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Characterizing Complete Erdős Space
Published online by Cambridge University Press: 20 November 2018
Abstract
The space now known as complete Erdős space${{\mathfrak{E}}_{\text{c}}}$ was introduced by Paul Erdős in 1940 as the closed subspace of the Hilbert space ${{\ell }^{2}}$ consisting of all vectors such that every coordinate is in the convergent sequence $\left\{ 0 \right\}\cup \left\{ 1/n:n\in \mathbb{N}\ \right\}$. In a solution to a problem posed by Lex $G$. Oversteegen we present simple and useful topological characterizations of ${{\mathfrak{E}}_{\text{c}}}$. As an application we determine the class of factors of ${{\mathfrak{E}}_{\text{c}}}$. In another application we determine precisely which of the spaces that can be constructed in the Banach spaces ${{\ell }^{p}}$ according to the ‘Erdős method’ are homeomorphic to ${{\mathfrak{E}}_{\text{c}}}$. A novel application states that if $I$ is a Polishable ${{F}_{\sigma }}$-ideal on $\omega $, then $I$ with the Polish topology is homeomorphic to either $\mathbb{Z}$, the Cantor set ${{2}^{\omega }},\,\mathbb{Z}\,\times \,{{2}^{\omega }}$, or ${{\mathfrak{E}}_{\text{c}}}$. This last result answers a question that was asked by Stevo Todorčević.
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- Copyright © Canadian Mathematical Society 2009