Li C. Tien, Three Pairs of Congruent Circles in a Circle,
Forum Geometricorum 4 (2004) 117--124.
Abstract: Consider a closed chain of three pairs of congruent circles
of radii a, b, c. The circle tangent internally to each
of the 6 circles has radius R = a + b + c if and only if there
is a pair of congruent circles whose centers are on a diameter of the enclosing
circle. Non-neighboring circles in the chain may overlap.
Conditions for nonoverlapping are established. There can be a ``central circle''
tangent to four of the circles in the chain.