A series in the form ${\displaystyle \sum _{n=1}^{\infty }{a}_n{e}^{-{\lambda }_{n}z}}$ where a_n and z are complex and {λ‎_n} is a monotonic increasing sequence of real numbers. When λ‎_n = lnn, the series reduces to ${\displaystyle \sum _{n=1}^{\infty }{a}_n{n}^{-z}}$, known as a Dirichlet L‐series.