  Given a graph $G$ on the vertex set $\{0,1,\ldots,n\}$ with the root vertex
$0$, 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 $\mathbb{K}$ such
that $\dim_{\mathbb{K}}(R/\mathcal{M}_G)=\det\widetilde L_G$, where $\widetilde
L_G$ is the truncated Laplacian of $G$. Dochtermann introduced the $1$-skeleton
ideal $\mathcal{M}_G^{(1)}$ of $\mathcal{M}_G$ which satisfies the property
that $\dim_{\mathbb{K}}(R/\mathcal{M}_G^{(1)})\ge\det\widetilde Q_G$, where
$\widetilde Q_G$ is the truncated signless Laplacian of $G$. In this paper we
characterize all subgraphs of the multigraph $K_{n+1}^{a,1}$, in particular all
simple graphs $G$, such that
$\dim_{\mathbb{K}}(R/\mathcal{M}_G^{(1)})=\det\widetilde Q_G$. Moreover, we
give examples of subgraphs $G$ of the complete multigraph $K_{n+1}^{a,b}$, in
which the equality $\dim_{\mathbb{K}}(R/\mathcal{M}_G^{(1)})=\det\widetilde
Q_G$ holds. We also provide a conjecture on the structure of a general
multigraph satisfying the above-mentioned equality.
