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Revista de la Unión Matemática Argentina
versión Online ISSN 16699637
Rev. Unión Mat. Argent. v.48 n.2 Bahía Blanca jul./dic. 2007
Group actions on algebras and module categories
J. A. de la Peña
Introduction and notations
Let be a field and a finite dimensional (associative with ) algebra. By we denote the category of finite dimensional left modules. In many important situations we may suppose that is presented as a quiver with relations (e.g. if is algebraically closed, then is Morita equivalent to ). We recall that if is presented by , then is a finite quiver and is an admissible ideal of the path algebra , that is, for some , where is the ideal of generated by the arrows of , see [6].
It is convenient to consider as a linear category with objects (= vertices of ) and morphisms given by linear maps , where is the trivial path at (for ). In this categorical approach we do not need to assume that is finite (therefore the algebra may not have unity). Ocasionally we write if we do not need to explicit the quiver .
The purpose of these notes is to present an introduction to the study of actions of groups on algebras and their module categories and to consider associated constructions that have proved useful in the Representation Theory of Algebras.
A symmetry of the quiver is a permutation of the set of vertices inducing an automorphism of . We denote by the group of all symmetries of . Those symmetries inducing a morphism such that form the group . In natural way, any induces an automorphism of the module category and on the AuslanderReiten quiver of (since the action commutes with the AuslanderReiten translation of ).
In section 1, we present some basic facts about the actions of groups on (orbits, stabilizers, Burnside's lemma) and show that for a representationfinite standard algebra , we have , where is formed by the symmetris of commuting with the translation . We recall that is standard if is representationfinite and for a choice of representatives of the isoclasses of indecomposables, the induced full subcategory of (denoted by ) is equivalent to which is the quotient of the path algebra by the ideal generated by the meshes of the almost split sequences . We recall that for , any representationfinite algebra is standard.
In section 2 we consider relations between the structure of and the Coxeter polynomial of (recall that, is the Coxeter polynomial associated to the Coxeter matrix which is invertible in case ).
In section 3 we present the main constructions associated to groups acting on algebras:
if acts freely on (that is, and for a vertex , implies ), then the Galois covering is a invariant functor of categories.
if is a graded category, the smash product is a category accepting the free action of . Moreover, .
Galois coverings were introduced by Bongartz and Gabriel [6, 7, 2] for the study of representation type of algebras. Smash products is a wellknown construction in ring theory (see [1]), but only recently was observed by Cibils and Marcos [4] that it yields the inverse operation to Galois coverings. Indeed, if acts freely on , then is a graded category such that .
In section 4, following [2], we introduce functors relating the module categories of and when acts freely on . The main results in these notes (section 5) relate the representation types of and (which was the original purpose of the introduction of Galois coverings). Indeed, given a Galois covering with a finite dimensional algebra, then is representationfinite if and only if is locally representationfinite (that is, for each , there are only finitely many indecomposable modules , up to isomorphism, with ). The proof of this result was partially given in [7] and completed in [10], and provides an efficient tool to deal with representationfinite algebras.
The representationinfinite situation is more involved. We recall that is said to be tame if for every there are finitely many bimodules wich are free finitely generated as right modules and such that any indecomposable module with dimension is of the form for some and some . We say that a tame algebra is domestic (resp. of polynomial growth) if can be chosen (resp. for some ) for all . It is not hard to show that is tame if is tame for a group acting freely on . The converse was shown to be false in [8]; nevertheless there are many interesting, general situations where it holds true.
In section 5 we give examples of results [5, 12] showing that for a Galois covering , the category is tame if and only if is tame, provided certain restrictions on the group or on the category are satisfied.
We denote by the Grothendieck group of , freely generated by representatives of the simple modules, where is the number of vertices of . We denote (resp. ) the projective cover (resp. injective envelope) of . With the categorical approach an module is a functor , and a morphism is a natural transformation. In case , the Coxeter matrix is defined by .
These notes follow closely the lectures given at the Workshop on Representation Theory in Mar del Plata, Argentina in March 2006. The intention of the lectures was to present an elementary introduction to the topic which would serve as a source of motivation and information on the techniques used. While we cannot provide complete proofs of every result, we tried to sketch some representative arguments. We thank the organizers of the Workshop for his hospitality.
