Let $R\subset A\times B$ be a relation.[1][2]
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The domain of $R$ is the subset $\mathrm{dom}\webleft (R\webright )$ of $A$ defined by
\[ \mathrm{dom}\webleft (R\webright )\mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\webleft\{ a\in A\ \middle |\ \begin{aligned} & \text{there exists some $b\in B$}\\ & \text{such that $a\sim _{R}b$}\\ \end{aligned} \webright\} . \]
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The range of $R$ is the subset $\mathrm{range}\webleft (R\webright )$ of $B$ defined by
\[ \mathrm{range}\webleft (R\webright )\mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\webleft\{ b\in B\ \middle |\ \begin{aligned} & \text{there exists some $a\in A$}\\ & \text{such that $a\sim _{R}b$}\\ \end{aligned} \webright\} . \]