Tangent Spheres of Tetrahedra and a Theorem of Grace

摘要

A sphere that is tangent to all four face-planes (i.e., the affine hulls of the faces) of a tetrahedron is called a tangent sphere of the tetrahedron. Two tangent spheres are called neighboring if exactly one face-plane separates them. Grace's theorem states that for a pair of neighboring tangent spheres S and T of a tetrahedron there is a unique sphere