Geometric Characterization
Automatic structures for groups have an elegant geometric characterization called the fellow traveller property (Epstein et al. 1992, ch. 2). Automatic structures for semigroups possess the fellow traveller property but are not in general characterized by it (Campbell et al. 2001). However, the characterization can be generalized to certain 'group-like' classes of semigroups, notably completely simple semigroups (Campbell et al. 2002) and group-embeddable semigroups (Cain et al. 2006).
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