Graphs represented by Ext

Abstract

This paper opens and discusses the question originally due to Daniel Herden, who asked for which graph $(\mu,R)$ we can find a family $\{\mathbb G_\alpha: \alpha < \mu\}$ of abelian groups such that for each $\alpha,\beta\in\mu$: $$Ext(\mathbb G_\alpha, \mathbb G_\beta) = 0 \Longleftrightarrow(\alpha,\beta) \in R.$$ In this regard, we present four results. First, we give a connection to Quillen's small object argument which helps $Ext$ vanishes and uses to present useful criteria to the question. Suppose $\lambda = \lambda^{\aleph_0}$ and $\mu = 2^\lambda$. We apply Jensen's diamond principle along with the criteria to present $\lambda$-free abelian groups representing bipartite graphs. Third, we use a version of the black box to construct in ZFC, a family of $\aleph_1$-free abelian groups representing bipartite graphs. Finally, applying forcing techniques, we present a consistent positive answer for general graphs.