Presentations of the unit group of an order in a non-split quaternion algebra

Abstract We give an algorithm to determine a finite set of generators of the unit group of an order in a non-split classical quaternion algebra H (K) over an imaginary quadratic extension K of the rationals. We then apply this method to obtain a presentation for the unit group of H ( Z [ 1+ −7 2 ]) . As a consequence a presentation is discovered for the orthogonal group SO 3 ( Z [ 1+ −7 2 ]) . These results provide the first examples of a characterization of the unit group of some group rings that have an epimorphic image that is an order in a non-commutative division algebra that is not a totally definite quaternion algebra.