Equalizers and kernels in categories of monoids

In dealing with monoids, the natural notion of kernel of a monoid morphism \(f:M\rightarrow N\) between two monoids M and N is that of the congruence \(\sim _f\) on M defined, for every \(m,m'\in M\), by \(m\sim _fm'\) if \(f(m)=f(m')\). In this paper, we study kernels and equalizers of monoid morphisms in the categorical sense. We consider the case of the categories of all monoids, commutative monoids, cancellative commutative monoids, reduced Krull monoids, inverse monoids and free monoids. In all these categories, the kernel of \(f:M\rightarrow N\) is simply the embedding of the submonoid \(f^{-1}(1_N)\) into M, but a complete characterization of kernels in these categories is not always trivial, and leads to interesting related notions.     

Constructions of categories of setoids from proof-irrelevant families

When formalizing mathematics in constructive type theories, or more practically in proof assistants such as Coq or Agda, one is often using setoids (types with explicit equivalence relations). In this note we consider two categories of setoids with equality on objects and show, within intensional Martin-Löf type theory, that they are isomorphic. Both categories are constructed from a fixed proof-irrelevant family F of setoids. The objects of the categories form the index setoid I of the family, whereas the definition of arrows differs. The first category has for arrows triples \((a,b,f:F(a)\,\rightarrow \,F(b))\) where f is an extensional function. Two such arrows are identified if appropriate composition with transportation maps (given by F) makes them equal. In the second category the arrows are triples \((a,b,R \hookrightarrow \Sigma (I,F)^2)\) where R is a total functional relation between the subobjects \(F(a), F(b) \hookrightarrow \Sigma (I,F)\) of the setoid sum of the family. This category is simpler to use as the transportation maps disappear. Moreover we also show that the full image of a category along an E-functor into an E-category is a category.     

Derived categories of Keum's fake projective planes

We conjecture that derived categories of coherent sheaves on fake projective n  -spaces have a semi-orthogonal decomposition into a collection of n+1$n+1$ exceptional objects and a category with vanishing Hochschild homology. We prove this for fake projective planes with non-abelian automorphism group (such as Keum's surface). Then by passing to equivariant categories we construct new examples of phantom categories with both Hochschild homology and Grothendieck group vanishing.     

Invertible bimodule categories over the representation category of a Hopf algebra

For any finite-dimensional Hopf algebra H   we construct a group homomorphism BiGal(H)→BrPic(Rep(H))$\mathrm{BiGal}(H)\to \text{BrPic}(\mathrm{Rep}(H))$, from the group of equivalence classes of H  -biGalois objects to the group of equivalence classes of invertible exact Rep(H)$\mathrm{Rep}(H)$-bimodule categories. We discuss the injectivity of this map. We exemplify in the case H=_Tq$H=T_q$ is a Taft Hopf algebra and for this we classify all exact indecomposable Rep(_Tq)$\mathrm{Rep}(T_q)$-bimodule categories.     

Constellations and their relationship with categories

Constellations are partial algebras that are one-sided generalisations of categories. Indeed, we show that a category is exactly a constellation that also satisfies the left-right dual axioms. Constellations have previously appeared in the context of inductive constellations: the category of inductive constellations is known to be isomorphic to the category of left restriction semigroups. Here we consider constellations in full generality, giving many examples. We characterise those small constellations that are isomorphic to constellations of partial functions. We examine in detail the relationship between constellations and categories. In particular, we characterise those constellations that arise as (sub-)reducts of categories. We demonstrate that the notion of substructure can be captured within constellations but not within categories. We show that every constellation P gives rise to a category \({\mathcal{C}(P)}\), its canonical extension, in a simplest possible way, and that P is a quotient of \({\mathcal{C}(P)}\) in a natural sense. We also show that many of the most common concrete categories may be constructed from simpler quotient constellations using this construction. We characterise the canonical congruences \({\delta}\) on a given category \({K}\) (those for which \({K \cong \mathcal{C}(K/\delta))}\), and show that the category of constellations is equivalent to the category of \({\delta}\)-categories, that is, categories equipped with distinguished canonical congruence \({\delta}\).The main observation of this paper is that category theory as it applies to the familiar concrete categories of modern mathematics (which come equipped with natural notions of substructures and indeed are \({\delta}\)-categories) may be subsumed by constellation theory.     

Tensor envelopes of regular categories

We extend the calculus of relations to embed a regular category A into a family of pseudo-abelian tensor categories T(A,δ) depending on a degree function δ. Assume that all objects have only finitely many subobjects. Then our results are as follows:

1.

Let N be the maximal proper tensor ideal of T(A,δ). We show that T(A,δ)/N is semisimple provided that A is exact and Mal'cev. Thereby, we produce many new semisimple, hence abelian, tensor categories.

2.

Using lattice theory, we give a simple numerical criterion for the vanishing of N.

3.

We determine all degree functions for which T(A,δ)/N is Tannakian. As a result, we are able to interpolate the representation categories of many series of profinite groups such as the symmetric groups Sn, the hyperoctahedral groups , or the general linear groups GL(n,Fq) over a fixed finite field.

This paper generalizes work of Deligne, who first constructed the interpolating category for the symmetric groups Sn.     

