Hilbert functions of filtered graphical zonotopal algebras
The external zonotopal algebra of a graph is generated by nilpotent vertex elements $X_v$ and determines the graphical matroid after deleting loops. Replacing these generators by $f(X_v)$, for a power series $f(u)$, and filtering by word length gives a family of filtered algebras. We extend filtered graph reconstruction from simple graphs to loopless multigraphs without isolated vertices when the linear and quadratic coefficients of $f$ are nonzero. Nevertheless, even the family of Hilbert functions indexed by all polynomials does not determine the graph: two nonisomorphic ten-vertex trees have the same Hilbert function for every $f$, answering a question of Nenashev and the first author. For forests and series whose first nonconstant term has degree $k\ge 2$, we give a monomial presentation indexed by partial score vectors whose entries are divisible by $k$. For normalized nondegenerate functions, a boundary-state calculus gives a common finite parameter partition for all trees of a fixed maximum degree. In maximum degrees three and four, exact symbolic calculations identify these partitions by the vanishing of specified coefficients of $f^{-1}$. For functions with nonzero quadratic coefficient, the same calculus gives explicit recurrences for attached or connecting paths. Symmetry reductions, including commuting actions of $sl_2$ and the symmetric group on the leaves of a star, determine the generic Hilbert series of stars and two-vertex multigraphs and also apply to parallel edges and leaves in larger graphs. For cycles, we determine the highest-degree term and the first three nonconstant coefficients in the stable range. A weighted independent-set formula describes the trace sequence in the remaining conjecture.