Relation/Examples/Sisterhood

Example of Relation
Let $S$ be the set of all human females.

Let $T$ be the set of all human beings.

Let $\RR$ be the set defined as:


 * $\RR = \set {\tuple {a, b}: a \in S, b \in T, \text {$a$ is a sister of $b$} }$

Then $\RR$ is a relation in $S$ to $T$.