Models for singularity categories

In this article we construct various models for singularity categories of modules over differential graded rings. The main technique is the connection between abelian model structures, cotorsion pairs and deconstructible classes, and our constructions are based on more general results about localization and transfer of abelian model structures. We indicate how recollements of triangulated categories can be obtained model categorically, discussing in detail Krause?s recollement _Kac(Inj(R))→K(Inj(R))→D(R)${\mathrm{K}}_{\mathrm{ac}}(\mathrm{Inj}(R))\to \mathrm{K}(\mathrm{Inj}(R))\to \mathrm{D}(R)$. In the special case of curved mixed Z$\mathbb{Z}$-graded complexes, we show that one of our singular models is Quillen equivalent to Positselski?s contraderived model for the homotopy category of matrix factorizations.     

n-angulated quotient categories induced by mutation pairs

Geiss, Keller and Oppermann (2013) introduced the notion of n-angulated category, which is a “higher dimensional” analogue of triangulated category, and showed that certain (n-2)-cluster tilting subcategories of triangulated categories give rise to n-angulated categories. We define mutation pairs in n-angulated categories and prove that given such a mutation pair, the corresponding quotient category carries a natural n-angulated structure. This result generalizes a theorem of Iyama-Yoshino (2008) for triangulated categories.     

Homological algebra i $${\varvec{}}$$-abelia categories

In this paper, we study the homological theory in n-abelian categories. First, we prove some useful properties of n-abelian categories, such as \((n+2)\times (n+2)\)-lemma, 5-lemma and n-Horseshoes lemma. Secondly, we introduce the notions of right(left) n-derived functors of left(right) n-exact functors, n-(co)resolutions, and n-homological dimensions of n-abelian categories. For an n-exact sequence, we show that the long n-exact sequence theorem holds as a generalization of the classical long exact sequence theorem. As a generalization of \(\textsf {Ext}^*(-,-)\), we study the n-derived functor \(\textsf {nExt}^*(-,-)\) of hom-functor \(\mathrm {Hom}(-,-)\). We give an isomorphism between the abelian group of equivalent classes of m-fold n-extensions \(\textsf {nE}^m(A,B)\) of A, B and \(\textsf {nExt}_{\mathcal A}^m(A,B)\) using n-Baer sum for \(m,n\ge 1\).     

Gorensteinness,homological invariants and Gorenstein derived categories

Relations between Gorenstein derived categories, Gorenstein defect categories and Gorenstein stable categories are established. Using these, the Gorensteinness of an algebra A and invariants with respect to recollements of the bounded Gorenstein derived category D _gp ^b (A-mod) of A are investigated. Specifically, the Gorensteinness of A is characterized in terms of recollements of D _gp ^b (A-mod) and Gorenstein derived equivalences. It is also shown that Cohen-Macaulay-finiteness is invariant with respect to the recollements of D _gp ^b (A-mod).     

Artin-Schelter regular algebras and categories

Motivated by constructions in the representation theory of finite dimensional algebras we generalize the notion of Artin-Schelter regular algebras of dimension n to algebras and categories to include Auslander algebras and a graded analogue for infinite representation type. A generalized Artin-Schelter regular algebra or a category of dimension n is shown to have common properties with the classical Artin-Schelter regular algebras. In particular, when they admit a duality, then they satisfy Serre duality formulas and the -category of nice sets of simple objects of maximal projective dimension n is a finite length Frobenius category.     

Categorical resolutions of a class of derived categories

We clarify the relation between the subcategory D_(hf)~b(A) of homological finite objects in D~b(A)and the subcategory K~b(P) of perfect complexes in D~b(A), by giving two classes of abelian categories A with enough projective objects such that D_(hf)~b(A) = K~b(P), and finding an example such that D_(hf)~b(A)≠K~b(P). We realize the bounded derived category D~b(A) as a Verdier quotient of the relative derived category D_C~b(A), where C is an arbitrary resolving contravariantly finite subcategory of A. Using this relative derived categories, we get categorical resolutions of a class of bounded derived categories of module categories of infinite global dimension.We prove that if an Artin algebra A of infinite global dimension has a module T with inj.dimT ∞ such that ~⊥T is finite, then D~b(modA) admits a categorical resolution; and that for a CM(Cohen-Macaulay)-finite Gorenstein algebra, such a categorical resolution is weakly crepant.     

<Emphasis Type="Italic">n</Emphasis>-Abelian and <Emphasis Type="Italic">n</Emphasis>-exact categories

We introduce n-abelian and n-exact categories, these are analogs of abelian and exact categories from the point of view of higher homological algebra. We show that n-cluster-tilting subcategories of abelian (resp. exact) categories are n-abelian (resp. n-exact). These results allow to construct several examples of n-abelian and n-exact categories. Conversely, we prove that n-abelian categories satisfying certain mild assumptions can be realized as n-cluster-tilting subcategories of abelian categories. In analogy with a classical result of Happel, we show that the stable category of a Frobenius n-exact category has a natural \((n+2)\)-angulated structure in the sense of Geiß–Keller–Oppermann. We give several examples of n-abelian and n-exact categories which have appeared in representation theory, commutative algebra, commutative and non-commutative algebraic geometry.