When a group *G* has a permutation action on a set *S*,
the orbit of an element *s* is defined to be the subset of
*S* consisting of those elements to which some element of *G*
can send *s*.

In a directed graph, we say that a vertex has outdegree *x* if
there are (exactly) *x* edges leaving that vertex.

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An On-line Dictionary of Combinatorics