Wiener–Ikehara theorem

Let A(x) be a non-negative, monotonic nondecreasing function of x, defined for 0 ≤ x f(s)=\int_0^\infty A(x) e^{-xs}\,dx converges for ℜ(s) > 1 to the function ƒ(s) and that, for some non-negative number c, :f(s) - \frac{c}{s-1} has an extension as a continuous function for ℜ(s) ≥ 1. Then the limit as x goes to infinity of e−x A(x) is equal to c. == One Particular Application ==

An important number-theoretic application of the theorem is to Dirichlet series of the form :\sum_{n=1}^\infty a(n) n^{-s} where a(n) is non-negative. If the series converges to an analytic function in :\Re(s) \ge b with a simple pole of residue c at s = b, then :\sum_{n\le X}a(n) \sim \frac{c}{b} X^b. Applying this to the logarithmic derivative of the Riemann zeta function, where the coefficients in the Dirichlet series are values of the von Mangoldt function, it is possible to deduce the Prime number theorem from the fact that the zeta function has no zeroes on the line :\Re(s)=1. ==References==