This paper establishes connections between the boundary behaviour of functions representable as absolutely convergent Dirichlet series in a half-plane and the convergence properties of partial sums of the Dirichlet series on the boundary. This yields insights into the boundary behaviour of Dirichlet series and Taylor series which have universal approximation properties.
General Dirichlet seriesWe consider a general Dirichlet series of the form f (s) = ∞ n=1 a n e −λns , where (λ n ) is an unbounded strictly increasing sequence of nonnegative real numbers. If this series converges when s = s 0 , then, as is well known, it converges uniformly throughout any angular region of the form {s ∈ C : |arg(s − s 0 )| < π/2 − δ}, where δ ∈ (0, π/2). In particular, f is defined on the half-plane {Re s > Re s 0 } and has a nontangential limit at s 0 . No such conclusion may be drawn if we know merely that some subsequence of the partial sums of the series converges at s = s 0 (see, for example, Bayart [3]). Nevertheless, we show below that there are strong connections between the nontangential boundary behaviour of f and the limiting behaviour of such subsequences. This has implications for the boundary behaviour of universal Dirichlet series and universal Taylor series.A typical point of the complex plane will be written as s = σ + it. We denote by C + the right-hand half-plane {σ > 0}, and by D(C + ) the space of all holomorphic functions on C + which can be represented there as an absolutely convergent Dirichlet series