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Descartes Circle Theorem


A special case of Apollonius' problem requiring the determination of a circle touching three mutually tangent circles (also called the kissing circles problem). There are two solutions: a small circle surrounded by the three original circles, and a large circle surrounding the original three. Frederick Soddy gave the formula for finding the radius of the so-called inner and outer Soddy circles given the radii of the other three. The relationship is

 2(kappa_1^2+kappa_2^2+kappa_3^2+kappa_4^2)=(kappa_1+kappa_2+kappa_3+kappa_4)^2,

where kappa_i=1/r_i are the curvatures of the circles with radii r_i. Here, the negative solution corresponds to the outer Soddy circle and the positive solution to the inner Soddy circle.

This formula was known to Descartes and Viète (Boyer and Merzbach 1991, p. 159), but Soddy extended it to spheres. In n-dimensional space, n+2 mutually touching n-spheres can always be found, and the relationship of their curvatures is

 n(sum_(i=1)^(n+2)kappa_i^2)=(sum_(i=1)^(n+2)kappa_i)^2.

See also

Apollonius' Problem, Four Coins Problem, Sangaku Problem, Soddy Circles, Sphere Packing, Tangent Circles

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References

Boyer, C. B. and Merzbach, U. C. A History of Mathematics, 2nd ed. New York: Wiley, 1991.Coxeter, H. S. M. "The Problem of Apollonius." Amer. Math. Monthly 75, 5-15, 1968.Coxeter, H. S. M. Introduction to Geometry, 2nd ed. New York: Wiley, pp. 13-16, 1969.Fukagawa, H. and Pedoe, D. "The Descartes Circle Theorem." §1.7 in Japanese Temple Geometry Problems. Winnipeg, Manitoba, Canada: Charles Babbage Research Foundation, pp. 16-17 and 92, 1989.Rothman, T. "Japanese Temple Geometry." Sci. Amer. 278, 85-91, May 1998.Wilker, J. B. "Four Proofs of a Generalization of the Descartes Circle Theorem." Amer. Math. Monthly 76, 278-282, 1969.Williams, R. The Geometrical Foundation of Natural Structure: A Source Book of Design. New York: Dover, pp. 50-51, 1979.

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Descartes Circle Theorem

Cite this as:

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

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