Laplacian matrix of a graph
Let be a finite graph with vectices and let be the incidence matrix (http://planetmath.org/IncidenceMatrixWithRespectToAnOrientation) of with respect to some orientation.The Laplacian matrix of is defined to be .
If we let be the adjacency matrix of then it can be shownthat ,where and is the degree ofthe vertex . As a result, the Laplacian matrix is independent of what orientation ischosen for .
The Laplacian matrix is usually denoted by . It is a positive semidefinitesingular matrix, so that the smallest eigenvalue
is 0.