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free semigroup with involution
Let be two disjoint sets in bijective correspondence given by the map . Denote by (here we use instead of to remind that the union is actually a disjoint union) and by the free semigroup on . We can extend the map to an involution on in the following way: given , we have for some letters ; then we define
It is easily verified that this is the unique way to extend to an involution on . Thus, the semigroup with the involution is a semigroup with involution. Moreover, it is the free semigroup with involution on , in the sense that it solves the following universal problem: given a semigroup with involution and a map , a semigroup homomorphism exists such that the following diagram commutes:
where is the inclusion map. It is well known from universal algebra that is unique up to isomorphisms.
If we use instead of , where and is the empty word (i.e. the identity of the monoid ), we obtain a monoid with involution that is the free monoid with involution on .
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