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Title: On plausible counterexamples to Lehnert's conjecture
Author: Bennett, Daniel
ISNI:       0000 0004 7234 390X
Awarding Body: University of St Andrews
Current Institution: University of St Andrews
Date of Award: 2018
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A group whose co-word problem is a context free language is called coCF. Lehnert's conjecture states that a group G is coCF if and only if G embeds as a finitely generated subgroup of R. Thompson's group V. In this thesis we explore a class of groups, Faug, proposed by Berns-Zieze, Fry, Gillings, Hoganson, and Mathews to contain potential counterexamples to Lehnert's conjecture. We create infinite and finite presentations for such groups and go on to prove that a certain subclass of Faug consists of groups that do embed into V. By Anisimov a group has regular word problem if and only if it is finite. It is also known that a group G is finite if and only if there exists an embedding of G into V such that its natural action on C₂:= {0,1}w is free on the whole space. We show that the class of groups with a context free word problem, the class of CF groups, is precisely the class of finitely generated demonstrable groups for V. A demonstrable group for V is a group G which is isomorphic to a subgroup in V whose natural action on C₂ acts freely on an open subset. Thus our result extends the correspondence between language theoretic properties of groups and dynamical properties of subgroups of V. Additionally, our result also shows that the final condition of the four known closure properties of the class of coCF groups also holds for the set of finitely generated subgroups of V.
Supervisor: Bleak, Collin Patrick ; Quick, Martyn R. Sponsor: Engineering and Physical Sciences Research Council (EPSRC)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QA174.2B4 ; Formal languages ; Group theory ; Semigroups