Bounds on entries in Bianchi group generators

Abstract

Upper and lower bounds are given for the maximum Euclidean curvature among faces in Bianchi's fundamental polyhedron for $PS{L}_2(O)$ in the upper-half space model of hyperbolic space, where $O$ is an imaginary quadratic ring of integers with discriminant $\Delta$. We prove these bounds are asymptotically within $(\log |\Delta |{)}^{8.54}$ of one another. This improves on the previous best upper-bound, which is roughly off by a factor between ${\Delta }^2$ and $|\Delta {|}^{5/2}$ depending on the smallest prime dividing $\Delta$. The gap between our upper and lower bounds is determined by an analog of Jacobsthal's function, introduced here for imaginary quadratic fields.

22 pages, 5 figures