This theorem is a stronger result about an upper bound for the chromatic number due to Rowland Leonard Brooks (1916 – 1993).
Let $G$ be a connected simple graph with the maximum vertex degree $d.$ If $G$ is neither a cycle with an odd number of vertices, nor a complete graph, then the chromatic number of $G$ obeys the following inequality: $$\chi(G)\le d.$$
Proofs: 1