Warning: this proof requires a bit of familiarity with the terminology of propositional logic and graph theory.

Problem: Let $ G$ be an infinite graph. Show that $ G$ is $ n$-colorable if and only if every finite subgraph $ G_0 \subset G$ is $ n$-colorable.

Solution: One of the many equivalent versions of the Compactness Theorem for the propositional calculus states that if $ \Sigma \subset \textup{Prop}(A)$, where $ A$ is a set of propositional atoms, then $ \Sigma$ is satisfiable if and only if any finite subset of $ \Sigma$ is satisfiable. (Recall that a set of propositions is satisfiable if for some truth valuation $ t:A \to \left \{ 0,1 \right \}$ the unique extension $ \hat{t}:\Sigma \to \left \{ 0,1 \right \}$ satisfies $ \hat{t}(p) = 1$ for all $ p \in \Sigma$. The function $ t$ is called a model for $ \Sigma$). This is equivalent to the Completeness Theorem, which says that if one can use $ \Sigma$ to prove $ p$, then every model of $ \Sigma$ satisfies $ \hat{t}(p) = 1$. Both are fundamental results in logic.

And so we will convert this graph coloring problem into a logical set of propositions, and use the Compactness Theorem against it. We want a set of propositions $ \Sigma$ which has a model if and only if the corresponding graph is n-colorable. Then we will use the n-colorability of finite subgraphs, to show that all finite subsets of $ \Sigma$ have models, and this implies by the compactness theorem that $ \Sigma$ has a model, so the original infinite graph is n-colorable.

We may think of a coloring of a graph $ G$ as a function on the set of vertices: $ c:V \to \left \{ 1, 2, \dots, n \right \}$. Define our set of propositional atoms as $ A = V \times \left \{ 1, 2, \dots, n \right \}$. In other words, we identify a proposition $ p_{v,i}$ to each vertex and possible color. So we will define three sets of propositions using these atoms, which codify the conditions of a valid coloring:

Let $ \Sigma$ be the union of the above three sets. Take $ \Sigma_0 \subset \Sigma$ to be any finite set of the above propositions. Let $ V_0$ be the finite subset of vertices of $ G$ which are involved in some proposition of $ \Sigma_0$ (i.e., $ p_{v,i} \in \Sigma_0$ if and only if $ v \in V_0$). Since every proposition involves finitely many atoms, $ V_0$ is finite, and hence the subgraph of vertices of $ V_0$ is n-colorable, with some coloring $ c: V_0 \to \left \{ 1, 2, \dots, n \right \}$. We claim that this $ c$ induces a model on $ \Sigma_0$.

Define a valuation $ t:A \to \left \{ 0,1 \right \}$ as follows. If $ v \notin V_0$, then we (arbitrarily) choose $ t(p_{v,i}) = 1$. If $ v \in V_0$ and $ c(v) = i$ then $ t(p_{v,1}) = 1$. Finally, if $ v \in V_0$ and $ c(v) \neq i$ then $ t(p_{v,i} = 0)$.

Clearly each of the possible propositions in the above three sets is true under the extension $ \hat{t}$, and so $ \Sigma_0$ has a model. Since $ \Sigma_0$ was arbitrary, $ \Sigma$ is finitely satisfiable. So by the Compactness Theorem, $ \Sigma$ is satisfiable, and any model $ s$ for $ \Sigma$ gives a valid graph coloring, simply by choosing $ i$ such that the proposition $ p_{v,i}$ satisfies $ s(p_{v,i}) = 1$. Our construction forces that such a proposition exists, and hence $ G$ is n-colorable. $ _\square$.

Note that without translating this into a logical system, we would be left with the mess of combining n-colorable finite graphs into larger n-colorable finite graphs. The number of cases we imagine encountering are mind-boggling. Indeed, there is probably a not-so-awkward graph theoretical approach to this problem, but this proof exemplifies the elegance that occurs when two different fields of math interact.

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