Unary FA-presentable binary relations: transitivity and classification results

Automatic presentations, also called FA-presentations, were introduced to extend finite model theory to infinite structures whilst retaining the solubility of fundamental decision problems. A particular focus of research has been the classification of those structures of some species that admit FA-presentations. Whilst some successes have been obtained, this appears to be a difficult problem in general. A restricted problem, also of significant interest, is to ask this question for unary FA-presentations: that is, FA-presentations over a one-letter alphabet. This paper studies unary FA-presentable binary relations. It is proven that transitive closure of a unary FA-presentable binary relation is itself unary FA-presentable. Characterizations are then given of unary FA-presentable binary relations, quasi-orders, partial orders, tournaments, directed trees and forests, undirected trees and forests, and the orbit structures of unary FA-presentable partial and complete mappings, injections, surjections, and bijections.

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