arXiv subjects
Publications and source records attributed to Claus Michael Ringel.
A famous result of Zimmermann-Huisgen, Hille and Reineke asserts that any projective variety occurs as a quiver Grassmannian for a suitable representation of some wild acyclic quiver. We show that this happens for any wild acyclic quiver.
The 3-Kronecker quiver has two vertices, namely a sink and a source, and 3 arrows. A regular representation of a representation-infinite quiver such as the 3-Kronecker quiver is said to be elementary provided it is non-zero and not a proper extension of two regular representations. Of course, any regular representation has a filtration whose factors are elementary, thus the elementary representations may be considered as the building blocks for obtaining all the regular representations. We are going to determine the elementary $3$-Kronecker modules. It turns out that all the elementary modules are combinatorially defined.
We call a triangulated category \emph{hereditary} provided that it is equivalent to the bounded derived category of a hereditary abelian category, where the equivalence is required to commute with the translation functors. If the triangulated category is algebraical, we may replace the equivalence by a triangle equivalence. We give two intrinsic characterizations of hereditary triangulated categories using a certain full subcategory and the non-existence of certain paths. We apply them to piecewise hereditary algebras.
These notes provide three contributions to the (well-established) representation theory of Dynkin and Euclidean quivers. They should be helpful as part of a direct approach to study representations of quivers, and they may shed some new light on properties of Dynkin and Euclidean quivers. Part 1 deals with the case A (see arXiv:1304.5720). Part 2 concerns the case D. We show that the category of representation of D_n contains a full subcategory which is equivalent to the category of representations of a quiver of type D_{n-1} such that the remaining indecomposable representations are thin. This suggests an inductive procedure to deal with the D-cases, starting with D_3 = A_3. Part 3 deals with the cases E. It provides a uniform way to construct the maximal indecomposable representation for these Dynkin quivers as well as a unified method to deal with the corresponding Euclidean quivers. The level of the presentation varies considerably and increases throughout the discussion. Whereas Part 1 is completely elementary, just based on some results in linear algebra, the further text is less self-contained and uses one-point extensions, simplification, perpendicular categories, Auslander-Reiten quivers and hammocks.
Given a finite acyclic quiver Q with path algebra kQ, Ingalls and Thomas have exhibited a bijection between the set of Morita equivalence classes of support-tilting modules and the set of thick subcategories of mod kQ and they have collected a large number of further bijections with these sets. We add some additional bijections and show that all these bijections hold for arbitrary hereditary artin algebras. The proofs presented here seem to be of interest also in the special case of the path algebra of a quiver.
The Dynkin algebras are the hereditary artin algebras of finite representation type. The paper exhibits the number of support-tilting modules for any Dynkin algebra. Since the support-tilting modules for a Dynkin algebra correspond bijectively to the non-crossing partitions of the same type, the calculations presented here may also be considered as a categorification of results concerning non-crossing partitions.
This is a survey on the categorification of the poset of generalized non-crossing partitions, using the representation theory of a hereditary artin algebra H, looking at the set P of exceptional subcategories in mod H. This categorification is due to Ingalls and Thomas, and a subsequent paper by Igusa and Schiffler. Starting point is a refinement of the classical tilting theory for mod H, replacing torsion pairs by torsion triples, thus putting it into the realm of the stability theory of King. The torsion pairs in mod H correspond nicely to the perpendicular pairs of exceptional subcategories and there is a wealth of bijections, the Ingalls-Thomas bijections, between sets of modules and subcategories. If H is representation-finite, one may look at the corresponding numbers of modules or subcategories. Such Dynkin functions (they attach to a Dynkin diagram an integer) are displayed in chapter 1. In a mysterious way, many Dynkin functions can be described using the exponents of the Weyl group. According to Shapiro and Kostant, the exponents are given by the height partition of the root poset. A recent result of Abe-Barakat-Cuntz-Hoge-Terao allows to determine them inductively, going up in a chain of ideals in the root poset, looking at the corresponding hyperplane arrangements. Chapter 4 deals with the case of the linearly oriented quiver of Dynkin type A. Here P is identified with the lattice NC of non-crossing partitions as introduced by Kreweras (now an important tool in several parts of mathematics, for example in free probability theory). We review some classical problems which are related to the maximal chains in NC: to count labeled trees as well as parking functions. The combinatorics of the Dynkin case A is just the combinatorics of the Catalan numbers; in an appendix, we discuss the nature of classical Catalan combinatorics.
Let k be a field, let A a finite-dimensional hereditary k-algebra. We consider the category of all finite-dimensional A-modules. We are going to characterize the representation type of A (tame or wild) in terms of the possible subcategories of all M-static modules, where M is an indecomposable A-module.
An (additive) functor F from an additive category A to an additive category B is said to be objective, provided any morphism f in A with F(f) = 0 factors through an object K with F(K) = 0. In this paper we concentrate on triangle functors between triangulated categories. The first aim of this paper is to characterize objective triangle functors F in several ways. Second, we are interested in the corresponding Verdier quotient functors V_F, in particular we want do know under what conditions V_F is full. The third question to be considered concerns the possibility to factorize a given triangle functor F = F_2F_1 with F_1 a full and dense triangle functor and F_2 a faithful triangle functor. It turns our that the behaviour of splitting monomorphisms (and splitting epimorphisms) plays a decisive role.
