Laplacian matrix

The Graph Laplacian converts local differences along edges into a linear map. Throughout this section, $G=(V,E)$ is a finite undirected graph with no self-loops or repeated edges, and each edge has weight one. In a fixed order of vertices, let $A$ be the adjacency matrix and $D$ the degree matrix. The combinatorial Graph Laplacian isThus the value at a vertex is compared with its neighbours. Every row sums to zero, so $L1=0$: a constant vector produces no differences. This definition and the associated Graph Laplacian quadratic form are developed in Spielman's authored survey.^[ref]

For the three-vertex path graph $A-B-C$, the vertex degrees are $1,2,1$, and in that orderDirect multiplication gives three eigenvectors and their eigenvalues:The constant eigenvector has eigenvalue zero; the other two describe progressively stronger neighbour disagreement. Dividing these eigenvectors by $\sqrt{3},\sqrt{2},\sqrt{6}$, respectively, gives an orthonormal basis. The four-vertex cycle graph is already worked out under circulant matrix, where Fourier diagonalization of a circulant matrix gives its eigenvalues.

Every formula here uses $L=D-A$ without degree normalization. For comparison, Chung's treatment uses the degree-scaled matrix $D^{-1/2}L{D}^{-1/2}$, with a convention for isolated vertices; its eigenvalues must not be substituted into the examples here.^[ref]