Barnes G-function

In mathematics, the Barnes G-function G(z) is a function that is an extension of superfactorials to the complex numbers. It is related to the gamma function, the K-function and the Glaisher–Kinkelin constant, and was named after mathematician Ernest William Barnes.[1] It can be written in terms of the double gamma function.

The Barnes G function along part of the real axis

Formally, the Barnes G-function is defined in the following Weierstrass product form:

where is the Euler–Mascheroni constant, exp(x) = ex, and ∏ is capital pi notation.

Functional equation and integer argumentsEdit

The Barnes G-function satisfies the functional equation

 

with normalisation G(1) = 1. Note the similarity between the functional equation of the Barnes G-function and that of the Euler gamma function:

 

The functional equation implies that G takes the following values at integer arguments:

 

(in particular,  ) and thus

 

where   denotes the gamma function and K denotes the K-function. The functional equation uniquely defines the G function if the convexity condition:   is added.[2]

Value at 1/2Edit

 

Reflection formula 1.0Edit

The difference equation for the G-function, in conjunction with the functional equation for the gamma function, can be used to obtain the following reflection formula for the Barnes G-function (originally proved by Hermann Kinkelin):

 

The logtangent integral on the right-hand side can be evaluated in terms of the Clausen function (of order 2), as is shown below:

 

The proof of this result hinges on the following evaluation of the cotangent integral: introducing the notation   for the logcotangent integral, and using the fact that  , an integration by parts gives

 

Performing the integral substitution   gives

 

The Clausen function – of second order – has the integral representation

 

However, within the interval  , the absolute value sign within the integrand can be omitted, since within the range the 'half-sine' function in the integral is strictly positive, and strictly non-zero. Comparing this definition with the result above for the logtangent integral, the following relation clearly holds:

 

Thus, after a slight rearrangement of terms, the proof is complete:

 

Using the relation   and dividing the reflection formula by a factor of   gives the equivalent form:

 


Ref: see Adamchik below for an equivalent form of the reflection formula, but with a different proof.

Reflection formula 2.0Edit

Replacing z with (1/2) − z'' in the previous reflection formula gives, after some simplification, the equivalent formula shown below (involving Bernoulli polynomials):

 
 

Taylor series expansionEdit

By Taylor's theorem, and considering the logarithmic derivatives of the Barnes function, the following series expansion can be obtained:

 

It is valid for  . Here,   is the Riemann Zeta function:

 

Exponentiating both sides of the Taylor expansion gives:

 

Comparing this with the Weierstrass product form of the Barnes function gives the following relation:

 

Multiplication formulaEdit

Like the gamma function, the G-function also has a multiplication formula:[3]

 

where   is a constant given by:

 

Here   is the derivative of the Riemann zeta function and   is the Glaisher–Kinkelin constant.

Asymptotic expansionEdit

The logarithm of G(z + 1) has the following asymptotic expansion, as established by Barnes:

 

Here the   are the Bernoulli numbers and   is the Glaisher–Kinkelin constant. (Note that somewhat confusingly at the time of Barnes [4] the Bernoulli number   would have been written as  , but this convention is no longer current.) This expansion is valid for   in any sector not containing the negative real axis with   large.

Relation to the Loggamma integralEdit

The parametric Loggamma can be evaluated in terms of the Barnes G-function (Ref: this result is found in Adamchik below, but stated without proof):

 

The proof is somewhat indirect, and involves first considering the logarithmic difference of the gamma function and Barnes G-function:

 

where

 

and   is the Euler–Mascheroni constant.

Taking the logarithm of the Weierstrass product forms of the Barnes function and gamma function gives:

 

A little simplification and re-ordering of terms gives the series expansion:

 

Finally, take the logarithm of the Weierstrass product form of the gamma function, and integrate over the interval   to obtain:

 

Equating the two evaluations completes the proof:

 

And since   then,

 

ReferencesEdit

  1. ^ E. W. Barnes, "The theory of the G-function", Quarterly Journ. Pure and Appl. Math. 31 (1900), 264–314.
  2. ^ M. F. Vignéras, L'équation fonctionelle de la fonction zêta de Selberg du groupe mudulaire SL , Astérisque 61, 235–249 (1979).
  3. ^ I. Vardi, Determinants of Laplacians and multiple gamma functions, SIAM J. Math. Anal. 19, 493–507 (1988).
  4. ^ E. T. Whittaker and G. N. Watson, "A Course of Modern Analysis", CUP.
  • Askey, R.A.; Roy, R. (2010), "Barnes G-function", in Olver, Frank W. J.; Lozier, Daniel M.; Boisvert, Ronald F.; Clark, Charles W. (eds.), NIST Handbook of Mathematical Functions, Cambridge University Press, ISBN 978-0-521-19225-5, MR 2723248
  • Adamchik, Viktor S. (2003). "Contributions to the Theory of the Barnes function". arXiv:math/0308086.