  The core group of a classical link was introduced independently by A.J. Kelly
in 1991 and M. Wada in 1992. It is a link invariant defined by a presentation
involving the arcs and crossings of a diagram, related to Wirtinger's
presentation of the fundamental group of a link complement. Two close relatives
of the core group are defined by presentations involving regions rather than
arcs; one of them is related to Dehn's presentation of a link group. The
definitions are extended to virtual link diagrams and properties of the
resulting invariants are discussed.
