Some Results on Polish Groups

Abstract

We prove that no quantifier-free formula in the language of group theory can define the ${\aleph }_1$-half graph in a Polish group, thus generalising some results from [6]. We then pose some questions on the space of groups of automorphisms of a given Borel complete class, and observe that this space must contain at least one uncountable group. Finally, we prove some results on the structure of the group of automorphisms of a locally finite group: firstly, we prove that it is not the case that every group of automorphisms of a graph of power $\lambda$ is the group of automorphism of a locally finite group of power $\lambda$; secondly, we conjecture that the group of automorphisms of a locally finite group of power $\lambda$ has a locally finite subgroup of power $\lambda$, and reduce the problem to a problem on $p$-groups, thus settling the conjecture in the case $\lambda ={\aleph }_0$.