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Given four mutually tangent circles, their bends are defined as the signed Curvatures of the Circles. If the contacts are all external, the signs are all taken as Positive, whereas if one circle surrounds the other three, the sign of this circle is taken as Negative (Coxeter 1969).
See also Curvature, Descartes Circle Theorem, Soddy Circles
References
Coxeter, H. S. M. Introduction to Geometry, 2nd ed. New York: Wiley, pp. 13-14, 1969.