General Dirichlet series

In the field of mathematical analysis, a general Dirichlet series is an infinite series that takes the form of where , are complex numbers and is a strictly increasing sequence of nonnegative real numbers that tends to infinity. A simple observation shows that an 'ordinary' Dirichlet series is obtained by substituting while a power series is obtained when . If a Dirichlet series is convergent at , then it is uniformly convergent in the domain and convergent for any where . There are now three possibilities regarding the convergence of a Dirichlet series, i.e. it may converge for all, for none or for some values of s. In the latter case, there exist a such that the series is convergent for and divergent for . By convention, if the series converges nowhere and if the series converges everywhere on the complex plane. The abscissa of convergence of a Dirichlet series can be defined as above. Another equivalent definition is The line is called the line of convergence. The half-plane of convergence is defined as The abscissa, line and half-plane of convergence of a Dirichlet series are analogous to radius, boundary and disk of convergence of a power series. On the line of convergence, the question of convergence remains open as in the case of power series. However, if a Dirichlet series converges and diverges at different points on the same vertical line, then this line must be the line of convergence. The proof is implicit in the definition of abscissa of convergence. An example would be the series which converges at (alternating harmonic series) and diverges at (harmonic series). Thus, is the line of convergence. Suppose that a Dirichlet series does not converge at , then it is clear that and diverges. On the other hand, if a Dirichlet series converges at , then and converges. Thus, there are two formulas to compute , depending on the convergence of which can be determined by various convergence tests. These formulas are similar to the Cauchy–Hadamard theorem for the radius of convergence of a power series. If is divergent, i.