The characterization of graphs with two trivial distance ideals

Abstract

The distance ideals of graphs are algebraic invariants that generalize the Smith normal form (SNF) and the spectrum of several distance matrices associated with a graph. In general, distance ideals are not monotone under taking induced subgraphs. However, in [7] the characterizations of connected graphs with one trivial distance ideal over $\mathbb{Z}[X]$ and over $\mathbb{Q}[X]$ were obtained in terms of induced subgraphs, where $X$ is a set of variables indexed by the vertices. Later, in [3], the first attempt was made to characterize the family of connected graphs with at most two trivial distance ideals over $\mathbb{Z}[X]$. There, it was proven that these graphs are $\{ \mathcal {F},\textsf{odd-holes}_{7}\}$-free, where $\textsf{odd-holes}_{7}$ consists of the odd cycles of length at least seven and $\mathcal{F}$ is a set of sixteen graphs. Here, we give a characterization of the $\{\mathcal{F},\textsf{odd-holes}_{7}\}$-free graphs and prove that the $\{\mathcal{F},\textsf{odd-holes}_{7}\}$-free graphs are precisely the graphs with at most two trivial distance ideals over $\mathbb{Z}[X]$. As byproduct, we also find that the determinant of the distance matrix of a connected bipartite graph is even, this suggests that it is possible to extend, to connected bipartite graphs, the Graham-Pollak-Lov\'asz celebrated formula $\det(D(T_{n+1}))=(-1)^nn2^{n-1}$, and the Hou-Woo result stating that $\text{SNF}(D(T_{n+1}))=\textsf{I}_2\oplus 2\textsf{I}_{n-2}\oplus (2n)$, for any tree $T_{n+1}$ with $n+1$ vertices. Finally, we also give the characterizations of graphs with at most two trivial distance ideals over $\mathbb{Q}[X]$, and the graphs with at most two trivial distance univariate ideals.