# Graph of a matrix

How to define the graph of a square matrix $\mathbf{G}$ with real entries?

I know that given a graph $\Gamma(V, E)$, one can define its adjacency matrix $\mathbf{A}$. But given a matrix $\mathbf{G}$ how to define its graph?

P.S. I was reading in this book Matrix Analysis. There is a theorem that asserts that $\mathbf{A}$ is irreducible $\Leftrightarrow$ $\Gamma(\mathbf{A})$ is strongly connected (where $\Gamma(\mathbf{A})$is the graph of $\mathbf{A}$). I do not know what is the definiton of the graph of a square matrix.

• In no expert in graph theory, but doesn't it depends on what your problem is ? For example you can consider that a square matrix $G$ with elements $G_{i,j}$ represents the graph ie : $G_{i,j}\neq 0$ iif $i$ and $j$ are connected, with a weight $G_{i,j}$ for this edge. – Bertrand R Apr 10 '14 at 21:41
• I was reading in this book Matrix Analysis. There is a theorem that asserts that $\mathbf{A}$ is irreducible $\Leftrightarrow$ $\Gamma(\mathbf{A})$ is strongly connected (where $\Gamma(\mathbf{A})$is the graph of $\mathbf{A}$). I do not know what is the definiton of the graph of a square matrix. – zighalo Apr 10 '14 at 21:52
• There are two binary matrices that are often associated with a given simple graph, the incidence matrix and the adjacency matrix. I would expect the book you are reading to define the notion of "the graph of a square matrix" to give a variation on the adjacency matrix, since these are always square. Perhaps you can add a page reference? – hardmath Aug 5 '17 at 17:13
• Did you perhaps overlook Definition 6.2.11 in that book (page 399), which associates a directed graph $G$ with $n\times n$ square matrix $A$ having $n$ nodes with a directed edge from the $i$th node to the $j$th node whenever $a_{ij} \neq 0$? – hardmath Aug 6 '17 at 19:05

In this context, if the matrix is $n\times n$ we define a directed graph with vertex set $\{1,\ldots,n\}$, where there is an arc from $i$ to $j$ if $A_{i,j}\ne0$. This directed graph may have loops, but they do not affect whether the directed graph is strongly connected. It is not weighted.