Nikolaos Dergiades, The Soddy circles,
Forum Geoemtricorum, 7 (2007) 191--197.
Abstract: Given three circles externally tangent to each other, we investigate
the construction of the two so called Soddy circles, that are tangent to the
given three circles. From this construction we get easily the formulas of
the radii and the barycentric coordinates of Soddy centers relative to the
triangle ABC that has vertices the centers of the three given circles.