An "unavoidable set" means the same for multivariate splines as for the 4 color map problem.
It is true but not trivial statement that any triangulation can be built by adding one flap or fill at a time, and maintaining a triangulation at every step.
A corresponding statement is not true for three dimensional
triangulations (i.e., tesselations of a three dimensional
domain by tetrahedra).
Click here for details.
[01-Nov-2023]