Skeleton Ideals of Certain Graphs, Standard Monomials and Spherical Parking Functions

Abstract: Let $G$ be an (oriented) graph on the vertex set $V = { 0, 1,\ldots,n}$ with root $0$. Postnikov and Shapiro associated a monomial ideal $\mathcal{M}G$ in the polynomial ring $ R = {\mathbb{K}}[x_1,\ldots,x_n]$ over a field $\mathbb{K}$. A subideal $\mathcal{M}_G^{(k)}$ of $\mathcal{M}_G$ generated by subsets of $\widetilde{V}=V\setminus {0}$ of size at most $k+1$ is called a $k$-skeleton ideal of the graph $G$. Many interesting homological and combinatorial properties of $1$-skeleton ideal $\mathcal{M}_G^{(1)}$ are obtained by Dochtermann for certain classes of simple graph $G$. A finite sequence $\mathcal{P}=(p_1,\ldots,p_n) \in \mathbb{N}^n$ is called a spherical $G$-parking function if the monomial $\mathbf{x}^{\mathcal{P}} = \prod{i=1}^{n} x_i^{p_i} \in \mathcal{M}G \setminus \mathcal{M}_G^{(n-2)}$. Let ${\rm sPF}(G)$ be the set of all spherical $G$-parking functions. In this paper, a combinatorial description for all multigraded Betti numbers of the $k$-skeleton ideal $\mathcal{M}{K_{n+1}}^{(k)}$ of the complete graph $K_{n+1}$ on $V$ are given. Also, using DFS burning algorithms of Perkinson-Yang-Yu (for simple graph) and Gaydarov-Hopkins (for multigraph), we give a combinatorial interpretation of spherical $G$-parking functions for the graph $G = K_{n+1}- {e}$ obtained from the complete graph $K_{n+1}$ on deleting an edge $e$. In particular, we showed that $|{\rm sPF}(K_{n+1}- {e_0} )|= (n-1)^{n-1}$ for an edge $e_0$ through the root $0$, but $|{\rm sPF}(K_{n+1} - {e_1})| = (n-1)^{n-3}(n-2)^2$ for an edge $e_1$ not through the root.