For the chromatic number $\chi(G)$ of a simple, undirected graph, there is a ["compactness" theorem by Erdos and De Bruijn](https://en.wikipedia.org/wiki/De_Bruijn%E2%80%93Erd%C5%91s_theorem_(graph_theory)) stating that if an infinite graph $G$ has finite chromatic number, then there is a finite subgraph $G\_0\subseteq G$ such that $\chi(G\_0) = \chi(G)$.


For any simple, undirected graph $G=(V,E)$ we set $\text{col}(G) = \sup\{\delta(H): H\subseteq G\}+1$, where $\delta(\cdot)$ denotes the minimal degree. We have $\chi(G) \leq \text{col}(G)$ for any graph $G$. Does the compactness theorem above hold for $\text{col}(\cdot)$, that is, if $\text{col}(G)$ is finite, is there $G\_0\subseteq G$ such that $\text{col}(G\_0) = \text{col}(G)$?



"Compactness" theorem for the coloring number
A line infinite in both directions has minimal degree $2$, but any its proper subgraph has minimal degree $1$ or $0$. Thus, $\mathrm{col}(G)=3$, but $\mathrm{col}(G\_0)\le2$ for any proper subgraph $G\_0$ of $G$.


More generally, if $G$ is the $d$-regular infinite tree, then $\mathrm{col}(G)=d+1$, but $\mathrm{col}(G\_0)\le d$ for any proper subgraph $G\_0$ of $G$.


