# Degree matrix

In the mathematical field of graph theory the degree matrix is a diagonal matrix which contains information about the degree of each vertex—that is, the number of edges attached to each vertex.[1] It is used together with the adjacency matrix to construct the Laplacian matrix of a graph.[2]

## Definition

Given a graph $G=(V,E)$ with $\|V\|=n$ the degree matrix $D$ for $G$ is a $n \times n$ diagonal matrix defined as[1]

$d_{i,j}:=\left\{ \begin{matrix} \deg(v_i) & \mbox{if}\ i = j \\ 0 & \mbox{otherwise} \end{matrix} \right.$

where the degree $\deg(v_i)$ of a vertex counts the number of times an edge terminates at that vertex. In an undirected graph, this means that each new loop increases the degree of a vertex by two. In a directed graph, the term degree may refer either to indegree (the number of incoming edges at each vertex) or outdegree (the number of outgoing edges at each vertex).

## Example

Vertex labeled graph Degree matrix
$\begin{pmatrix} 4 & 0 & 0 & 0 & 0 & 0\\ 0 & 3 & 0 & 0 & 0 & 0\\ 0 & 0 & 2 & 0 & 0 & 0\\ 0 & 0 & 0 & 3 & 0 & 0\\ 0 & 0 & 0 & 0 & 3 & 0\\ 0 & 0 & 0 & 0 & 0 & 1\\ \end{pmatrix}$

## Properties

The degree matrix of a k-regular graph has a constant diagonal of $k$.

## References

1. ^ a b Chung, Fan; Lu, Linyuan; Vu, Van (2003), "Spectra of random graphs with given expected degrees", Proceedings of the National Academy of Sciences of the United States of America 100 (11): 6313–6318, doi:10.1073/pnas.0937490100, MR 1982145.
2. ^ Mohar, Bojan (2004), "Graph Laplacians", in Beineke, Lowell W.; Wilson, Robin J., Topics in algebraic graph theory, Encyclopedia of Mathematics and its Applications 102, Cambridge University Press, Cambridge, pp. 113–136, ISBN 0-521-80197-4, MR 2125091.