A perspective collineation with center 
and axis 
not incident is called a geometric
homology. A geometric 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.
Harmonic Homology
See also
Geometric Homology, Perspective CollineationExplore with Wolfram|Alpha
References
Coxeter, H. S. M. Introduction to Geometry, 2nd ed. New York: Wiley, p. 248, 1969.Referenced on Wolfram|Alpha
Harmonic HomologyCite this as:
Weisstein, Eric W. "Harmonic Homology." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/HarmonicHomology.html