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One approach to cross ratio interprets it as a homography that takes three designated points to 0, 1, and ∞. Under restrictions having to do with inverses, it is possible to generate such a mapping with ring operations in the projective line over a ring. The cross ratio of four points is the evaluation of this homography at the fourth point.
The cross-ratio (,;,) = () is a ratio of division ratios, or a double ratio. Setting the double ratio to minus one means that when t(c) + t(d) = 0, then c and d are harmonic conjugates with respect to a and b. So the division ratio criterion is that they be additive inverses.
The last compound ratio (namely JD : GD & BG : JB) is what is known today as the cross ratio of the collinear points J, G, D, and B in that order; it is denoted today by (J, G; D, B). So we have shown that this is independent of the choice of the particular straight line JD that crosses the three straight lines that concur at A.
The cross-ratio between 4 points ,,, is invariant under an inversion. In particular if O is the centre of the inversion and r 1 {\displaystyle r_{1}} and r 2 {\displaystyle r_{2}} are distances to the ends of a line L, then length of the line d {\displaystyle d} will become d / ( r 1 r 2 ) {\displaystyle d/(r_{1}r_{2})} under an inversion with ...
The metric distance between two points inside the absolute is the logarithm of the cross ratio formed by these two points and the two intersections of their line with the absolute. In mathematics, a Cayley–Klein metric is a metric on the complement of a fixed quadric in a projective space which is defined using a cross-ratio.
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A ratio that has integers for both quantities and that cannot be reduced any further (using integers) is said to be in simplest form or lowest terms. Sometimes it is useful to write a ratio in the form 1:x or x:1, where x is not necessarily an integer, to enable comparisons of different ratios. For example, the ratio 4:5 can be written as 1:1 ...
Key Points from 24/7 Wall St. The average dividend yield of an S&P 500 company is less than what savings accounts are paying today.. Given that the index is up around 24% over the past year, it's ...
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