1. The group of automorphisms of an algebra
1.1. Let be a finite dimensional algebra:
denotes the group of automorphisms of . By we denote the group of symmetries of and by the group of symmetries of fixing (that is, induces such that ).
 Lemma.
 is a subgroup of .
Each symmetry gives rise to a matrix sending to . This representation is called the canonical representation.
1.2. Let be a subgroup. For a given , denotes the orbit of and subgroup of , denotes the stabilizer of . Clearly is a subgroup of .
 Lemma.
 The mapping , is a bijection from the orbit to the set of left cosets .
1.3. Let be the Grothendieck group of . Consider
Let be a set of representatives of the irreducible representations of (here is the number of conjugacy classes of ). Let be the trivial representation.
Consider the character corresponding to (that is, , ). The characters form an orthonormal basis of the class group , with the scalar product .
 Lemma.
 [Burnside's lemma]. where , is the character of the canonical representation .
Proof. Observe that is the number of fixed points of .
1.4. Let be a subgroup of then acts on as follows:
Clearly, this action preserves indecomposable modules and induces an action of of the AuslanderReiten quiver , satisfying:
 the action preserves projective, injective and simple modules;
 the action preserves AuslanderReiten sequences (in particular, );
 is a subgroup of , the group of automorphisms of the quiver (commuting with the structure).
For (c), let be an element inducing a trivial action on ; we shall prove . Indeed, implies for every vertex .
Let , be all arrows between and , then establishes a permutation of the . Let be the dimensional module with . Since is indecomposable, or . Therefore .
 Proposition.
 Let be an algebra satisfying:
(a) is representation finite, (b) is standard
then .
Proof. We already know that is a subgroup of . Let and consider the induced automorphism of the mesh category . Clearly, restricts to an automorphism of (considering the full embedding , ). By definition there is an automorphism inducing , and therefore inducing . □
2. The canonical representation and the Coxeter matrix
2.1. Let and assume that . For example, this happens if is triangular, that is, has no oriented cycles.
Recall that , is the Coxeter matrix if , which is invertible. In case (i.e. ), then for any nonprojective indecomposable .
The characteristic polynomial is called the Coxeter polynomial.
 Examples:

 Dynkin type, then .
 extended Dynkin type, then and is a root of multiplicity .

is irreducible (over ).
 Proposition [11].

 is an automorphism of the representation .
 If is not trivial, then is not irreducible.
Proof. (a): It suffices to observe that for any .
(b): Let be a set of representatives of the irreducible representations of . Let be the trivial representation. Up to conjugation (with )
By Schur's lemma , where is an automorphism. Hence . Therefore if is irreducible then for .
If , then . Moreover, the characters
Finally, , with implies the existence of with . Therefore is not irreducible. □
Consider and the canonical representation. It is not hard to calculate the character table of :
Then
3. Constructions of algebras asociated to groups of automorphisms (coverings and smash products)
3.1. Let be a category given as . Let be a subgroup of . We say that acts freely on if for some implies .
 Lemma.
 Let be a group acting freely on . Then there exists a category and a functor satisfying:
 (invariant): for every
 (universal invariant): for any functor which is invariant, there exists a unique functor such that
 is formed by the orbits of vertices in and for any and
Proof. Let be defined as in (c) with composition maps
3.2. If acts freely on , the functor as in (2.1) is called a Galois covering defined by and is a Galois quotient of .
We say that a category is graded if for each pair of objects there is a vector space decomposition such that the composition induces linear maps
 Lemma.
 is a graded category.
3.3. For a graded category we define the smash product as the category with objects and for any pair , the morphisms are
 Theorem
 [4]. The category accepts a free action of such that .
Moreover, if acts freely on , then .
Proof. Clearly there is a invariant functor inducing a functor which is the identity on objects. Check that is an isomorphism.
Assume acts freely on and let be the induced Galois covering.
Since acts freely, there is a bijection which commutes with the action. Moreover, if , , then
In other words, smash products and Galois quotients are inverse operations in the class of categories.
