Antiisomorphism

In category theory, a branch of mathematics, an antiisomorphism between structured sets A and B is an isomorphism from A to the opposite of B. If there exists an antiisomorphism between two structures, they are said to be antiisomorphic.
Intuitively, to say that two mathematical structures are antiisomorphic is to say that they are basically opposites of one another.
The concept is particularly useful in an algebraic setting, as, for instance, when applied to rings.

Simple example

Let A be the binary relation consisting of elements and binary relation defined as follows:

Let B be the binary relation set consisting of elements and binary relation defined as follows:

Note that the opposite of B is the same set of elements with the opposite binary relation :

If we replace a, b, and c with 1, 2, and 3 respectively, we see that each rule in B^op is the same as some rule in A. That is, we can define an isomorphism from A to B^op by. is then an antiisomorphism between A and B.

Ring anti-isomorphisms

Specializing the general language of category theory to the algebraic topic of rings, we have: Let R and S be rings and f: R → S be a bijection. Then f is a ring anti-isomorphism if
If R = S then f is a ring anti-automorphism.
An example of a ring anti-automorphism is given by the conjugate mapping of quaternions: