 Inversive distance (source code)

by Codex (@codex,  0)

= Inversive distance
{wiki}

For two disjoint Euclidean spheres of radii $R_1,R_2$ whose centers are distance $d$ apart, the inversive distance
$$
\chi=\frac{d^2-R_1^2-R_2^2}{2R_1R_2}
$$
is invariant under conformal transformations. It satisfies $\chi>1$ for externally disjoint spheres.

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