# Khinchin's constant

In number theory, Khinchin's constant is a mathematical constant related to the simple continued fraction expansions of many real numbers. In particular Aleksandr Yakovlevich Khinchin proved that for almost all real numbers x, the coefficients a$_i$ of the continued fraction expansion of x have a finite geometric mean that is independent of the value of x. It is known as Khinchin's constant and denoted by K$_0$.
That is, for
    $x=a_{0}+{\cfrac {1}{a_{1}+{\cfrac {1}{a_{2}+{\cfrac {1}{a_{3}+{\cfrac {1}{\ddots }}}}}}}}\;$
it is almost always true that
    $\lim _{n\rightarrow \infty }\left(a_{1}a_{2}...a_{n}\right)^{1/n}=K_{0}.$
The decimal value of Khinchin's constant is given by:
    $K_{0}=2.68545\,20010\,65306\,44530\dots$ (sequence A002210 in the OEIS)
Although almost all numbers satisfy this property, it has not been proven for any real number not specifically constructed for the purpose. The following numbers whose continued fraction expansions apparently do have this property (based on empirical data) are:
* π
* Roots of equations with a degree > 2, e.g. cubic roots and quartic roots
* Natural logarithms, e.g. ln(2) and ln(3)
* The Euler-Mascheroni constant γ
* Apéry's constant ζ(3)
* The Feigenbaum constants δ and α
* Khinchin's constant
Among the numbers x whose continued fraction expansions are known not to have this property are:
* Rational numbers
* Roots of quadratic equations, e.g. the square roots of integers and the golden ratio ⁠$\varphi$⁠ (however, the geometric mean of all coefficients for square roots of nonsquare integers from 2 to 24 is about 2.708, suggesting that quadratic roots collectively may give the Khinchin constant as a geometric mean);
* The base of the natural logarithm e.
Khinchin is sometimes spelled Khintchine (the French transliteration of Russian Хинчин) in older mathematical literature.

## Series expressions

Khinchin's constant can be given by the following infinite product:
    $K_{0}=\prod _{r=1}^{\infty }{\left(1+{1 \over r(r+2)}\right)}^{\log _{2}r}$
This implies:
    $\ln K_{0}=\sum _{r=1}^{\infty }\ln {\left(1+{1 \over r(r+2)}\right)}{\log _{2}r}$
Khinchin's constant may also be expressed as a rational zeta series in the form
    $\ln K_{0}={\frac {1}{\ln 2}}\sum _{n=1}^{\infty }{\frac {\zeta (2n)-1}{n}}\sum _{k=1}^{2n-1}{\frac {(-1)^{k+1}}{k}}$
or, by peeling off terms in the series,
    $\ln K_{0}={\frac {1}{\ln 2}}\left[-\sum _{k=2}^{N}\ln \left({\frac {k-1}{k}}\right)\ln \left({\frac {k+1}{k}}\right)+\sum _{n=1}^{\infty }{\frac {\zeta (2n,N+1)}{n}}\sum _{k=1}^{2n-1}{\frac {(-1)^{k+1}}{k}}\right]$
where N is an integer, held fixed, and ζ(s, n) is the complex Hurwitz zeta function. Both series are strongly convergent, as ζ(n) − 1 approaches zero quickly for large n. An expansion may also be given in terms of the dilogarithm:
    $\ln {\frac {K_{0}}{2}}={\frac {1}{\ln 2}}\left[{\mbox{Li}}_{2}\left({\frac {-1}{2}}\right)+{\frac {1}{2}}\sum _{k=2}^{\infty }(-1)^{k}{\mbox{Li}}_{2}\left({\frac {4}{k^{2}}}\right)\right].$

## Integrals

There exist a number of integrals related to Khinchin's constant:
    $\int _{0}^{1}{\frac {\log _{2}\lfloor x^{-1}\rfloor }{x+1}}\mathrm {d} x=\ln {K_{0}}$
    $\int _{0}^{1}{\frac {\log _{2}(\Gamma (2+x)\Gamma (2-x))}{x(x+1)}}\mathrm {d} x=\ln K_{0}-\ln 2$
    $\int _{0}^{1}{\frac {1}{x(x+1)}}\log _{2}\left({\frac {\pi x(1-x^{2})}{\sin \pi x}}\right)\mathrm {d} x=\ln K_{0}-\ln 2$
    $\int _{0}^{\pi }{\frac {\log _{2}(x|\cot x|)}{x}}\mathrm {d} x=\ln K_{0}-{\frac {1}{2}}\ln 2-{\frac {\pi ^{2}}{12\ln 2}}$

