Rotations and units in quaternion algebras

Abstract

Unit groups of orders in quaternion algebras over number fields provide important examples of non-commutative arithmetic groups. Let K = Q(d ) be a quadratic field with d < 0 a squarefree integer such that d ≡ 1(mod 8), and let R be its ring of integers. In this note we study, through its representation in SO3(R), the group of units of several orders in the quaternion algebra over K with basis {1, i, j,k} satisfying the relations i2 =j2 =−1, i j =−ji =k.