Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. In mathematics, in semigroup theory, an involution in a semigroup is a transformation of the semigroup which is its own inverse and which is an anti-automorphism of the semigroup. A semigroup in which an involution is defined is called a semigroup with involution. In the multiplicative semigroup of real square matrices of order n, the map which sends a matrix to its transpose is an involution. In the free semigroup generated by a nonempty set the operation which reverses the order of the letters in a word is an involution. A semigroup with an involution is also called a semigroup.