Diophantine approximation on Bianchi groups
The author uses techniques which go back to L. Ford to give an upper bound on the Hurwitz constants for the Bianchi groups, \(\text{PSL} (2,\mathbb{Q}(\sqrt {-d}))\) with \(d\) a positive integer. The Hurwitz constant is the supremum of the Lagrange values, thus it gives a measurement of how well one can approximate complex numbers by elements of the orbit of infinity under the particular group in question. The geometric approach is clearly explained and expertly applied in this well-written paper.
scientific article; zbMATH DE number 809108