3.4. Galois correspondence. Let be a category. There is a correspondence
( Galois covering defined by ) group grading , such that for any
two Galois coverings defined by and defined by , there exist a commutative diagram
A Galois triple of is a Galois covering defined by the (free) action of a group . There is a universal object in this set of Galois triples. Indeed, let and consider the set of all walks in starting and ending at a fixed vertex . Let be the equivalence relation induced by the following elementary relations:
 and for any arrow ;
 if with , such that for any we have , then for ;
 if , then , whenever the products are defined.
Then has a group structure such that is graded. The group is called the fundamental group of .
 Proposition
 [9]. Let and be defined by . The triple is a universal Galois covering, that is, for any Galois covering defined by the action of a group , there exists a covering defined by such that .
4. Actions induced on module categories
4.1. Let be a Galois covering of categories defined by the action of . We shall denote by
 the category of left modules;
 those with for every ;
 those with .
There are naturally defined functors:
Recall that acts on and is stable if for every . The category of stable modules is denoted by .
4.2. Proposition [7, 2]. Let be a Galois covering defined by the action of . Then the following happens:
 The categories and are equivalent.
 For any and , we have . Moreover,
as modules.  Let be a subgroup of and . Then has a natural structure as module. If is a finite group and , then decomposes as a direct sum of at least factors.
Proof. (a): clear.
(b): Observe that canonically.
Hence and correspondingly in morphisms.
(c): Choose a set of representatives in of the right cosets . Then and correspondingly in morphisms.
Hence we get a group homomorphism such that for each idempotent of , is idempotent and there is a factorization of .
In case is a finite group with , by Mashké theorem, the group algebra is semisimple (with idempotents). The result follows. □
4.3. Assume acts freely on and is the corresponding Galois covering.
Let , the stabilizer is the subgroup of formed by those such that . That is, if .
 Proposition [7].

 If and is torsion free, then .
 If and , then is indecomposable and for any module with , then for some .
Proof. (a): Let for some , then establishes a permutation of (a finite set). Then for some , on . Since acts freely on , then . Since is torsion free, then and .
(b): Assume , then . Assume is a direct summand of , then and . Therefore is indecomposable.
If , then is indecomposable and for some . □
5. Coverings and the representation type of an algebra
(a) Let
Let be the infinite category with quiver
and generated by all relations of the form . Then we get a Galois covering defined by the action of on .
The module
It is stable under the action of , .
The universal Galois covering of is given by
where the free group in two generators acts on . The normal subgroup of acts on inducing the covering .
Observe that for , we have and therefore .
(b) [13] Let be a standard algebra of finite representation type. Then the AuslanderReiten quiver is finite and equipped with the mesh relations: , for each almost split sequence .
There is an universal Galois cover of translation quivers defined by the action of a free group . Moreover, is the AuslanderReiten quiver of a category such that .
We illustrate the situation in the following example:
A full subcategory of is convex in if for a quiver which is path closed in (i.e. with implies ).
 Theorem.
 Let be a representationfinite algebra. Then
 [13] There exists a universal Galois covering defined by the action of a free group . Moreover for a category . If is standard, then is a universal Galois covering.
 [3] For every finite convex subcategory of , we have and is representationdirected (i.e. is representationfinite and is a preprojective component).
5.2. We say that the group acts freely on indecomposable classes of modules if for implies . Observe that for the algebra with quiver
 Theorem.
 Let be a Galois covering defined by a group
 [2] If acts freely on indecomposable classes of modules, then induces an injection ( and preserves AuslanderReiten sequences.
 [2, 10] If is finite (hence a algebra), then is representationfinite if and only if (i) acts freely on and (ii) is locally representationfinite (i.e. for each , is finite). In that case, .
Proof. (a): Let , then we know and . Moreover implies for some . For an almost split sequence with we get an exact sequence
(b): Assume is representationfinite and are representatives of . Let and suppose is such that . We shall prove that . Let be a covering defined by a free group as in (5.1), that is, we get commutative diagrams:
Then induces an automorphism on such that and for some with . We get an automorphism of acting freely (since acts freely on ).
Let be the full subcategory of formed by . It is easy to see that is convex in and therefore is a preprojective component. Since , then we get with . Since commutes with , there is a projective module with , which is a contradiction unless .
We check that is locally representationfinite: let and let