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Inversion


InversePoints

Inversion is the process of transforming points P to a corresponding set of points P^' known as their inverse points. Two points P and P^' are said to be inverses with respect to an inversion circle having inversion center O=(x_0,y_0) and inversion radius k if P^' is the perpendicular foot of the altitude of DeltaOQP, where Q is a point on the circle such that OQ_|_PQ.

The analogous notation of inversion can be performed in three-dimensional space with respect to an inversion sphere.

If P and P^' are inverse points, then the line L through P and perpendicular to OP is sometimes called a "polar" with respect to point P^', known as the "inversion pole". In addition, the curve to which a given curve is transformed under inversion is called its inverse curve (or more simply, its "inverse"). This sort of inversion was first systematically investigated by Jakob Steiner.

From similar triangles, it immediately follows that the inverse points P and P^' obey

 (OP)/k=k/(OP^'),
(1)

or

 k^2=OP×OP^'
(2)

(Coxeter 1969, p. 78), where the quantity k^2 is known as the circle power (Coxeter 1969, p. 81).

The general equation for the inverse of the point (x,y) relative to the inversion circle with inversion center (x_0,y_0) and inversion radius k is given by

x^'=x_0+(k^2(x-x_0))/((x-x_0)^2+(y-y_0)^2)
(3)
y^'=y_0+(k^2(y-y_0))/((x-x_0)^2+(y-y_0)^2).
(4)

In vector form,

 x^'=x_0+(k^2(x-x_0))/(|x-x_0|^2).
(5)

Note that a point on the circumference of the inversion circle is its own inverse point. In addition, any angle inverts to an opposite angle.

InversionCircles

Treating lines as circles of infinite radius, all circles invert to circles (Lachlan 1893, p. 221). Furthermore, any two nonintersecting circles can be inverted into concentric circles by taking the inversion center at one of the two so-called limiting points of the two circles (Coxeter 1969), and any two circles can be inverted into themselves or into two equal circles (Casey 1888, pp. 97-98). Orthogonal circles invert to orthogonal circles (Coxeter 1969). The inversion circle itself, circles orthogonal to it, and lines through the inversion center are invariant under inversion. Furthermore, inversion is a conformal mapping, so angles are preserved.

Inversion

The property that inversion transforms circles and lines to circles or lines (and that inversion is conformal) makes it an extremely important tool of plane analytic geometry. By picking a suitable inversion circle, it is often possible to transform one geometric configuration into another simpler one in which a proof is more easily effected. The illustration above shows examples of the results of geometric inversion.

The inverse of a circle of radius a with center (x,y) with respect to an inversion circle with inversion center (x_0,y_0) and inversion radius k is another circle with center

x^'=x_0+s(x-x_0)
(6)
y^'=y_0+s(y-y_0)
(7)

and radius

 r^'=|s|a,
(8)

where

 s=(k^2)/((x-x_0)^2+(y-y_0)^2-a^2).
(9)

These equations can also be naturally extended to inversion with respect to a sphere in three-dimensional space.

CheckerInverseChecker

The above plot shows a chessboard centered at (0, 0) and its inverse about a small circle also centered at (0, 0) (Gardner 1984, pp. 244-245; Dixon 1991).


See also

Anamorphic Art, Arbelos, Circle Power, Conformal Mapping, Cyclide, Hexlet, Inverse Curve, Inverse Points, Inversion Circle, Inversion Operation, Inversion Pole, Inversion Radius, Inversion Sphere, Inversive Distance, Inversive Geometry, Limiting Point, Midcircle, Pappus Chain, Peaucellier Inversor, Permutation Inversion, Polar, Radical Line, Steiner Chain, Steiner's Porism

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References

Casey, J. "Theory of Inversion." §6.4 in 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., pp. 95-112, 1888.Coolidge, J. L. "Inversion." §1.2 in A Treatise on the Geometry of the Circle and Sphere. New York: Chelsea, pp. 21-30, 1971.Courant, R. and Robbins, H. "Geometrical Transformations. Inversion." §3.4 in What Is Mathematics?: An Elementary Approach to Ideas and Methods, 2nd ed. Oxford, England: Oxford University Press, pp. 140-146, 1996.Coxeter, H. S. M. "Inversion in a Circle" and "Inversion of Lines and Circles." §6.1 and 6.3 in Introduction to Geometry, 2nd ed. New York: Wiley, pp. 77-83, 1969.Coxeter, H. S. M. and Greitzer, S. L. "An Introduction to Inversive Geometry." Ch. 5 in Geometry Revisited. Washington, DC: Math. Assoc. Amer., pp. 103-131, 1967.Darboux, G. Leçons sur les systemes orthogonaux et les coordonnées curvilignes. Paris: Gauthier-Villars, 1910.Dixon, R. "Inverse Points and Mid-Circles." §1.6 in Mathographics. New York: Dover, pp. 62-73, 1991.Durell, C. V. "Inversion." Ch. 10 in Modern Geometry: The Straight Line and Circle. London: Macmillan, pp. 105-120, 1928.Fukagawa, H. and Pedoe, D. "Problems Soluble by Inversion." §1.8 in Japanese Temple Geometry Problems. Winnipeg, Manitoba, Canada: Charles Babbage Research Foundation, pp. 17-22 and 93-99, 1989.Gardner, M. The Sixth Book of Mathematical Games from Scientific American. Chicago, IL: University of Chicago Press, 1984.Jeans, J. H. The Mathematical Theory of Electricity and Magnetism, 5th ed. Cambridge, England: The University Press, 1925.Johnson, R. A. Modern Geometry: An Elementary Treatise on the Geometry of the Triangle and the Circle. Boston, MA: Houghton Mifflin, pp. 43-57, 1929.Kelvin, W. T. and Tait, P. G. Principles of Mechanics and Dynamics, Vol. 2. New York: Dover, p. 62, 1962.Lachlan, R. "The Theory of Inversion." Ch. 14 in An Elementary Treatise on Modern Pure Geometry. London: Macmillian, pp. 218-236, 1893.Liouville, J. "Note au sujet de l'article précédent." J. math. pures appl. 12, 265-290, 1847.Lockwood, E. H. "Inversion." Ch. 23 in A Book of Curves. Cambridge, England: Cambridge University Press, pp. 176-181, 1967.Maxwell, J. C. A Treatise on Electricity and Magnetism, Vol. 1, unabridged 3rd ed. New York: Dover, 1954.Maxwell, J. C. A Treatise on Electricity and Magnetism, Vol. 2, unabridged 3rd ed. New York: Dover, 1954.Morley, F. and Morley, F. V. Inversive Geometry. Boston, MA: Ginn, 1933.Ogilvy, C. S. Excursions in Geometry. New York: Dover, pp. 25-31, 1990.Schmidt, H. Die Inversion und ihre Anwendungen. Munich, Germany: Oldenbourg, 1950.Thomson, W. "Extrait d'un lettre de M.William Thomson a M.Liouville." J. math. pures appl. 10, 364-367, 1845.Thomson, W. "Extrait de deux lettres adressées à M. Liouville." J. math. pures appl. 12, 256, 1847.Wangerin, A. S.147 in Theorie des Potentials und der Kugelfunktionen, Bd. II. Berlin: de Gruyter, 1921.Weber, E. Electromagnetic Fields: Theory and Applications. New York: Wiley, p. 244, 1950.Wells, D. The Penguin Dictionary of Curious and Interesting Geometry. London: Penguin, pp. 119-121, 1991.

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Inversion

Cite this as:

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

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