A Perspective Collineation with center and axis not incident is called a Homology. A Homology is said to be harmonic if the points and on a line through are harmonic conjugates with respect to and . Every Perspective Collineation of period two is a harmonic homology.

**References**

Coxeter, H. S. M. *Introduction to Geometry, 2nd ed.* New York: Wiley, p. 248, 1969.

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1999-05-25