Coloring Grids

Abstract

A structure $\mathcal{A}=\left(A;E_i\right)_{i\in n}$ where each $E_i$ is an equivalence relation on $A$ is called an $n$-grid if any two equivalence classes coming from distinct $E_i$'s intersect in a finite set. A function $\chi: A \to n$ is an acceptable coloring if for all $i \in n$, the set $\chi^{-1}(i)$ intersects each $E_i$-equivalence class in a finite set. If $B$ is a set, then the $n$-cube $B^n$ may be seen as an $n$-grid, where the equivalence classes of $E_i$ are the lines parallel to the $i$-th coordinate axis. We use elementary submodels of the universe to characterize those $n$-grids which admit an acceptable coloring. As an application we show that if an $n$-grid $\mathcal{A}$ does not admit an acceptable coloring, then every finite $n$-cube is embeddable in $\mathcal{A}$.