in mathematics, a correspondence that associates to every element in a set a unique element of the set (perhaps itself) and for which there is a companion correspondence, known as its inverse, such that one followed by the other produces the identity correspondence (i); i.e., the correspondence that associates every element with itself. In symbols, if f is the original correspondence and g is its inverse, then g(f(a))=i(a)=a=i(a)=f(g(a)) for every a in the set. Furthermore, operations such as addition and multiplication must be preserved; for example, f(a+b)=f(a)+f(b) and f(a∙b)=f(a)∙f(b) for every a and b in the set.

The collection of all possible automorphisms for a given set A, denoted Aut(A), forms a group, which can be examined to determine various symmetries in the structure of the set A.