Extension of almost periodic functions and the distribution of zeros

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Using the ergodic theory and the theory of function algebras, we investigate the property of Dirichlet series by regarding as analytic functions on Bohr group. In connection with analytic number theory, we especially restrict our attention to the case of the Riemann ζ-function and obtain a mean-value theorem in a weak sense and some results on value distribution of ζ-function.Let K be the dual group of the discrete group {log γ ; γ positive rational}. Then a one-parameter group {T_t}_<t∈R> of homeomorphisms of K is defined naturally. Fix 1/2 < u and putZ_u(x) = Σ^^∞___<n=1>(1)/(n^u)x_<log n>^(x), x∈K,where x_<log n> denotes the character by log n. Then t → Z_u(T_tO) represents ζ(u+it), and Z_u(x) is an outer function of H^2(K). Since (K, {T_t}_<t∈R>) is an ergodic flow, we have the following mean-value theorem of the Riemann ζ-function : Theorem Let 0 < k < ∞, and let l > 0. Then there is a subset J of Z^+ of density zero such that<lim>___<J【∋!/】N→∞>(1)/(Nl)Σ^^^<N-1>___<n=0>∫^^^<(n+1)l>___<nl>|ζ(u+it)|^<2k>dt=∫_K|Z_u(x)|^<2k>dσ(x). This theorem shows that Lindelof hypothesis holds in a weak sense. We also study the class of all Dirichlet series t → Z_u(T_t x) with Euler products. This enables us to understand the peculiarity of the Riemann ζ-function.