Khinchin's constant

In number theory, Aleksandr Yakovlevich Khinchin proved that for almost all real numbers x, coefficients a_i of the continued fraction expansion of x have a finite geometric mean that is independent of the value of x and is known as Khinchin's constant.

That is, for

${\displaystyle x=a_{0}+{\cfrac {1}{a_{1}+{\cfrac {1}{a_{2}+{\cfrac {1}{a_{3}+{\cfrac {1}{\ddots }}}}}}}}\;}$

it is almost always true that

${\displaystyle \lim _{n\rightarrow \infty }\left(a_{1}a_{2}...a_{n}\right)^{1/n}=K_{0}}$

where ${\displaystyle K_{0}}$ is Khinchin's constant

${\displaystyle K_{0}=\prod _{r=1}^{\infty }{\left(1+{1 \over r(r+2)}\right)}^{\log _{2}r}\approx 2.6854520010\dots }$ (sequence A002210 in the OEIS)

(with ${\displaystyle \prod }$ denoting the product over all sequence terms).

Although almost all numbers satisfy this property, it has not been proven for any real number not specifically constructed for the purpose. Among the numbers whose continued fraction expansions apparently do have this property (based on numerical evidence) are π, the Euler-Mascheroni constant γ, Apéry's constant ζ(3), and Khinchin's constant itself. However, this is unproven.

Among the numbers x whose continued fraction expansions are known not to have this property are rational numbers, roots of quadratic equations (including the golden ratio Φ and the square roots of integers), and the base of the natural logarithm e.

Khinchin is sometimes spelled Khintchine (the French transliteration of Russian Хинчин) in older mathematical literature.