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Zbl 1115.20043
Cain, Alan J.; Robertson, Edmund F.; Ruškuc, Nik
Subsemigroups of virtually free groups: finite Malcev presentations and testing for freeness.
(English)
[J] Math. Proc. Camb. Philos. Soc. 141, No. 1, 57-66 (2006). ISSN 0305-0041; ISSN 1469-8064

Let $F$ be a finitely generated virtually free group, i.e., it has a free subgroup of finite index. The authors prove that it is algorithmically solvable whether a given finite subset $X\subseteq F$ generates a free subsemigroup of $F$. For the proof of this result, the authors introduce a theorem on context-free languages.\par Also, it is shown that each finitely generated subsemigroup of $F$ has a finite Malcev presentation and such a presentation can be effectively found.
[Tero J. Harju (Turku)]
MSC 2000:
*20M05 Free semigroups
20M35 Semigroups in automata theory, linguistics, etc.

Keywords: free subsemigroups; virtually free groups; Malcev presentations; context-free languages; pushdown automata; algorithmic solvability

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