The union of $R$ and $S$1 is the relation $R\cup S$ from $A$ to $B$ defined as follows:
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Viewing relations from $A$ to $B$ as subsets of $A\times B$, we define2
\[ R\cup S\mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\left\{ (a,b)\in B\times A\ \middle |\ \text{we have $a\sim _{R}b$ or $a\sim _{S}b$}\right\} . \] -
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Viewing relations from $A$ to $B$ as functions $A\to \mathcal{P}(B)$, we define
\[ [R\cup S](a)\mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}R(a)\cup S(a) \]for each $a\in A$.
- 1Further Terminology: Also called the binary union of $R$ and $S$, for emphasis.
- 2This is the same as the union of $R$ and $S$ as subsets of $A\times B$.
