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Locality and Centrality: The Variety ZG

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arxiv 2102.07724 v5 pith:FSQ5ECVR submitted 2021-02-15 cs.FL

classification cs.FL
keywords languagesvarietyelementslocalmonoidsresultsemigroupsbelong
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We study the variety ZG of monoids where the elements that belong to a group are central, i.e., commute with all other elements. We show that ZG is local, that is, the semidirect product ZG * D of ZG by definite semigroups is equal to LZG, the variety of semigroups where all local monoids are in ZG. Our main result is thus: ZG * D = LZG. We prove this result using Straubing's delay theorem, by considering paths in the category of idempotents. In the process, we obtain the characterization ZG = MNil \vee Com, and also characterize the ZG languages, i.e., the languages whose syntactic monoid is in ZG: they are precisely the languages that are finite unions of disjoint shuffles of singleton languages and regular commutative languages.

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  1. Dynamic Membership for Regular Tree Languages

    cs.FL 2025-04 accept novelty 7.0 of 10

    Dynamic membership for any fixed regular tree language is maintained in O(log n / log log n) per relabeling, with an exact constant-time class under a standard hardness conjecture.

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