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Riemann Xi function

From Wikipedia, the free encyclopedia
Riemann xi function in the complex plane. The color of a point encodes the value of the function. Darker colors denote values closer to zero and hue encodes the value's argument.

In mathematics, the Riemann Xi function is a variant of the Riemann zeta function, and is defined so as to have a particularly simple functional equation. The function is named in honour of Bernhard Riemann.


Riemann's original lower-case "xi"-function, was renamed with an upper-case (Greek letter "Xi") by Edmund Landau. Landau's lower-case ("xi") is defined as[1]

for . Here denotes the Riemann zeta function and is the Gamma function.

The functional equation (or reflection formula) for Landau's is

Riemann's original function, rebaptised upper-case by Landau,[1] satisfies


and obeys the functional equation

Both functions are entire and purely real for real arguments.


The general form for positive even integers is

where Bn denotes the n-th Bernoulli number. For example:

Series representations[edit]

The function has the series expansion


where the sum extends over ρ, the non-trivial zeros of the zeta function, in order of .

This expansion plays a particularly important role in Li's criterion, which states that the Riemann hypothesis is equivalent to having λn > 0 for all positive n.

Hadamard product[edit]

A simple infinite product expansion is

where ρ ranges over the roots of ξ.

To ensure convergence in the expansion, the product should be taken over "matching pairs" of zeroes, i.e., the factors for a pair of zeroes of the form ρ and 1−ρ should be grouped together.


  1. ^ a b Landau, Edmund (1974) [1909]. Handbuch der Lehre von der Verteilung der Primzahlen [Handbook of the Study of Distribution of the Prime Numbers] (Third ed.). New York: Chelsea. §70-71 and page 894.

This article incorporates material from Riemann Ξ function on PlanetMath, which is licensed under the Creative Commons Attribution/Share-Alike License.