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A new hierarchy for automaton semigroups. (English) Zbl 1474.68168

Summary: We define a new strict and computable hierarchy for the family of automaton semigroups, which reflects the various asymptotic behaviors of the state-activity growth. This hierarchy extends that given by S. Sidki [J. Math. Sci., New York 100, No. 1, 1925–1943 (2000; Zbl 1069.20504)] for automaton groups, and also gives new insights into the latter. Its exponential part coincides with a notion of entropy for some associated automata.
We prove that the Order Problem is decidable whenever the state-activity is bounded. The Order Problem remains open for the next level of this hierarchy, that is, when the state-activity is linear. P. Gillibert [Int. J. Algebra Comput. 24, No. 1, 1–9 (2014; Zbl 1292.20040)] showed that it is undecidable in the whole family.
We extend the aforementioned hierarchy via a semi-norm making it more coarse but somehow more robust and we prove that the Order Problem is still decidable for the first two levels of this alternative hierarchy.

MSC:

68Q45 Formal languages and automata
20M35 Semigroups in automata theory, linguistics, etc.

Software:

FR
Full Text: DOI

References:

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