The spectrum of a finite graph can be defined in terms of its adjacency matrix. In "spectral graph theory," the eigenvalues of this matrix (or related matrices) are used to analyze properties of the graph, such as connectivity.The integer lattice discussed in Section 4.1.4 could be thought of as an infinite discrete graph, with lattice points as vertices and edges connecting nearest neighbors. The discrete Laplacian on Z n , as defined in (4.10), involves only neighboring vertices, and in fact differs from the adjacency matrix by a multiple of the identity. We can thus adapt the formula for Z n to define the discrete Laplacian on an arbitrary graph.Spectral graph theory has a long history as an important tool in combinatorics, with applications to network theory and computer science, as well as number theory, chemistry, and mathematical physics.Another way to build spectral models from graphs is by considering metric graphs, for which a length is assigned to each edge. This identifies the edges with intervals in R, allowing us to consider differential operators acting on a Hilbert space defined as the direct sum of the L 2 spaces for each edge interval. In physical applications, the operator is usually taken to be either the one-dimensional Laplacian or a Schrödinger operator. This combination of a metric graph equipped with a quantum Hamiltonian is called a quantum graph. Research in quantum graphs has been motivated by in large part by physical applications, such as understanding the electromagnetic properties of carbon nanostructures.Note that the key difference between the discrete and continuous cases lies in the support of the functions. In the discrete case the Hilbert space consists of functions on the vertices, while in the quantum graph case they live on the edges. It is possible to synthesize these two approaches and consider functions taking values on both edges and vertices.