As a special case of an $n$-ary relation on $S$, note that when $n = 1$ we define a unary relation on $S$ as:
- $\mathcal R \subseteq S$
That is, a unary relation is a subset of $S$.
- 1993: Keith Devlin: The Joy of Sets: Fundamentals of Contemporary Set Theory (2nd ed.) ... (previous) ... (next): $\S 1.5$: Relations