Introduction to Representation Theory
《Introduction to Representation Theory》 · 第 19—20 页 · Section 2.8「Quivers」，顶点集的记号约定及 Definition 2.8.4（省略中间内容）
We denote the set of vertices of the quiver $Q$ as $I$ and the set of edges as $E$. […] Definition 2.8.4. The path algebra $P_Q$ of a quiver $Q$ is the algebra whose basis is formed by oriented paths in $Q$, including the trivial paths $p_i$, $i\in I$, corresponding to the vertices of $Q$, and multiplication is the concatenation of paths: $ab$ is the path obtained by first tracing $b$ and then $a$. If two paths cannot be concatenated, the product is defined to be zero.