Adjacency-degree algebras and spectral determination of graphs

Abstract

McKay proved that the spectra of all polynomial functions of the adjacency matrix $A$ and the diagonal degree matrix $D$ determine a tree. We prove a principal version of this theorem. Let $\mathcal A(G)=\langle I,A_G,D_G\rangle$ and let $M_G=\mathcal A(G)\mathbf1$ be the cyclic module generated by the all-ones vector. For connected graphs the ideal $\mathcal A(G)J\mathcal A(G)$, where $J=\mathbf1\mathbf1^T$, acts on $M_G$ as the full endomorphism algebra. We show that every forest satisfies $M_G=U_G$, the automorphism-orbit module, and that the induced algebra on the orbit quotient of a tree is a full matrix algebra. It follows that the scalar moments $\mathbf1^Tw(A_T,D_T)\mathbf1$ determine every tree. For general graphs these moments are degree-decorated caterpillar homomorphism counts. The resulting moment-rigidity class lies inside the amenable, compact, refinable hierarchy of color refinement, and its first small-order failures are ten-vertex integral switchings invisible to $M_G$.