Abstract. We give a new method to construct isolated left orderings of groups whose positive cones are finitely generated. Our construction uses an amalgamated free product of two groups having an isolated ordering. We construct a lot of new examples of isolated orderings, and give an example of isolated left orderings having various properties which previously known isolated orderings do not have.
IntroductionThe set of all left orderings of G is denoted by LO(G). For g ∈ G, let U g be a subset of LO(G) defined byWe equip a topology on LO(G) so that {U g } g∈G is an open sub-basis of the topology. This topology is understood as follows. For a left ordering < G of G, G is decomposed as a disjoint union G = P (< G ) ⊔ {1} ⊔ P (< G ) −1 using the positive cone P (< G ). Conversely, a sub-semigroup P of G having this property is a positive cone of a left ordering of G: An ordering < P defined by g < P g ′ if g −1 g ′ ∈ P is a leftordering whose positive cone is P . Thus LO(G) is identified with a subset of the powerset 2 G−{1} . The topology of LO(G) defined as above coincides with the relative topology as the subspace of 2 G−{1} , equipped with the powerset topology. In this paper, we always consider countable groups, so we simply refer a countable group as a group unless otherwise specified. Then it is known that LO(G) is a compact, metrizable, and totally disconnected [10]. Moreover, LO(G) is either uncountable or finite [5]. Thus as a topological space, LO(G) is rather similar to the Cantor set: The main difference is that the space LO(G) might be non-perfect, that is, LO(G) might have isolated points. Indeed, if LO(G) has no isolated points and is not a finite set, then LO(G) is homeomorphic to the Cantor set. We call a left ordering which is an isolated point of LO(G) an isolated ordering.It is known that a left ordering < G whose positive cone is a finitely generated semigroup is isolated. In this paper we will concentrate our attention to study such an isolated ordering. We say a finite set of non-trivial elements of G, G = {g 1 , . . . , g r } defines an isolated left ordering < G of G if the positive cone of < G is generated by G as a semigroup. For an isolated left ordering < G of a group G, the 2010 Mathematics Subject Classification. Primary 20F60 , Secondary 06F15.