(_x_, _y_, _z_, _t_), and (_ξ_, _η_, _ζ_, _τ_) follows, as in the
sphere, from the equation
cos_s_ (_xξ_ + _yη_ + _zζ_ + _tτ_)
------ = --------------------------- (4.)
_R_ _R_²
One of the co-ordinates may be eliminated from the values given in 2 to
4, by means of equation 1, and the expressions then apply to space of
three dimensions.