Mathematical Research Letters

Volume 14 (2007)

Number 2

Metric Baumgartner theorems and universality

Pages: 215 – 226

DOI: http://dx.doi.org/10.4310/MRL.2007.v14.n2.a5

Authors

Stefan Geschke (Universität Berlin)

Menachem Kojman (University of the Negev)

Abstract

It is consistent with the axioms of set theory that for every metric space $X$ which is isometric to some separable Banach space or to Urysohn’s universal separable metric space $\mathbb U$ the following holds: \begin{quote} $ {(\star)_X}$ \emph{There exists a nowhere meager subspace of $X$ of cardinality $\aleph_1$ and any two nowhere meager subsets of $X$ of cardinality $\aleph_1$ are almost isometric to each other.} \end{quote} As a corollary, it is consistent that the Continuum Hypothesis fails and the following hold: \begin{enumerate} \item There exists an almost-isometry ultrahomogeneous and universal element in the class of separable metric spaces of size $\aleph_1$. \item For every separable Banach space $X$ there exists an almost-isometry conditionally ultrahomogeneous and universal element in the class of subspaces of $X$ of size $\aleph_1$. \item For every finite dimensional Banach space $X$, there is a \emph{unique} universal element up to almost-isometry in the class of subspaces of $X$ of size $\aleph_1$ \end{enumerate}

Full Text (PDF format)