Submission date: 18 February 2010.

Abstract:

We consider the question of when an expansion of a topological structure has the property that every open set definable in the expansion is definable in the original structure. This question is related to and inspired by recent work of Dolich, Miller and Steinhorn on the property of having o-minimal open core. We answer the question in a fairly general setting and provide conditions which in practice are often easy to check. We give a further characterisation in the special case of an expansion by a generic predicate.

Mathematics Subject Classification: 03C64

Keywords and phrases: first order topological structure, open core, dense pair, generic predicate

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