The inversive distance is the Natural Logarithm of the ratio of two concentric circles into which the given
circles can be inverted. Let be the distance between the centers of two nonintersecting Circles of
Radii and . Then the inversive distance is
The inversive distance between the Soddy Circles is given by
Coxeter, H. S. M. and Greitzer, S. L. Geometry Revisited. Washington, DC: Math. Assoc. Amer., pp. 123-124
and 127-131, 1967.