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Set-theoretic constructions of two-point sets

Ben Chad;  Robin Knight;  Rolf Suabedissen — 2009

Fundamenta Mathematicae

A two-point set is a subset of the plane which meets every line in exactly two points. By working in models of set theory other than ZFC, we demonstrate two new constructions of two-point sets. Our first construction shows that in ZFC + CH there exist two-point sets which are contained within the union of a countable collection of concentric circles. Our second construction shows that in certain models of ZF, we can show the existence of two-point sets without explicitly invoking the Axiom of Choice....

Nonhyperbolic one-dimensional invariant sets with a countably infinite collection of inhomogeneities

Chris Good;  Robin Knight;  Brian Raines — 2006

Fundamenta Mathematicae

We examine the structure of countable closed invariant sets under a dynamical system on a compact metric space. We are motivated by a desire to understand the possible structures of inhomogeneities in one-dimensional nonhyperbolic sets (inverse limits of finite graphs), particularly when those inhomogeneities form a countable set. Using tools from descriptive set theory we prove a surprising restriction on the topological structure of these invariant sets if the map satisfies a weak repelling or...

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