Vivanti–Pringsheim theorem
The Vivanti–Pringsheim theorem is a mathematical statement in complex analysis, that determines a specific singularity for a function described by certain type of power series. The theorem was originally formulated by Giulio Vivanti in 1893 and proved in the following year by Alfred Pringsheim. More precisely the theorem states the following: A complex function defined by a power series with non-negative real coefficients formula_2 and a radius of convergence formula_3 has a singularity at formula_4. A simple example is the (complex) geometric series with a singularity at formula_6.