# Gauss–Kuzmin–Wirsing operator

*"GKW" redirects here. For the Indian engineering firm see Guest Keen Williams.*

In mathematics, the **Gauss–Kuzmin–Wirsing operator**, named after Carl Gauss, Rodion Osievich Kuzmin and Eduard Wirsing, occurs in the study of continued fractions; it is also related to the Riemann zeta function.

## Introduction

The Gauss–Kuzmin–Wirsing operator is the transfer operator of the Gauss map

This operator acts on functions as

The first eigenfunction of this operator is

which corresponds to an eigenvalue of *λ*_{1}=1. This eigenfunction gives the probability of the occurrence of a given integer in a continued fraction expansion, and is known as the Gauss–Kuzmin distribution. This follows in part because the Gauss map acts as a truncating shift operator for the continued fractions: if

is the continued fraction representation of a number 0 < *x* < 1, then

Additional eigenvalues can be computed numerically; the next eigenvalue is *λ*_{2} = −0.3036630029... (sequence A038517 in OEIS)
and its absolute value is known as the **Gauss–Kuzmin–Wirsing constant**. Analytic forms for additional eigenfunctions are not known. It is not known if the eigenvalues are irrational.

## Eigenvalues

Let us arrange the eigenvalues of the Gauss–Kuzmin–Wirsing operator according to an absolute value:

It was conjectured in 1995 by Philippe Flajolet and Brigitte Vallée that

In 2014, Giedrius Alkauskas proved this conjecture.^{[1]} Moreover, the following asymptotic result holds:

here the function is bounded, and is the Riemann zeta function.

## Relationship to the Riemann zeta

The GKW operator is related to the Riemann zeta function. Note that the zeta can be written as

which implies that

by change-of-variable.

## Matrix elements

Consider the Taylor series expansions at x=1 for a function *f*(*x*) and . That is, let

and write likewise for *g*(*x*). The expansion is made about *x* = 1 because the GKW operator is poorly behaved at *x* = 0. The expansion is made about 1-x so that we can keep *x* a positive number, 0 ≤ *x* ≤ 1. Then the GKW operator acts on the Taylor coefficients as

where the matrix elements of the GKW operator are given by

This operator is extremely well formed, and thus very numerically tractable. Note that each entry is a finite rational zeta series. The Gauss–Kuzmin constant is easily computed to high precision by numerically diagonalizing the upper-left *n* by *n* portion. There is no known closed-form expression that diagonalizes this operator; that is, there are no closed-form expressions known for the eigenvalues or eigenvectors.

## Riemann zeta

The Riemann zeta can be written as

where the are given by the matrix elements above:

Performing the summations, one gets:

where is the Euler–Mascheroni constant. These play the analog of the Stieltjes constants, but for the falling factorial expansion. By writing

one gets: *a*_{0} = −0.0772156... and *a*_{1} = −0.00474863... and so on. The values get small quickly but are oscillatory. Some explicit sums on these values can be performed. They can be explicitly related to the Stieltjes constants by re-expressing the falling factorial as a polynomial with Stirling number coefficients, and then solving. More generally, the Riemann zeta can be re-expressed as an expansion in terms of Sheffer sequences of polynomials.

This expansion of the Riemann zeta is investigated in ^{[2]}^{[3]}^{[4]}^{[5]}^{[6]} The coefficients are decreasing as

## References

- ↑ Giedrius Alkauskas, http://arxiv.org/abs/1210.4083 Transfer operator for the Gauss' continued fraction map. I. Structure of the eigenvalues and trace formulas (2014).
- ↑ A. Yu. Eremin, I. E. Kaporin, and M. K. Kerimov, "The calculation of the Riemann zeta-function in the complex domain",
*U.S.S.R. Comput. Math. and Math. Phys.***25**(1985), no. 2, 111–119 - ↑ A. Yu. Yeremin, I. E. Kaporin, and M. K. Kerimov, "Computation of the derivatives of the Riemann zeta-function in the complex domain",
*U.S.S.R. Comput. Math. and Math. Phys.***28**(1988), no. 4, 115–124 - ↑ Luis Báez-Duarte, "A New Necessary and Sufficient Condition for the Riemann Hypothesis" (2003)
*ArXiv*math.NT/0307215 - ↑ Luis Báez-Duarte, "A sequential Riesz-like criterion for the Riemann hypothesis",
*Internation Journal of Mathematics and Mathematical Sciences*,**21**, pp. 3527–3537 (2005) - ↑ Philippe Flajolet and Linas Vepstas, "On differences of zeta values", J. Comput. Appl. Math. 220, No. 1-2, 58-73 (2008).

## General references

- A. Ya. Khinchin,
*Continued Fractions*, 1935, English translation University of Chicago Press, 1961 ISBN 0-486-69630-8*(See section 15).* - K. I. Babenko,
*On a Problem of Gauss*, Soviet Mathematical Doklady**19**:136–140 (1978) MR 57 #12436 - K. I. Babenko and S. P. Jur'ev,
*On the Discretization of a Problem of Gauss*, Soviet Mathematical Doklady**19**:731–735 (1978). MR 81h:65015 - A. Durner,
*On a Theorem of Gauss–Kuzmin–Lévy.*Arch. Math.**58**, 251–256, (1992). MR 93c:11056 - A. J. MacLeod,
*High-Accuracy Numerical Values of the Gauss–Kuzmin Continued Fraction Problem.*Computers Math. Appl.**26**, 37–44, (1993). - E. Wirsing,
*On the Theorem of Gauss–Kuzmin–Lévy and a Frobenius-Type Theorem for Function Spaces.*Acta Arith.**24**, 507–528, (1974). MR 49 #2637

## Further reading

- Keith Briggs,
*A precise computation of the Gauss–Kuzmin–Wirsing constant*(2003)*(Contains a very extensive collection of references.)* - Phillipe Flajolet and Brigitte Vallée,
*On the Gauss–Kuzmin–Wirsing Constant*(1995). - Linas Vepstas The Bernoulli Operator, the Gauss–Kuzmin–Wirsing Operator, and the Riemann Zeta (2004) (PDF)