On a topological Erdős similarity problem

Abstract

A pattern is called universal in another collection of sets, when every set in the collection contains some linear and translated copy of the original pattern. Paul Erdős proposed a conjecture that no infinite set is universal in the collection of sets with positive measure. This paper explores an analogous problem in the topological setting. Instead of sets with positive measure, we investigate the collection of dense 𝐺 𝛿 sets and the collection of generic sets (dense 𝐺 𝛿 and complement has Lebesgue measure zero). We refer to such pattern as topologically universal and generically universal, respectively. It is easy to show that any countable set is topologically universal, while any set containing an interior cannot be topologically universal. In this paper, we will show that Cantor sets on ℝ 𝑑 are not topologically universal and Cantor sets with positive Newhouse thickness on ℝ 1 are not generically universal. This gives a positive partial answer to a question by Svetic concerning the Erdős similarity problem on Cantor sets. Moreover, we also obtain a higher dimensional generalization of the generic universality problem.M S C 2 0 2 0 28A80 (primary) INTRODUCTIONA question that frequently arises has the following generic form: Does every 'large' (or unstructured) set possess a 'copy' of a 'small' (or structured) set? For example, Erdős and Turán