Hall's universal group is a subgroup of the abstract commensurator of a
free group
Hall's universal group is a subgroup of the abstract commensurator of a
free group
P. Hall constructed a universal countable locally finite group U, determined up to isomorphism by two properties: every finite group C is a subgroup of U, and every embedding of C into U is conjugate in U. Every countable locally finite group is a subgroup of U. We prove that …