Representations of a quiver can be interpreted as modules over a non-commutative algebra A(Q) whose elements are linear combinations of paths in Q.
Let Q be a quiver. A non-trivial path in Q is a sequence of arrows am…a0 such that h(ai−1) = t(ai) for i = 1,…, m:
The path is p = am…a0. Writing t(p) = t(a0) and saying that p starts at t(a0) and, similarly, writing h(p) = h(am) and saying that p finishes at h(am). For each vertex i ∈ Q0, we denote by ei the trivial path which starts and finishes at i. Two paths p and q are compatible if t(p) = h(q) and, in this case, the composition pq can defined by juxtaposition of p and q. The length l(p) of a path is the number of arrows it contains; in particular, a trivial path has length zero.
The path algebra A(Q) of a quiver Q is the complex vector space with basis consisting of all paths in Q, equipped with the multiplication in which the product pq of paths p and q is defined to be the composition pq if t(p) = h(q), and 0 otherwise. Composition of paths is non-commutative; in most cases, if p and q can be composed one way, then they cannot be composed the other way, and even if they can, usually pq ≠ qp. Hence the path algebra is indeed non-commutative.
Let us define Al ⊂ A to be the subspace spanned by paths of length l. Then A = ⊕l≥0Al is a graded C-algebra. The subring A0 ⊂ A spanned by the trivial paths ei is a semisimple ring in which the elements ei are orthogonal idempotents, in other words eiej = ei when i = j, and 0 otherwise. The algebra A is finite-dimensional precisely if Q has no directed cycles.
The category of finite-dimensional representations of a quiver Q is isomorphic to the category of finitely generated left A(Q)-modules. Let (V, φ) be a representation of Q. We can then define a left module V over the algebra A = A(Q) as follows: as a vector space it is
V = ⊕i∈Q0 Vi
and the A-module structure is extended linearly from
eiv = v, v ∈ Mi
= 0, v ∈ Mj for j ≠ i
for i ∈ Q0 and
av = φa(vt(a)), v ∈ Vt(a)
= 0, v ∈ Vj for j ≠ t(a)
for a ∈ Q1. This construction can be inverted as follows: given a left A-module V, we set Vi = eiV for i ∈ Q0 and define the map φa: Vt(a) → Vh(a) by v ↦ a(v). Morphisms of representations of (Q, V) correspond to A-module homomorphisms.