Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. In graph theory, a strong coloring, with respect to a partition of the vertices into (disjoint) subsets of equal sizes, is a (proper) vertex coloring in which every color appears exactly once in every partition. When the order of the graph G is not divisible by k, we add isolated vertices to G just enough to make the order of the new graph G divisible by k. In that case, a strong coloring of G minus the previously added isolated vertices is considered a strong coloring of G. A graph is strongly k-colorable if, for each partition of the vertices into sets of size k, it admits a strong coloring.