This is a glossary of graph theory. Graph theory is the study of graphs, systems of nodes or vertices connected in pairs by lines or edges.
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{{defn|The complement graph \bar{G} of a simple graph is another graph on the same vertex set as , with an edge for each two vertices that are not adjacent in .}} ==D==
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{{defn|A Ramanujan graph is a graph whose spectral expansion is as large as possible. That is, it is a -regular graph, such that the second-largest eigenvalue of its adjacency matrix is at most 2\sqrt{d-1}.}} ==S==