Graphs with two trivial critical ideals
The critical ideals of a graph are the determinantal ideals of the generalized Laplacian matrix associated to a graph. A basic property of the critical ideals of graphs asserts that the graphs with at most k k trivial critical ideals, ??k ? ? k , are closed under induced subgraphs. In this article we find the set of minimal forbidden subgraphs for ??2 ? ? 2 , and we use this forbidden subgraphs to get a classification of the graphs in ??2 ? ? 2 . As a consequence we give a classification of the simple graphs whose critical group has two invariant factors equal to one. At the end of this article we give two infinite families of forbidden subgraphs.
critical ideal generalized laplacian matrix critical group forbidden induced subgraph