegler spectrum of C, is associated.It is proved that the open subsets of the Ziegler spectrum ofC are in bijective correspondence with the Serre subcategoriesof coh C the subcategory of coherent objects of C. This is aNullstellensatz for locally coherent Grothendieck categories.If R is a ring, there is a canonical locally coherent Grothendieckcategory RC (respectively, CR) used for the study of left (respectively,right) R-modules. This category contains the category of R-modulesand its Ziegler spectrum is quasi-compact, a property used toconstruct large (not finitely generated) indecomposable modulesover an artin algebra. Two kinds of examples of locally coherentGrothendieck categories are given: the abstract category theoreticexamples arising from torsion and localization and the examplesthat arise from particular modules over the ring R. The dualitybetween coh-(RC) and coh-CR is shown to give an isomorphismbetween the topologies of the left and right Ziegler spectraof a ring R. The Nullstellensatz is used to give a proof ofthe result of Crawley-Boevey that every character : K₀(coh-C) Z is uniquely expressible as a Z-linear combination of irreduciblecharacters. 1991 Mathematics Subject Classification: 16D90,18E15.     

Balance in Stable Categories

We study when the stable category ${\mathcal A}/\langle{\mathcal T}\rangleWe study when the stable category A/áT?{\mathcal A}/\langle{\mathcal T}\rangle of an abelian category A{\mathcal A} modulo a full additive subcategory T{\mathcal T} is balanced and, in case T{\mathcal T} is functorially finite in A{\mathcal A}, we study a weak version of balance for A/áT?{\mathcal A}/\langle{\mathcal T}\rangle. Precise necessary and sufficient conditions are given in case T{\mathcal T} is either a Serre class or a class consisting of projective objects. The results in this second case apply very neatly to (generalizations of) hereditary abelian categories.     

Stable Tubes in Extriangulated Categories

Czechoslovak Mathematical Journal - Let χ be a semibrick in an extriangulated category. If χ is a τ-semibrick, then the Auslander-Reiten quiver $$Gamma ({cal F}({cal X}))$$ of the...     

Compactly Generated Relative Stable Categories

Let G be a finite group. The stable module category of G has been applied extensively in group representation theory. In particular, it has been used to great effect that it is a triangulated category which is compactly generated by the class of finitely generated modules. Let H be a subgroup of G. It is possible to define a stable module category of G relative to H. This is also a triangulated category, but no non-trivial examples have been known where it was compactly generated. While the finitely generated modules are compact objects, they do not necessarily generate the category. We show that the relative stable category is compactly generated if the group algebra of H has finite representation type. In characteristic p, this is equivalent to the Sylow p-subgroups of H being cyclic.     

Torsion Pairs in Stable Categories

Let 𝒞 be a triangulated category. When ω is a functorially finite subcategory of 𝒞, Jøtrgensen showed that the stable category 𝒞/ω is a pretriangulated category. A pair (𝒳, 𝒴) of subcategories of 𝒞 with ω ? 𝒳 ∩ 𝒴 gives rise to a pair (𝒳/ω, 𝒴/ω) of subcategories of 𝒞/ω. In this article, we find conditions for (𝒳/ω, 𝒴/ω) to be a torsion pair in terms of properties of the pair (𝒳, 𝒴). In particular, we obtain necessary and sufficient conditions for (𝒳/ω, 𝒴/ω) to be a torsion pair in the stable category 𝒞/ω when τω = ω, where τ is the Auslander–Reiten translation.     

The Grothendieck Groups and Stable Equivalences of Mesh Algebras

We deal with the finite-dimensional mesh algebras given by stable translation quivers. These algebras are self-injective, and thus the stable module categories have a structure of triangulated categories. Our main result determines the Grothendieck groups of these stable module categories. As an application, we give a complete classification of the mesh algebras up to stable equivalences.     

Classifying Dense Resolving and Coresolving Subcategories of Exact Categories Via Grothendieck Groups

Classification problems of subcategories have been deeply considered so far. In this paper, we discuss classifying dense resolving and dense coresolving subcategories of exact categories via their Grothendieck groups. This study is motivated by the classification of dense triangulated subcategories of triangulated categories due to Thomason.     

QF-Proper Classes and Relative Stable Categories

A relative version of Rickard's theorem is proved, namely, if is a quasi-Frobenius proper class of short sequences in an Abelian category , then the -stable category of the category is a quotient category of the relative bounded derived category ( ). Bibliography: 20 titles.