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

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

where the degree ${\displaystyle \deg(v_{i})}$ 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
${\displaystyle {\begin{pmatrix}4&0&0&0&0&0\\0&3&0&0&0&0\\0&0&2&0&0&0\\0&0&0&3&0&0\\0&0&0&0&3&0\\0&0&0&0&0&1\\\end{pmatrix}}}$

## Properties

The degree matrix of a k-regular graph has a constant diagonal of ${\displaystyle k}$.

## 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, PMC , PMID 12743375.
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.