Involution semigroup
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This category corresponds roughly to MSC {{{id}}} {{{title}}}; see {{{id}}} at MathSciNet and {{{id}}} at zbMATH.
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
- Ivan Rival (ed.),"Algorithms and Order", Kluwer (1989) ISBN 940107691X Template:ZBL
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