Markoff spectra of Hecke groups

Abstract

We study the initial discrete parts of the Markoff spectra of Hecke groups of index $q\ge 5$. Using a digit expansion associated with geodesics and a Perron-type formula, we classify all bi-infinite digit sequences whose Markoff values lie at or below the first accumulation point. We obtain explicit formulas for the discrete spectral values and identify the sequences realizing them, including those attaining the first accumulation point. We also prove that the infinite family of discrete values increases strictly to this point. Our results recover Vulakh's description of the initial discrete spectra through an explicit symbolic approach.