Published online by Cambridge University Press: 14 November 2011
If ℍ is a rational indefinite quaternion algebra and T is an order of ℍ, let G+(T) denote the subgroup of units u ∊ T with norm n(u) = + 1. For a certain class of orders, Eichler has determined the measure μ(G+(T)) of G+(T), viewed as a Fuchsian group. This is extended to arbitrary orders by methods depending only upon classical number-theory and group theory. As an application, an estimate of the magnitude of a small non-trivial solution of the diophantine Pellian equation
is supplied and the restrictions on the integers D and P associated with previous work on this question are eliminated.