Functions and Relations
G. P. McKeown, Victor J. Rayward-Smith · 1995
Both functions and relations are devices for associating elements of an input set with elements of an output set. They differ in that a function associates a particular input with a single output, whereas a relation may associate a particular input with many different elements of the output set. Although they may be regarded as special cases of relations, functions are so important in mathematics and computing that we treat them separately.