More precisely, let G be a graph with n vertices. It is assumed that G is simple, that is, it does not contain loops or parallel edges. Let A be the adjacency matrix of G and let , , be the eigenvalues of A. Then the energy of the graph is defined as:
- Cvetković, Dragoš M.; Doob, Michael; Sachs, Horst (1980), Spectra of graphs, Pure and Applied Mathematics 87, New York: Academic Press Inc. [Harcourt Brace Jovanovich Publishers], ISBN 0-12-195150-2, MR 572262.
- Gutman, Ivan (1978), "The energy of a graph", 10. Steiermärkisches Mathematisches Symposium (Stift Rein, Graz, 1978), Ber. Math.-Statist. Sekt. Forsch. Graz 103, pp. 1–22, MR 525890.
- Gutman, Ivan (2001), "The energy of a graph: old and new results", Algebraic combinatorics and applications (Gößweinstein, 1999), Berlin: Springer, pp. 196–211, MR 1851951.
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