  For a closed topological manifold M with dim (M) >= 5 the topological
structure set S(M) admits an abelian group structure which may be identified
with the algebraic structure group of M as defined by Ranicki. If dim (M) =
2d-1, M is oriented and M is equipped with a map to the classifying space of a
finite group G, then the reduced rho-invariant defines a function,
  \wrho : S(M) \to \QQ R_{hat G}^{(-1)^d}, to a certain sub-quotient of the
complex representation ring of G. We show that the function \wrho is a
homomorphism when 2d-1 >= 5. Along the way we give a detailed proof that a
geometrically defined map due to Cappell and Weinberger realises the 8-fold
Siebenmann periodicity map in topological surgery.
