THE QUATERNION FORMALISM FOR MOBIUS GROUPS IN 4 OR FEWER DIMENSIONS

The Mobius group of R(N) or {infinity} defines N-dimensional inversive geometry. This geometry can serve as an alternative to projective geo metry in providing a common foundation for spherical Euclidean and hyp erbolic geometry. Accordingly the Mobius group plays an important role in geometry and topology. The modem emphasis on low-dimensional topol ogy makes it timely to discuss a useful quaternion formalism for the M obius groups in four or fewer dimensions. The present account is self- contained. it begins 'with the representation of quaternions by 2 x 2 matrices of complex numbers. It discusses 2 X 2 matrices of quaternion s and how a suitably normalized subgroup of these matrices, extended b y a certain involution related to sense reversal, is 2-1 homomorphic t o the Mobius group acting on R4 or {infinity}. It provides details of this action and the relation of this action to various models of the c lassical geometries. In higher dimensions N greater-than-or-equal-to 5 , the best description of the Mobius group is probably by means of (N + 2) X (N + 2) Lorentz matrices. In the lower dimensions covered by th e quaternion formalism, this alternative Lorentz formalism is a source of interesting homomorphisms. A sampling of these homomorphisms is co mputed explicitly both for intrinsic interest and for an illustration of the ease with which one can handle the quaternion formalism.