Two sets A and B are said to be independent if their intersection A intersection B = ∅, where ∅ is the empty set. For example, {A, B, C} and {D, E} are independent, but {A, B, C} and {C, D, E} are not. Independent sets are also called disjoint or mutually exclusive. An independent vertex set of a graph G is a subset of the vertices such that no two vertices in the subset represent an edge of G. The figure above shows independent vertex sets consisting of two subsets for a number of graphs (the wheel graph W_8, utility graph K_(3, 3), Petersen graph, and Frucht graph). An independent edge set can be defined similarly.

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