An APL solution for the sphere tangent to four spheres
Gérard A. Langlet · Journal of Applied Crystallography · 1977
A differential formulation of the problem of the largest sphere inscribed between n + 1 spheres in a space with n dimensions leads to a linear system of n + 1 equations with n unknowns which can be solved by the least-squares method. This formulation is most efficient for APL computing.