Definition:Reachability Relation/Definition 2

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Let $G = \left({V, A}\right)$ be a directed graph.

Let $\mathcal R$ be the relation on $V$ defined by letting $x \mathrel {\mathcal R} y$ if and only if $y$ is reachable from $x$.

That is, $x \mathrel {\mathcal R} y$ if and only if there exists a directed walk from $x$ to $y$.

Then $\mathcal R$ is the reachability relation of $G$.