Steiner chain
= Steiner chain
{c}
A cyclic chain of distinct <circles> between two disjoint <circle> boundaries, each tangent to both boundaries and its two neighbours. In concentric coordinates with inner radius $\rho$ and outer radius $R$, the chain <circles> have radius $r=(R-\rho)/2$ and centre distance $d=(R+\rho)/2$. A simple disjoint chain with $n\geq3$ closes when $r/d=\sin(\pi/n)$.