Semigroup with involution: Difference between revisions

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: <math>w^\dagger=w_k^\dagger w_{k-1}^\dagger \cdots w_{2}^\dagger w_{1}^\dagger.</math>
 
This map is an [[#Formal definition|involution]] on the semigroup <math>Y^+\,</math>. Thus, the semigroup <math>(X\sqcup X^\dagger)^+</math> with the map <math>{ }^\dagger\,</math> is a semigroup with involution, called a '''free semigroup with involution''' on ''X''.<ref name="L172">Lawson p. 172</ref> (The irrelevance of the concrete identity of <math>X^\dagger</math> and of the bijection <math>\theta</math> in this choice of terminology is explained below in terms of the universal property of the construction.) Note that unlike in [[#ex6|exampleExample 6]], the involution ''of every letter'' is a distinct element in an alphabet with involution, and consequently the same observation extends to a free semigroup with involution.
 
If in the above construction instead of <math>Y^+\,</math> we use the [[free monoid]] <math>Y^*=Y^+\cup\{\varepsilon\}</math>, which is just the free semigroup extended with the [[empty word]] <math>\varepsilon\,</math> (which is the [[identity element]] of the [[monoid]] <math>Y^*\,</math>), and suitably extend the involution with <math>\varepsilon^\dagger = \varepsilon</math>,