2 Definition

A relation is simply a subset of a Cartesian product of sets. That's it! In other words, a relation $L$ is defined as $L \subseteq \prod_{i=1}^{n}X_i$ . Each one of $X_1, X_2, \ldots, X_n$ is a set. Together, the $X_i$ sets are the domains of the relation. The notation $\prod_{i=0}^{n}X_i$ is the Cartesian product of $X_1, X_2, X_3,\ldots X_n$ . Relation $L$ is said to be a relation over sets $X_1, X_2, X_3,\ldots X_n$ .