Involution semigroup

2020 Mathematics Subject Classification: Primary: 20M10 [MSN][ZBL]

A semigroup $(S,{\cdot})$ with an involution $*$, having the properties $(x\cdot y)^* = y^* \cdot x^*$ and $x^{{*}{*}} = x$.

A projection in an involution semigroup is an element $e$ such that $e\cdot e = e = e^*$. There is a partial order on projections given by $e \le f$ if $e\cdot f = e$.

A regular involution semigroup is one in which $x x^* x = x$: that is, it is also a regular semigroup in which the involution acts as generalised inverse.

#### References[edit]

* Ivan Rival (ed.),"Algorithms and Order", Kluwer (1989) ISBN 940107691X Zbl 0709.68004