Omega-transitive Representation of Free Groups
Jorge Bruno · arXiv (Cornell University) · 2012
Given a linear order $\Omega$ its automorphism group $\Aut(\Omega)$ forms a lattice-ordered group via pointwise order. Assuming the continuum to be a regular cardinal, we show that \emph{pathological} and \emph{$\omega$-transitive} (i.e. highly transitive) representations of free groups abound within \emph{large} permutation groups of linear orders. Consequently, under the Generalized Continuum Hypothesis it is then true that given any linear order $\Omega$ for which $|\Omega| = $ cof$(\Omega) = \aleph_i$ ($i \in \N$) then any permutation group that is large in $\Aut(\Omega)$ contains an $\omega$-transitive representation of $G_{\aleph_{i}^+}$ (i.e. the free group of rank $2^{\aleph_i}$). In particular, and working solely within ZFC, we show that any large subgroup of $\Aut(\Q)$ (resp. $\Aut(\R)$) contains an $\omega$-transitive and pathological representation of any free group of rank $\lambda \in [\aleph_0,2^{\aleph_0}]$ (resp. of rank $2^{\aleph_0}$). Lastly, we also find a bound on the rank of free subgroups of certain restricted direct products.