Ordering starlike trees by the totality of their spectral moments

Abstract

The $k$-th spectral moment $M_k(G)$ of the adjacency matrix of a graph~$G$ represents the number of closed walks of length~$k$ in~$G$. We study here the partial order $\preceq$ of graphs, defined by $G\preceq H$ if $M_k(G)\leq M_k(H)$ for all $k\geq 0$, and are interested in the question when is $\preceq$ a linear order within a specified set of graphs? Our main result is that $\preceq$ is a linear order on each set of starlike trees with constant number of vertices. Recall that a connected graph $G$ is a starlike tree if it has a vertex~$u$ such that the components of $G-u$ are paths, called the branches of~$G$. It turns out that the $\preceq$ ordering of starlike trees with constant number of vertices coincides with the shortlex order of sorted sequence of their branch lengths.