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Relation**

A relation is a proposition of two variables, where typically each variable may belong to a fixed set or class; for \(R\) a relation on \(X,Y\) we typically write this proposition as either \(R(x,y)\) or \(xRy\). One often identifies a relation \(R\) with its graph \(\Gamma_R=\{(x,y)\in X\times Y|xRy\}\), and if \(Y\) is not specified one interprets '\(R\) is a relation on \(X\)' as \(\Gamma_R\subseteq X\times X\).