Bounding the multiplicities of eigenvalues of graph matrices in terms of circuit rank using a new approach

Abstract: Let $G$ be a simple undirected graph, $\theta(G)$ be the circuit rank of $G$, $\eta_M(G)$ and $m_M(G,\lambda)$ be the nullity and the multiplicity of eigenvalue $\lambda$ of a graph matrix $M(G)$, respectively. In the case $M(G)$ is the adjacency matrix $A(G)$, (the Laplacian matrix $L(G)$, the signless Laplacian matrix $Q(G)$) we find bounds to $m_M(G,\lambda)$ in terms of $\theta(G)$ when $\lambda$ is an integer (even integer, respectively). We also show that when $\alpha$ and $\lambda$ are rational numbers similar bounds can be found for $m_{A_{\alpha}}(G,\lambda)$ where $A_{\alpha}(G)$ is the generalized adjaceny matrix of $G$. Our bounds contain only $\theta(G)$, not a multiple of it. Up to now only bounds of $m_A(G,\lambda)$ (and later $m_{A_\alpha}(G,\lambda)$) have been found in terms of the circuit rank and all of them contains $2\theta(G)$. There is only one exception in the case $\lambda=0$. Wong et al. (2022) showed that $\eta_A(G_c)\leq \theta(G_c)+1$, where $G_c$ is a connected cactus whose blocks are even cycles. Our result, in particular, generalizes and extends this result to the multiplicity of any even eigenvalue of A(G) of any even connected graph $G$, and of any even eigenvalue of $L(G)$ and $Q(G)$ of any connected graph $G$. They also showed that $\eta_A(G_c)\leq 1$ when every block of the cactus is an odd cycle. This also corresponds a special case of our bound.