Spectral asymmetry and odd cycles in integral graphs

Source abstract

We prove that the triangle $C_3$ is the only integral unicyclic graph with an odd cycle. The main tool is a spectral inequality that does not require integrality: if $G$ is a connected non-bipartite graph and $G-e$ is bipartite for some edge $e$, then the eigenvalues $λ_1\ge\cdots\geλ_n$ of $G$ satisfy $\sum_{i}(λ_i+λ_{n+1-i})^+\le 2$ and $0<λ_1+λ_n\le 1$. It follows from comparing $A(G)$ with the signed adjacency matrix obtained by reversing the sign of $e$, which is similar to $-A(G)$, through a common rank-one perturbation and Weyl's monotonicity theorem. For integral graphs in this class, an arithmetic lemma then gives $λ_n=1-λ_1$ and at least $λ_1-1\ge1$ triangles, which yields the classification. We also prove that an integral graph of odd girth $g$ has at least $(g-1)!/2$ cycles of length $g$, with equality for the Petersen graph when $g=5$, and we derive consequences for quantum periodicity a