  Let S be a subset of the unit disk, and let F(s) denote the class of
completely multiplicative functions f such that f(p) is in S for all primes p.
The authors' main concern is which numbers arise as mean-values of functions in
F(s). More precisely, let Gamma_N(S) = {1/N sum_{n <= N} f(n): f in F(S)} and
Gamma(S) = lim_{N -> infinity} Gamma_N(s). The authors call Gamma(S) the
spectrum of the set S, and study its properties.
