Strang's Conjecture: Positive Result on Strong Collapsible Complexes and a Code to Check the Conjecture

Abstract: Splines, which are piecewise polynomial functions with given smoothness, are used in numerous applications such as computer aided geometric design, algebraic geometry and for implementation of the finite element method. In many applications, it is important to know the dimension of the space of splines over a domain, but this continues to be an open problem. Strang's conjecture predicts the dimension of the space of bivariate $C<sup>1$ splines of degree at most $k$ over a polygonal domain with no holes. For the cases, $k=4$ and $k \geq 5$ the conjecture has been proven true. For the case $k=2$, John Morgan and L. Ridgway Scott provided a counterexample. In this project, we begin by relating the space of bivariate $C<sup>1$ splines of degree at most $k$ to various other function spaces. With the aid of some intrinsic operators on these spaces, we obtain a complex that yields an equivalence statement to Strang's conjecture. The equivalent statement admits a computational approach to the conjecture and thus we present an NGSolve code to check Strang's conjecture for a given mesh. In addition, we prove that if the simplicial complex associated to a triangulation is strongly collapsible (with a non-collinear condition), then Strang's conjecture holds for $k=2$.