A graph G is a pair (V,E) where

• V is a set of vertices
• E is a set of edges

Example:

• V = {1, 2, 3, 4, 5}
• E = {(1,2), (1,5), (2,4), (2,3), (2,5), (3,4), (4,5)}

E is a binary relation on V.

• If the edges in E are unordered pairs (u,v), the graph G is undirected
• If the edges in E are ordered pairs ⟨u,v⟩, the graph G is directed