Let $G = (V, E)$ be a digraph, and consider the set $\left\{ v \in V \; \middle| \; \left(\{v\} \times V \right) \cap E \neq \varnothing  \right\}$, the set of vertices which form edges with other vertices; or if you will, the set of heads of edges.Is there a commonly-used term for this set?
A commonly-used symbol for the set of all heads of directed edges?
graphs;graph theory;reference request;notation