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单词 FreeSemigroupWithInvolution
释义

free semigroup with involution


Let X,X be two disjoint sets in bijectiveMathworldPlanetmathPlanetmath correspondence given by the map :XX. Denote by Y=XX (here we use instead of to remind that the union is actually a disjoint unionMathworldPlanetmathPlanetmath) and by Y+ the free semigroupMathworldPlanetmath on Y. We can extend the map to an involution :Y+Y+ on Y+ in the following way: given wY+, we have w=w1w2wk for some letters wiY; then we define

w=wkwk-1w2w1.

It is easily verified that this is the unique way to extend to an involution on Y. Thus, the semigroupPlanetmathPlanetmath (XX)+ with the involution is a semigroup with involution. Moreover, it is the free semigroup with involution on X, in the sense that it solves the following universal problem: given a semigroup with involution S and a map Φ:XS, a semigroup homomorphism Φ¯:(XX)+S exists such that the following diagram commutes:

\\xymatrix&X\\ar[r]ι\\ar[d]Φ&(XX)+\\ar[dl]Φ¯&S&

where ι:X(XX)+ is the inclusion mapMathworldPlanetmath. It is well known from universal algebraMathworldPlanetmathPlanetmath that (XX)+ is unique up to isomorphismsMathworldPlanetmathPlanetmathPlanetmathPlanetmathPlanetmathPlanetmathPlanetmath.

If we use Y* instead of Y+, where Y*=Y+{ε} and ε is the empty wordPlanetmathPlanetmath (i.e. the identityPlanetmathPlanetmathPlanetmathPlanetmath of the monoid Y*), we obtain a monoid with involution (XX)* that is the free monoid with involution on X.

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更新时间:2025/5/5 2:46:59