Paul Yiu, Rational Steiner porism,
Forum Geometricorum, 11 (2011) 237--249.

Abstract.   We establish formulas relating the radii of neighboring circles in a Steiner chain, a chain of mutually tangent circles each tangent to two given ones, one in the interior of the other. From such we parametrize, for n = 3, 4, 6, all configurations of Steiner n-cycles of rational radii.

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