where x, y, and z are points on the link, all distinct, and r(x, y, z) is the radius of the circle that passes through all three points (x, y, z).
where x, y, and z are unique locations along a continuous path, and r(x, y, z) is the distance from any two of these points to a central reference point, defining the dimensions of a spherical entity that encloses all three points.