On a Quaternionic Analogue of the Cross-Ratio
  In this article we study an exact analogue of the cross-ratio for the algebra of quaternions H and use it to derive several interesting properties of quaternionic fractional linear transformations. In particular, we show that there exists a fractional linear transformation T on H mapping four distinct quaternions q_1, q_2, q_3 and q_4 into q'_1, q'_2, q'_3 and q'_4 respectively if and only if the quadruples (q_1, q_2, q_3, q_4) and (q'_1, q'_2, q'_3, q'_4) have the same cross-ratio. If such a fractional linear transformation T exists it is never unique. However, we prove that a fractional linear transformation on H is uniquely determined by specifying its values at five points in general position. We also prove some properties of the cross-ratio including criteria for four quaternions to lie on a single circle (or a line) and for five quaternions to lie on a single 2-sphere (or a 2-plane). As an application of the cross-ratio, we prove that fractional linear transformations on H map spheres (or affine subspaces) of dimension 1, 2 and 3 into spheres (or affine subspaces) of the same dimension. 
Ewain Gwynne and Matvei Libine
math.CV math.DG