Cross-ratio Criterion
The four points are sometimes called a harmonic range (on the real projective line) as it is found that D always divides the segment AB internally in the same proportion as C divides AB externally. That is:
If these segments are now endowed with the ordinary metric interpretation of real numbers they will be signed and form a double proportion known as the cross ratio (sometimes double ratio)
for which a harmonic range is characterized by a value of -1, We therefore write:
The value of a cross ratio in general is not unique, as it depends on the order of selection of segments (and there are six such selections possible). But for a harmonic range in particular there are just three values of cross ratio:{−1, 1/2, 2} since -1 is self-inverse - so exchanging the last two points merely reciprocates each of these values but produces now new value, and is known classically as the harmonic cross-ratio.
In terms of a double ratio, given points a and b on an affine line, the division ratio of a point x is
Note that when a < x < b, then t(x) is negative, and that it is positive outside of the interval. The cross-ratio (c,d;a,b) = t(c)/t(d) is a ratio of division ratios, or a double ratio. Setting the double ratio to minus one means that when, then c and d are projective harmonic conjugates with respect to a and b. So the division ratio criterion is that they be additive inverses.
In some school studies the configuration of a harmonic range is called harmonic division.
Read more about this topic: Projective Harmonic Conjugate
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