Generalizations and extensions
More than four points in the plane With five or more points in the plane
there is no simple description of the hub. The difficulty stems from the fact that
the set {(u1, . . . , uk) | ui = 1,
ui = 0} ⊂ (R2)k has dimension k − 2. The onedimensional
orthogonal group O(2) acts on the set by rotations; the quotient space is
the space of geometrically distinct configurations and has dimension k − 3. For k = 3
there is a unique configuration up to the action of O(2), and for k = 4 there is a onedimensional
set of geometrically distinct configurations parameterized by the smallest
angle between two of the ui. For k ≥ 5 the dimension of the set of geometrically
distinct configurations is two or more, and there are too many possibilities. Figure 6
is a typical example with k = 6, showing the unit vectors and the associated hexagon
that has no obvious symmetry. The hexagon can be deformed continuously into other
hexagons with three degrees of freedom. (Challenge to the reader: identify the degrees of freedom.)