Given two sets \(X\) and \(Y\), \(X^Y\) denotes the set of all functions defined from \(Y\) to \(X\).

The notation is motivated by the fact that given two sets \(X\) and \(Y\) with cardinalities \(m\) and \(n\), respectively. There exists \(m^n\) functions from \(Y\) to \(X\).