Let S(n) be the category of invariant subspaces of nilpotent operators with nilpotency index at most n. Such submodule categories have been studied already in 1936 by Birkhoff, they have attracted a lot of attention in recent years, for example in connection with some weighted projective lines (Kussin, Lenzing, Meltzer). On the other hand, we consider the preprojective algebra of type A_n; the preprojective algebras were introduced by Gelfand and Ponomarev, they are now of great interest, for example they form an important tool to study quantum groups (Lusztig) or cluster algebras (Geiss, Leclerc, Schroeer). Direct connections between the submodule category S(n) and the module category of the preprojective algebra of type A_{n-1} have been established quite a long time ago by Auslander and Reiten, and recently also by Li and Zhang, but apparently this remained unnoticed. The aim of this note is to provide details on this relationship. As a byproduct we see that here we deal with ideals I in triangulated categories T such that I is generated by an idempotent and T/I is abelian.
Let A be a connected hereditary artin algebra. We show that the set of functorially finite torsion classes of A-modules is a lattice if and only if A is either representation-finite (thus a Dynkin algebra) or A has only two simple modules. For the case of A being the path algebra of a quiver, this result has recently been established by Iyama-Reiten-Thomas-Todorov and our proof follows closely their considerations.
The representations of a quiver Q over a field k have been studied for a long time. It seems to be worthwhile to consider also representations of Q over arbitrary finite-dimensional k-algebras A. Here we draw the attention to the case when A = k[ε] is the algebra of dual numbers, thus to the Λ-modules with Λ= kQ[ε]. The algebra Λ is a 1-Gorenstein algebra, therefore the torsionless Λ-modules are known to be of special interest (as the Gorenstein-projective or maximal Cohen-Macaulay modules). They form a Frobenius category \Cal L, thus the corresponding stable category is a triangulated category. As we show, the category \Cal L is the category of perfect differential kQ-modules and the stable category of \Cal L is triangle equivalent to the orbit category of the derived category D^b(mod kQ) modulo the shift. The homology functor H from mod Λ to \mod kQ yields a bijection between the indecomposables in the stable category of \Cal L and those in mod kQ. Our main interest lies in the inverse, it is given by the minimal \Cal L-approximation. Also, we determine the kernel of the restriction of the functor H to \Cal L and we describe the Auslander-Reiten quiver of \Cal L.
Let Q be a connected directed quiver with n vertices. We show that Q is representation-infinite if and only if there do exist n isomorphism classes of exceptional modules of some fixed length at least 2.
We consider Dynkin algebras, these are the hereditary artin algebras of finite representation type. The indecomposable modules for a Dynkin algebra correspond bijectively to the positive roots of a Dynkin diagram. Given a Dynkin algebra with n simple modules, a complete exceptional sequence is a sequence M_1,..., M_n of indecomposable modules such that Hom(M_i,M_j) = 0 = Ext(M_i,M_j) for i > j. The aim of this paper is to determine the number of complete exceptional sequences for any Dynkin algebra. There are direct connections between the representation theory of a Dynkin algebra A and the lattice L of non-crossing partitions of the same Dynkin type: As Ingalls and Thomas have shown, the lattice of the thick subcategories of mod A can be identified with L. Hubery and Krause have pointed out that this identification provides a bijection between the complete exceptional sequences for A and the maximal chains in L. Thus, our calculations may also be considered as a categorification of results concerning non-crossing partitions.
Let k be a field and A a finite-dimensional k-algebra (associative, with 1). A recent preprint by Chindris, Kinser and Weyman draws the attention to the present sheer ignorance concerning the possible behavior of wild algebras. The aim of this note is to exhibit two examples which answer questions mentioned in the paper.
We show that any generic representation of a wild connected directed quiver is the union of its subrepresentations of finite length which are regular. As a consequence, we see that the direct limit closure of the preprojective component does not contain any generic module.
Let k be an algebraically closed field and A a finite-dimensional k-algebra. Given an A-module M, the set G_e(M) of all submodules of M with dimension vector e is called a quiver Grassmannian. If D,Y are A-modules, then we consider Hom(D,Y) as a B-module, where B is the opposite of the endomorphism ring of D, and the Auslander varieties for A are the quiver Grassmannians of the form G_e Hom(D,Y). Quiver Grassmannians, thus also Auslander varieties are projective varieties and it is known that every projective variety occurs in this way. There is a tendency to relate this fact to the wildness of quiver representations and the aim of this note is to clarify these thoughts: We show that for an algebra A which is (controlled) wild, any projective variety can be realized as an Auslander variety, but not necessarily as a quiver Grassmannian.
Let A be an artin algebra. In his seminal Philadelphia Notes published in 1978, M. Auslander introduced the concept of morphisms being determined by modules. Auslander was very passionate about these ivestigations (they also form part of the final chapter of the Auslander-Reiten-Smaloe book and could and should be seen as its culmination), but the feedback until now seems to be somewhat meager. The theory presented by Auslander has to be considered as an exciting frame for working with the category of A-modules, incorporating all what is known about irreducible maps (the usual Auslander-Reiten theory), but the frame is much wider and allows for example to take into account families of modules - an important feature of module categories. What Auslander has achieved is a clear description of the poset structure of the category of A-modules as well as a blueprint for interrelating individual modules and families of modules. Auslander has subsumed his considerations under the heading of "morphisms being determined by modules". Unfortunately, the wording in itself seems to be somewhat misleading, and the basic definition may look quite technical and unattractive, at least at first sight. This could be the reason that for over 30 years, Auslander's powerful results did not gain the attention they deserve. The aim of this survey is to outline the general setting for Auslander's ideas and to show the wealth of these ideas by exhibiting many examples.