The $LaTeX: n \times n$ binary matrix, say $LaTeX: A$, that represents node adjacency in a simple graph: $LaTeX: A_{i,j} = 1$ if node $LaTeX: i$ is adjacent to node $LaTeX: j$. In the undirected case, $LaTeX: A$ is symmetric. In the directed case, the adjacency means there is an arc from $LaTeX: i$ to $LaTeX: j$. In the case of a multigraph, $LaTeX: A_{i,j}$ is the number of edges whose endpoints are nodes $LaTeX: i$ and $LaTeX: j$.