1. J. L. Coolidge, A Treatise on the Circle and the Sphere, Oxford University Press, Oxford, 1916.
2. Similarities and conformal transformations
3. H. S. M. Coxeter, Introduction to Geometry, Wiley, New York, 1969.
4. A new characteristic of Möbius transformations by use of Apollonius quadrilaterals
5. S. Katok, Fuchsian Groups, University of Chicago Press, Chicago, 1992.