The Homomorphism Submodule Graph

Abstract

Let $M$ be a left $R$-module. We define the \emph{homomorphism submodule graph} $\hat{I}{“}_{\mathrm{Hom}}(M)$ as the simple graph whose vertices are the proper submodules of $M$, with an edge between distinct vertices $N_1$ and $N_2$ if and only if ${\mathrm{Hom}}_R(N_1,M/N_2)\ne 0$ or ${\mathrm{Hom}}_R(N_2,M/N_1)\ne 0$. This graph encodes homological information about $M$ and reflects its internal structure. We compute $\hat{I}{“}_{\mathrm{Hom}}(M)$ for semisimple and uniserial modules, establish precise correspondences between graph-theoretic and algebraic properties, and prove that for modules over Artinian local rings, the isomorphism type of $M$ is determined by $\hat{I}{“}_{\mathrm{Hom}}(M)$. We also show that over commutative rings with identity, the graph is always chordal, and we relate its spectral radius to composition length in natural families.

9 pages