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Riemann Xi function

Inmathematics, 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.

Definition[edit]

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

${\displaystyle \xi ($s)={\frac {1}{2}}s(s-1)\pi ^{-s/2}\Gamma \left({\frac {s}{2}}\right)\zeta (s)}

for ${\displaystyle s\in \mathbb {C} }$. Here ${\displaystyle \zeta ($s)} denotes the Riemann zeta function and ${\displaystyle \Gamma ($s)} is the Gamma function.

The functional equation (orreflection formula) for Landau's ${\displaystyle ~\xi ~}$is

${\displaystyle \xi (1-s)=\xi ($s)~.}

Riemann's original function, rebaptised upper-case ${\displaystyle ~\Xi ~}$ by Landau,^[1] satisfies

${\displaystyle \Xi ($z)=\xi \left({\tfrac {1}{2}}+zi\right)} ,

and obeys the functional equation

${\displaystyle \Xi (-z)=\Xi ($z)~.}

Both functions are entire and purely real for real arguments.

Values[edit]

The general form for positive even integers is

${\displaystyle \xi (2n)=(-1)^{n+1}{\frac {n!}{(2n)!}}B_{2n}2^{2n-1}\pi ^{n}(2n-1)}$

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

${\displaystyle \xi ($2)={\frac {\pi }{6}}}

Series representations[edit]

The ${\displaystyle \xi }$ function has the series expansion

${\displaystyle {\frac {d}{dz}}\ln \xi \left({\frac {-z}{1-z}}\right)=\sum _{n=0}^{\infty }\lambda _{n+1}z^{n},}$

where

${\displaystyle \lambda _{n}={\frac {1}{(n-1)!}}\left.{\frac {d^{n}}{ds^{n}}}\left[s^{n-1}\log \xi ($s)\right]\right|_{s=1}=\sum _{\rho }\left[1-\left(1-{\frac {1}{\rho }}\right)^{n}\right],}

where the sum extends over ρ, the non-trivial zeros of the zeta function, in order of ${\displaystyle |\Im (\rho )|}$.

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

Hadamard product[edit]

A simple infinite product expansion is

${\displaystyle \xi ($s)={\frac {1}{2}}\prod _{\rho }\left(1-{\frac {s}{\rho }}\right),\!}

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.

References[edit]

• Weisstein, Eric W. "Xi-Function". MathWorld.
• Keiper, J.B. (1992). "Power series expansions of Riemann's xi function". Mathematics of Computation. 58 (198): 765–773. Bibcode:1992MaCom..58..765K. doi:10.1090/S0025-5718-1992-1122072-5.

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