1. Howard Eves, A Survey of Geometry, Allyn and Bacon, Boston, 1963.
2. Incenters and Excenters Viewed from the Euler Line
3. G. H. Hardy and E. M. Wright, An Introduction to the Theory of Numbers, 3rd Ed., Clarendon Press, Oxford, 1954.
4. George E. Martin, The Foundations of Geometry and the Non-Euclidean Plane, Springer-Verlag, New York, 1975.