Reachable (from a) vertex

Reachable (from [math]\displaystyle{ a }[/math]) vertex --- достижимая (из [math]\displaystyle{ a }[/math]) вершина. Given a digraph [math]\displaystyle{ G = (V,A) }[/math], a vertex [math]\displaystyle{ w \in V }[/math] is called reachable from [math]\displaystyle{ v \in V }[/math] iff there exists a path from [math]\displaystyle{ v }[/math] to [math]\displaystyle{ w }[/math].