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 ai 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 K0. That is, for
x = a 0 + 1 a 1 + 1 a 2 + 1 a 3 + 1 ⋱ {\displaystyle 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 → ∞ ( a 1 a 2 . . . a n ) 1 / n = K 0 . {\displaystyle \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 … {\displaystyle 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 itself (which would mean it is irrational) 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 φ {\displaystyle \varphi } ; 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 = ∏ r = 1 ∞ ( 1 + 1 r ( r + 2 ) ) log 2 r {\displaystyle K_{0}=\prod _{r=1}^{\infty }{\left(1+{1 \over r(r+2)}\right)}^{\log _{2}r}}
This implies:
ln K 0 = ∑ r = 1 ∞ ln ( 1 + 1 r ( r + 2 ) ) log 2 r {\displaystyle \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 = 1 ln 2 ∑ n = 1 ∞ ζ ( 2 n ) − 1 n ∑ k = 1 2 n − 1 ( − 1 ) k + 1 k {\displaystyle \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 = 1 ln 2 [ − ∑ k = 2 N ln ( k − 1 k ) ln ( k + 1 k ) + ∑ n = 1 ∞ ζ ( 2 n , N + 1 ) n ∑ k = 1 2 n − 1 ( − 1 ) k + 1 k ] {\displaystyle \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 K 0 2 = 1 ln 2 [ Li 2 ( − 1 2 ) + 1 2 ∑ k = 2 ∞ ( − 1 ) k Li 2 ( 4 k 2 ) ] . {\displaystyle \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:
∫ 0 1 log 2 ⌊ x − 1 ⌋ x + 1 d x = ln K 0 {\displaystyle \int _{0}^{1}{\frac {\log _{2}\lfloor x^{-1}\rfloor }{x+1}}dx=\ln {K_{0}}}
∫ 0 1 log 2 ( Γ ( 2 + x ) Γ ( 2 − x ) ) x ( x + 1 ) d x = ln K 0 − ln 2 {\displaystyle \int _{0}^{1}{\frac {\log _{2}(\Gamma (2+x)\Gamma (2-x))}{x(x+1)}}dx=\ln K_{0}-\ln 2}
∫ 0 1 1 x ( x + 1 ) log 2 ( π x ( 1 − x 2 ) sin π x ) d x = ln K 0 − ln 2 {\displaystyle \int _{0}^{1}{\frac {1}{x(x+1)}}\log _{2}\left({\frac {\pi x(1-x^{2})}{\sin \pi x}}\right)dx=\ln K_{0}-\ln 2}
∫ 0 π log 2 ( x | cot x | ) x d x = ln K 0 − 1 2 ln 2 − π 2 12 ln 2 {\displaystyle \int _{0}^{\pi }{\frac {\log _{2}(x|\cot x|)}{x}}dx=\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 a0 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 ] ∖ Q {\displaystyle I=[0,1]\setminus \mathbb {Q} } . These numbers are in bijection with infinite continued fractions of the form [0; a1, a2, ...], which we simply write [a1, a2, ...], where a1, a2, ... are positive integers. Define a transformation T:I → I by
T ( [ a 1 , a 2 , … ] ) = [ a 2 , a 3 , … ] . {\displaystyle 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
μ ( E ) = 1 ln 2 ∫ E d x 1 + x . {\displaystyle \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 ( T k x ) {\displaystyle f\left(T^{k}x\right)} is the same for almost all x {\displaystyle x} :
lim n → ∞ 1 n ∑ k = 0 n − 1 ( f ∘ T k ) ( x ) = ∫ I f d μ for μ -almost all x ∈ I . {\displaystyle \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([a1, a2, ...]) = ln(a1), we obtain that
lim n → ∞ 1 n ∑ k = 1 n ln a k = ∫ I f d μ = ∑ r = 1 ∞ ln [ 1 + 1 r ( r + 2 ) ] log 2 r {\displaystyle \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 [a1, a2, ...] 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 {an}, the Hölder mean of order p of the series is given by
K p = lim n → ∞ [ 1 n ∑ k = 1 n a k p ] 1 / p . {\displaystyle K_{p}=\lim _{n\to \infty }\left[{\frac {1}{n}}\sum _{k=1}^{n}a_{k}^{p}\right]^{1/p}.}
When the {an} are the terms of a continued fraction expansion, the constants are given by
K p = [ ∑ k = 1 ∞ − k p log 2 ( 1 − 1 ( k + 1 ) 2 ) ] 1 / p . {\displaystyle 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 {\displaystyle p<1} . The arithmetic average diverges: lim n → ∞ 1 n ∑ k = 1 n a k = K 1 = + ∞ {\displaystyle \lim _{n\to \infty }{\frac {1}{n}}\sum _{k=1}^{n}a_{k}=K_{1}=+\infty } , and so the coefficients grow arbitrarily large: lim sup n a n = + ∞ {\displaystyle \limsup _{n}a_{n}=+\infty } . The value for K0 is obtained in the limit of p → 0. The harmonic mean (p = −1) is
K − 1 = 1.74540566240 … {\displaystyle 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 → ∞ ( a 1 a 2 . . . a n ) 1 / n {\displaystyle \lim _{n\rightarrow \infty }\left(a_{1}a_{2}...a_{n}\right)^{1/n}} converges to Khinchin's constant. However, none of these limits has 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.
See also Lochs' theorem Lévy's constant Somos' constant List of mathematical constants
References
David H. Bailey; Jonathan M. Borwein; Richard E. Crandall (1995). "On the Khinchine constant" (PDF). Mathematics of Computation. 66 (217): 417–432. doi:10.1090/s0025-5718-97-00800-4. Jonathan M. Borwein; David M. Bradley; Richard E. Crandall (2000). "Computational Strategies for the Riemann Zeta Function" (PDF). J. Comput. Appl. Math. 121 (1–2): 11. Bibcode:2000JCoAM.121..247B. doi:10.1016/s0377-0427(00)00336-8. Archived from the original (PDF) on 2006-09-25. Retrieved 2005-07-08. Thomas Wieting (2007). "A Khinchin Sequence". Proceedings of the American Mathematical Society. 136 (3): 815–824. doi:10.1090/S0002-9939-07-09202-7. Aleksandr Ya. Khinchin (1997). Continued Fractions. New York: Dover Publications.
External links
110,000 digits of Khinchin's constant 10,000 digits of Khinchin's constant