Classification of prime graphs with 2-switch-degree at most 4

Abstract

The 2-switch-degree $\text{deg}(G)$ of a graph $G$ is the number of 2-switches that can be performed on $G$; equivalently, it is the degree of $G$ as a vertex of the realization graph $\mathcal{G}(d)$ of its degree sequence $d$. We classify the prime graphs of 2-switch-degree at most 4, where a graph is prime if it is indecomposable with respect to the Tyshkevich composition and every vertex takes part in some 2-switch. From this classification we derive a sharp dichotomy that recovers the global shape of $\mathcal{G}(d)$ from a single one of its local degrees: if $d$ has a realization $X$ with $\text{deg}(X)=k\le3$, then $\mathcal{G}(d)$ is vertex-transitive and $k$-regular if and only if neither $T_{221}$ nor $\overline{T_{221}}$ is an induced subgraph of $X$. Moreover, for every $k\geq 4$ some prime graph of degree $k$ carries a 2-switch raising its degree to $2k-2$. As a further consequence, up to isomorphism there are only 10 realization graphs of prime graphs with degree at most 4.