# Automorphism group

Especially in geometric contexts, an automorphism group is also called a symmetry group. A subgroup of an automorphism group is sometimes called a **transformation group**.

Automorphism groups are studied in a general way in the field of category theory.

If *X* is a set with no additional structure, then any bijection from *X* to itself is an automorphism, and hence the automorphism group of *X* in this case is precisely the symmetric group of *X*. If the set *X* has additional structure, then it may be the case that not all bijections on the set preserve this structure, in which case the automorphism group will be a subgroup of the symmetric group on *X*. Some examples of this include the following:

If *X* is an object in a category, then the automorphism group of *X* is the group consisting of all the invertible morphisms from *X* to itself. It is the unit group of the endomorphism monoid of *X*. (For some examples, see PROP.)

In general, however, an automorphism group functor may not be represented by a scheme.