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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