Definition:Rooted Tree/Child Node

Definition
Let $T$ be a rooted tree with root $r_T$.

Let $t$ be a node of $T$.

The child nodes of $t$ are the elements of the set:
 * $\set {s \in T: \map \pi s = t}$

where $\map \pi s$ denotes the parent mapping of $s$.

That is, the children of $t$ are all the nodes of $T$ of which $t$ is the parent.