**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.

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