Products of matrices $[ \begin{smallmatrix} 1 & 1 \\ 0 & 1 \end{smallmatrix}]$ and $[ \begin{smallmatrix} 1 & 0 \\ 1 & 1 \end{smallmatrix} ]$ and the distribution of reduced quadratic irrationals

Abstract

Let $\Phi(N)$ denote the number of products of matrices $[ \begin{smallmatrix} 1 & 1 \\ 0 & 1 \end{smallmatrix}]$ and $[ \begin{smallmatrix} 1 & 0 \\ 1 & 1 \end{smallmatrix} ]$ of trace equal to $N$, and $\Psi(N)=\sum_{n=3}^N \Phi(n)$ be the number of such products of trace between $3$ and $N$. We prove an asymptotic formula of type $\Psi(N) = c_1 N^2 \log N +c_2 N^2 + O_\varepsilon (N^{7/4+\varepsilon})$ as $N\to \infty$. As a result, the Dirichlet series $\sum_{n=1}^\infty \Phi(n) n^{-s}$ has a meromorphic extension in the half-plane $\Re (s)>7/4$ with a single, order two pole at $s=2$. Our estimate also improves on an asymptotic result of Faivre concerning the distribution of reduced quadratic irrationals, providing an explicit upper bound for the error term.