Glasgow Mathematical Journal

Research Article

Diophantine approximation on Hecke groups

J. Lehnera1

a1 Institute for Advanced Study Princeton, New Jersey 08540, U.S.A.

If α is a real irrational number, there exist infinitely many reduced rational fractions p/q for which


and √5 is the best constant possible. This result is due to A. Hurwitz. The following generalization was proposed in [2]. Let Г be a finitely generated fuchsian group acting on H+, the upper half of the complex plane. Let xs2112 be the limit set of Г P and the set of cusps (parabolic vertices). Assume ∞xs220AP. Then if αxs220Axs2112P, we have


for infinitely many p/qxs220AГ(∞), where k depends only on Г. Attention centers on


k running over the set for which (1.2) holds. We call hthe Hurwitz constant for Г. When Г=Г(1), the modular group, (1.2) reduces to (1.1) and h(Г(l))=√5. A proof of (1.2) when Г is horocyclic (i.e., xs2112=xs211D, the real axis) was furnished by Rankin [4]; he also found upper and lower bounds for h. See also [3, pp. 334–5], where the theorem is proved for arbitrary finitely generated Г.