  Let G be a group, let U(G) denote the set of unbounded operators on L^2(G)
which are affiliated to the group von Neumann algebra W(G) of G, and let D(G)
denote the division closure of CG in U(G). Thus D(G) is the smallest subring of
U(G) containing CG which is closed under taking inverses. If G is a free group
then D(G) is a division ring, and in this case we shall give a criterion for an
element of U(G) to be in D(G). This extends a result of Duchamp and Reutenauer,
which was concerned with proving a conjecture of Connes.