## Sketch of proof

The proof presented here was arranged by Czesław Ryll-Nardzewski and is much simpler than Khinchin's original proof which did not use ergodic theory.
Since the first coefficient a$_0$ of the continued fraction of x plays no role in Khinchin's theorem and since the rational numbers have Lebesgue measure zero, we are reduced to the study of irrational numbers in the unit interval, i.e., those in $I=[0,1]\setminus \mathbb {Q}$. These numbers are in bijection with infinite continued fractions of the form [0; a$_1$, a$_2$, ...], which we simply write [a$_1$, a$_2$, ...], where a$_1$, a$_2$, ... are positive integers. Define a transformation T:I → I by
    $T([a_{1},a_{2},\dots ])=[a_{2},a_{3},\dots ].\,$
The transformation T is called the Gauss–Kuzmin–Wirsing operator. For every Borel subset E of I, we also define the Gauss–Kuzmin measure of E
    $\mu (E)={\frac {1}{\ln 2}}\int _{E}{\frac {dx}{1+x}}.$
Then μ is a probability measure on the σ-algebra of Borel subsets of I. The measure μ is equivalent to the Lebesgue measure on I, but it has the additional property that the transformation T preserves the measure μ. Moreover, it can be proved that T is an ergodic transformation of the measurable space I endowed with the probability measure μ (this is the hard part of the proof). The ergodic theorem then says that for any μ-integrable function f on I, the average value of $f\left(T^{k}x\right)$ is the same for almost all $x$:
    $\lim _{n\to \infty }{\frac {1}{n}}\sum _{k=0}^{n-1}(f\circ T^{k})(x)=\int _{I}fd\mu \quad {\text{for }}\mu {\text{-almost all }}x\in I.$
Applying this to the function defined by f([a$_1$, a$_2$, ...]) = ln(a$_1$), we obtain that
    $\lim _{n\to \infty }{\frac {1}{n}}\sum _{k=1}^{n}\ln a_{k}=\int _{I}f\,d\mu =\sum _{r=1}^{\infty }\ln \left[1+{\frac {1}{r(r+2)}}\right]\log _{2}r$
for almost all [a$_1$, a$_2$, ...] in I as n → ∞.
Taking the exponential on both sides, we obtain to the left the geometric mean of the first n coefficients of the continued fraction, and to the right Khinchin's constant.

## Generalizations

The Khinchin constant can be viewed as the first in a series of the Hölder means of the terms of continued fractions. Given an arbitrary series {a$_n$}, the Hölder mean of order p of the series is given by
    $K_{p}=\lim _{n\to \infty }\left[{\frac {1}{n}}\sum _{k=1}^{n}a_{k}^{p}\right]^{1/p}.$
When the {a$_n$} are the terms of a continued fraction expansion, the constants are given by
    $K_{p}=\left[\sum _{k=1}^{\infty }-k^{p}\log _{2}\left(1-{\frac {1}{(k+1)^{2}}}\right)\right]^{1/p}.$
This is obtained by taking the p-th mean in conjunction with the Gauss–Kuzmin distribution. This is finite when $p<1$.
The arithmetic average diverges: $\lim _{n\to \infty }{\frac {1}{n}}\sum _{k=1}^{n}a_{k}=K_{1}=+\infty$, and so the coefficients grow arbitrarily large: $\limsup _{n}a_{n}=+\infty$.
The value for K$_0$ is obtained in the limit of p → 0.
The harmonic mean (p = −1) is
    $K_{-1}=1.74540566240\dots$ (sequence A087491 in the OEIS).

## Open problems

Many well known numbers, such as π, the Euler–Mascheroni constant γ, and Khinchin's constant itself, based on numerical evidence, are thought to be among the numbers for which the limit $\lim _{n\rightarrow \infty }\left(a_{1}a_{2}...a_{n}\right)^{1/n}$ converges to Khinchin's constant. However, none of these limits have been rigorously established. In fact, it has not been proven for any real number, which was not specifically constructed for that exact purpose.
The algebraic properties of Khinchin's constant itself, e. g. whether it is a rational, algebraic irrational, or transcendental number, are also not known.