# nLab quiver

Contents

### Context

#### Graph theory

graph theory

graph

category of simple graphs

### Extra structure

#### Representation theory

representation theory

geometric representation theory

# Contents

## Idea

A quiver (Gabriel 72) is a collection of edges which may stretch between (ordered) pairs of “points”, called vertices. Hence a quiver is a kind of graph, often called a directed graph (or digraph) by category theorists.

The term “quiver” (German: Köcher) for a directed graph is due to Gabriel 72, where it says on the first page:

For such a 4-tuple $\big[$ $V \underoverset{\underset{t}{\longrightarrow}}{\overset{s}{\longrightarrow}}{} E$ $\big]$ we propose the term quiver, and not graph, since there are already too many notions attached to the latter word.

While therefore, as concepts in themselves, quiver and directed graph are just the same, using the term quiver serves to indicate certain constructions that one is interested in (a concept with an attitude), notably the corresponding quiver representations which much of the theory revolves around. The key result here is Gabriel's theorem from the same article Gabriel 72 that introduced the terminology quiver.

On the other hand, beware that in graph theory, the term “directed graph” is often taken to mean that there is at most one edge from one vertex to another. To emphasize that a quiver allows more edges between vertices one may speak of directed pseudographs. See at directed graph for more.

From yet another perspective, quivers/directed pseudographs are also like small categories with identity morphisms and composition forgotten. Conversely, a small category may be regarded as a quiver/directed pseudograph equipped with extra structure. Formally this is witnessed by a forgetful functor

$U\colon Cat \to Quiv$

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

$F\colon Quiv \to Cat$

sending each quiver to the free category on that quiver.

## Definitions

### Slick definition

The walking quiver1 $X$ is the category with

• one object $X_0$, called the object of vertices;

• one object $X_1$, called the object of edges;

• two morphisms $s, t\colon X_1 \to X_0$, called the source and target;

• together with identity morphisms.

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

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

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

• objects are functors $G\colon X \to C$,

• morphisms are natural transformations between such functors.

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

### Nuts-and-bolts definitions

A quiver $G$ consists of two sets $E$ (the set of edges of $G$), $V$ (the set of vertices of $G$) and two functions

$s, t\colon E \rightrightarrows V$

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

If $G = (E, V, s, t)$ and $G' = (E', V', s', t')$ are two quivers in a category $C$, a morphism $g\colon G \to G'$ is a pair of morphisms $g_0\colon V \to V'$, $g_1\colon E \to E'$ such that $s' \circ g_1 = g_0 \circ s$ and $t' \circ g_1 = g_0 \circ t$.

In graph theory, a quiver is often (cf. p. 4 or Figure 2-2) called a directed pseudograph (or some variation on that theme), but category theorists often just call them directed graphs.

## Remarks

Let $G_0 = G(X_0)$ and $G_1 = G(X_1)$.

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

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

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

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

## 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 $G$ in the sense of quiver representation is nothing but a functor $\rho\colon Q := F(G) \to Vect$ from the free category $F(G)$ on the quiver $G$:

Given a graph $G$ with collection of vertices $G_0$ and collection of edges $G_1$, there is the free category $F(G)$ 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 $G$ 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:

$U\colon Cat \to Quiv$

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

$F\colon Quiv\to Cat$

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 $Q = F(G)$ is a functor

$R\colon Q \to Vect$

Concretely, such a thing assigns a vector space to each vertex of the graph $G$, 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 $F\colon Quiv \to Cat$ is an embedding ($k$-surjective for all $k \gt 0$) 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 $U$ 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 $G$ and a category $D$, then a representation of $G$ in $D$ is precisely a functor from $F(G)$ to $D$ (or equivalently a quiver morphism from $G$ to $U(D)$), and we may well want to think of this as being a morphism (a heteromorphism) from $G$ to $D$. As long as $D$ is not itself a free category, this is unlikely to cause confusion.

### Relation to representation theory of algebras

For $Q$ a quiver, write $k Q$ for the path algebra of $Q$ over a ground field $k$. That is, $k Q$ is an algebra with $k$-basis given by finite composable sequences of arrows in $Q$, 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 $k Q$ is the same thing as a representation of $Q$, so the theory of representations of quivers can be viewed within the broader context of representation theory of (associative) algebras.

If $Q$ is acyclic, then $k Q$ is finite-dimensional as a vector space, so in studying representations of $Q$, 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 (Gabriel 72) says that connected quivers with a finite number of indecomposable quiver representations over an algebraically closed fieldare precisely the Dynkin quivers: 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).

The concept of quiver in the context of quiver representation (and their classification in Gabriel's theorem) originates with

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.

Quivers (referred to as directed pseudographs) were a tool in parts of the work of Ringel and Youngs in the second half the twentieth century to prove Heawood’s formula for every finite genus, cf. e.g. Fig. 2.3 the monograph

• Gerhard Ringel: Map Color Theorem. Springer. Grundlehren Band 209. 1974

Beware that, strictly speaking, for Ringel, “quiver” means “embedded quiver” (into a given surface); in particular the author distinguishes between the two possible orientations of an embedded loop.

Quivers embedded in surfaces are studied in:

• Arthur T. White: Graphs, Groups and Surfaces. North Holland. Completely revised and enlarged edition (1985)

A special kind of quiver (finite, no loops, no parallel arcs) is treated in

• Gregory Gutin, Jørgen Bang-Jensen: Digraphs: Theory, Algorithms and Applications. Springer Monographs in Mathematics. Second Edition (2009)

• William Lawvere: Qualitative Distinctions Between Some Toposes of Generalized Graphs, Contemporary Mathematics 92 (1989)

Some introductory material on the relation between quivers (there called multigraphs) and categories can be found in

1. Also called “the elementary ”parallel process“ ” by Lawvere in p. 272, “a category that has no compositions in it” by Lawvere, in his lecture at Como.