Line Involution

Pairs of points of a line, the product of whose distances from a fixed point is a given constant. This is more concisely defined as a projectivity of period two.

If ${AA^',BB^',CC^'}$ is a range in involution, then the ranges ${AA^',BC}$ and ${A^'A,B^'C^'}$ are equicross, and conversely.

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Anger J
cos(x) + 1/2 cos(2x) + 1/4 cos(4x)
int e^(-a t) dt, t=0..a

References

Casey, J. A Sequel to the First Six Books of the Elements of Euclid, Containing an Easy Introduction to Modern Geometry with Numerous Examples, 5th ed., rev. enl. Dublin: Hodges, Figgis, & Co., p. 133, 1888.Lachlan, R. "Theory of Involutions" and "Involution." Ch. 5 and §426-427 in An Elementary Treatise on Modern Pure Geometry. London: Macmillian, pp. 272-274, 1893.

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Line Involution

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Weisstein, Eric W. "Line Involution." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/LineInvolution.html

Line Involution

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References

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