On Number of Rich Words

Abstract

Any finite word $w$ of length $n$ contains at most $n+1$ distinct palindromic factors. If the bound $n+1$ is reached, the word $w$ is called rich. The number of rich words of length $n$ over an alphabet of cardinality $q$ is denoted $R_n(q)$. For binary alphabet, Rubinchik and Shur deduced that ${R_n(2)}\leq c 1.605^n $ for some constant $c$. We prove that $\lim\limits_{n\rightarrow \infty }\sqrt[n]{R_n(q)}=1$ for any $q$, i.e. $R_n(q)$ has a subexponential growth on any alphabet.