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## 4.2 Background on Graph Spectra

A graph G = (V, E) is a set of nodes V connected by means of the elements of the set of links E. Here we are dealing only with simple graphs , that is, an undirected graph without multiple links or self-loops. Thus, by graph we mean a simple graph. A node vV is a terminal point of a link and represents an abstraction of an entity in a complex network such as a person, a city, a protein, an atom, etc. The links represent the relations between these entities.

A graph G = (V, E) can be represented by different kinds of matrices . The (ordinary) spectrum of a graph always refers to the spectrum of the adjacency matrix of the graph . Thus, we will be concerned here only with this matrix. Excellent reviews about the Laplacian spectrum of graphs can be found in the literature . The adjacency matrix A = A(G) of a graph G = (V, E) is a symmetric matrix of order n = |V|, where |···| means the cardinality of the set and where Aij = 1 if there is a link between nodes i and j, and Aij = 0 otherwise.

The “spectrum” of a network is a listing of the eigenvalues of the adjacency matrix of such a network. It is well known that every n × n real symmetric matrix A has a spectrum of n orthonormal eigenvectors ϕ12,...,ϕn with eigenvalues λ1 ≥ λ2 ≥ 0 ... ≥ λn . The largest eigenvalue of graph λ1 is known as the principal eigenvalue, the spectral radius, or the Perron–Frobenius eigenvalue . The eigenvector associated with this eigenvalue is also ...

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