  The difference between the quadratic L-groups L_*(A) and the symmetric
L-groups L^*(A) of a ring with involution A is detected by generalized Arf
invariants. The special case A=Z[x] gives a complete set of invariants for the
Cappell UNil-groups UNil_*(Z;Z,Z) for the infinite dihedral group
D_{\infty}=Z_2*Z_2, extending the results of Connolly and Ranicki
(math.AT/0304016) and Connolly and Davis.
