-vertex coloring, chromatic number $\chi(G)$ edge coloring, chromatic index (=edge chromatic number) $\chi'(G)$ $χ'(G)$ は $\Delta(G)$ or $\Delta(G) + 1$ Coloring maps and planar graphs 4 色定理（5 色なら証明簡単） triangle-free -> 3-colo…