nLab
quiver

Contents

Idea

A quiver is a collection of edges which may stretch between (ordered) pairs of “points”, called vertices.

A quiver is like a category with units and composition forgotten. Indeed, a category is a quiver with extra structure. To formalize this idea, we say there is a forgetful functor

U:CatQuivU\colon Cat \to Quiv

where Quiv is the category of quivers and Cat is the category of (small strict) categories. Moreover, this forgetful functor has a left adjoint

F:QuivCatF\colon Quiv \to Cat

sending each quiver to the free category on that quiver.

A quiver is a kind of graph and is often called a directed graph (or digraph) by category theorists. However, in the context of graph theory, the term “directed graph” is often taken to mean that there is at most one edge from one vertex to another. See directed graph.

Definitions

Slick definition

The walking quiver XX is the category with

  • one object X 0X_0, called the object of vertices;

  • one object X 1X_1, called the object of edges;

  • two morphisms s,t:X 1X 0s, t\colon X_1 \to X_0, called the source and target;

  • together with identity morphisms.

A quiver is a functor G:XG\colon X \to Set.

More generally, a quiver in a category CC is a functor G:XCG\colon X \to C.

The category of quivers in CC, Quiv(C)(C), is the functor category C XC^{X}, where:

In the basic case where CC is Set, the category Quiv(Set) is equivalent to the category of presheaves on X opX^{op}. So: the category of quivers, Quiv, is the category of presheaves on the category X opX^{op}.

Nuts-and-bolts definitions

A quiver GG consists of two sets EE (the set of edges of GG), VV (the set of vertices of GG) and two functions

s,t:EVs, t\colon E \rightrightarrows V

(the source and target functions). More generally, a quiver internal to a category (more simply, in a category) CC consists of two objects EE, VV and two morphisms s,t:EVs, t\colon E \rightrightarrows V.

If G=(E,V,s,t)G = (E, V, s, t) and G=(E,V,s,t)G' = (E', V', s', t') are two quivers in a category CC, a morphism g:GGg\colon G \to G' is a pair of morphisms g 0:VVg_0\colon V \to V', g 1:EEg_1\colon E \to E' such that sg 1=g 0ss' \circ g_1 = g_0 \circ s and tg 1=g 0tt' \circ g_1 = g_0 \circ t.

In graph theory, a quiver may be called a directed pseudograph (or some variation on that theme), but category theorists often just call them directed graphs.

Remarks

Let G 0=G(X 0)G_0 = G(X_0) and G 1=G(X 1)G_1 = G(X_1).

  • A quiver in CC is a presheaf on X opX^{op} with values in CC.

  • A quiver is a globular set which is concentrated in the first two degrees.

  • A quiver can have distinct edges e,eG 1e,e'\in G_1 such that s(e)=s(e)s(e) = s(e') and t(e)=t(e)t(e) = t(e'). A quiver can also have loops, namely, edges with s(e)=t(e)s(e) = t(e).

  • A quiver is complete? if for any pair of vertices v,vG 0v,v'\in G_0, there exists a unique directed edge eG 1e\in G_1 with s(e)=v,t(e)=vs(e) = v, t(e) = v'.

Terminology

Saying quiver instead of directed (multi)graph indicates focus on a certain set of operation intended on that graph. Notably there is the notion of a quiver representation.

Now, one sees that a representation of a graph GG in the sense of quiver representation is nothing but a functor ρ:Q:=F(G)Vect\rho\colon Q := F(G) \to Vect from the free category F(G)F(G) on the quiver GG:

Given a graph GG with collection of vertices G 0G_0 and collection of edges G 1G_1, there is the free category F(G)F(G) on the graph whose collection of objects coincides with the collection of vertices, and whose collection of morphisms consists of finite sequences of edges in GG that fit together head-to-tail (also known as paths). The composition operation in this free category is the concatenation of sequences of edges.

Here we are taking advantage of the adjunction between Cat (the category of small categories) and Quiv (the category of directed graphs). Namely, any category has an underlying directed graph:

U:CatQuivU\colon Cat \to Quiv

and the left adjoint of this functor gives the free category on a directed graph:

F:QuivCatF\colon Quiv\to Cat

Since this is the central operation on quivers that justifies their distinction from the plain concept of directed graph, we may adopt here the point of view that quiver is synonymous with free category.

So a representation of a quiver Q=F(G)Q = F(G) is a functor

R:QVectR\colon Q \to Vect

Concretely, such a thing assigns a vector space to each vertex of the graph GG, and a linear operator to each edge. Representations of quivers are fascinating things, with connections to ADE theory, quantum groups, string theory, and more.

Properties

Relation to free categories

It may be handy to identify a quiver with its free category. This can be justified in the sense that the functor F:QuivCatF\colon Quiv \to Cat is an embedding (kk-surjective for all k>0k \gt 0) on the cores. In other words, isomorphisms between quivers may be identified with equivalences between free categories with no ambiguity.

However, at the level of noninvertible morphisms, this doesn't work; while UU is faithful, it is not full. In other words, there are many functors between free categories that are not morphisms of quivers.

Nevertheless, if we fix a quiver GG and a category DD, then a representation of GG in DD is precisely a functor from F(G)F(G) to DD (or equivalently a quiver morphism from GG to U(D)U(D)), and we may well want to think of this as being a morphism (a heteromorphism?) from GG to DD. As long as DD is not itself a free category, this is unlikely to cause confusion.

Relation to representation theory of algebras

For QQ a quiver, write kQk Q for the path algebra of QQ over a ground field kk. That is, kQk Q is an algebra with kk-basis given by finite composable sequences of arrows in QQ, including a “lazy path” of length zero at each vertex. The product of two paths composable paths is their composite, and the product of non-composable paths is zero.

A module over kQk Q is the same thing as a representation of QQ, so the theory of representations of quivers can be viewed within the broader context of representation theory of (associative) algebras.

If QQ is acyclic, then kQk Q is finite-dimensional as a vector space, so in studying representations of QQ, we are really studying representations of a finite dimensional algebra, for which many interesting tools exist (Auslander-Reiten theory, tilting, etc.).

Classification

Gabriel's theorem says that connected quivers of finite type are precisely those whose underlying undirected graph is a Dynkin diagram? in the ADE series, and that the indecomposable quiver representations are in bijection with the positive roots in the root system of the Dynkin diagram. (Gabriel 72).

References

Some general-purpose references include

  • Harm Derksen, Jerzy Weyman, Quiver representations AMS Notices, 2005.

  • William Crawley-Boevy, Lectures on quiver representations (pdf).

  • Alistair Savage, Finite-dimensional algebras and quivers (arXiv:math/0505082), Encyclopedia of Mathematical Physics, eds. J.-P. Françoise, G.L. Naber and Tsou S.T., Oxford, Elsevier, 2006, volume 2, pp. 313-320.

Classification results for quivers appeared in

Revised on May 27, 2014 01:49:18 by Urs Schreiber (82.136.246.44)