J\'onsson groups of various cardinalities

Abstract

A group $G$ is J\'onsson if $|H| < |G|$ whenever $H$ is a proper subgroup of $G$. Using an embedding theorem of Obraztsov it is shown that there exists a J\'onsson group $G$ of infinite cardinality $\kappa$ if and only if there exists a J\'onsson algebra of cardinality $\kappa$. Thus the question as to which cardinals admit a J\'onsson group is wholly reduced to the well-studied question of which cardinals are not J\'onsson. As a consequence there exist J\'onsson groups of arbitrarily large cardinality. Another consequence is that the infinitary edge-orbit conjecture of Babai is true.