Szegő inequality

The Komornik–Loreti constant is a mathematical constant that represents the smallest number for which there still exists a unique q-development.

Definition

Given a real number q > 1, the series

$x=\sum _{n=0}^{\mathrm{\infty }}{a}_n{q}^{-n}$

is called the q-expansion, or $\beta$-expansion, of the positive real number x if, for all $n\ge 0$, $0\le a_n\le \lfloor q\rfloor$, where $\lfloor q\rfloor$ is the floor function and $a_n$ need not be an integer. Any real number $x$ such that $0\le x\le q\lfloor q\rfloor /(q-1)$ has such an expansion, as can be found using the greedy algorithm.

The special case of $x=1$, $a_0=0$, and $a_n=0$ or 1 is sometimes called a $q$-development. $a_n=1$ gives the only 2-development. However, for almost all $1<q<2$, there are an infinite number of different $q$-developments. Even more surprisingly though, there exist exceptional $q\in (1,2)$ for which there exists only a single $q$-development. Furthermore, there is a smallest number $1<q<2$ known as the Komornik–Loreti constant for which there exists a unique $q$-development.

The Komornik–Loreti constant is the value $q$ such that

$1=\sum _{n=1}^{\mathrm{\infty }}\frac{t_k}{q^k}$

where $t_k$ is the Thue–Morse sequence, i.e., $t_k$ is the parity of the number of 1's in the binary representation of $k$. It has approximate value

$q=1.787231650\dots .$

The constant $q$ is also the unique positive real root of

$\prod _{k=0}^{\mathrm{\infty }}(1-\frac{1}{q^{2^k}})={(1-\frac{1}{q})}^{-1}-2.$

This constant is transcendental.

References

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