# Incidence matrix

A matrix, say $LaTeX: M$, that represents the incidence of edges or arcs to nodes in a graph or network. In the undirected case, $LaTeX: M$ is binary valued: if edge $LaTeX: k$ has endpoints $LaTeX: i$ and $LaTeX: j$, then $LaTeX: M_{ik} = M_{jk} = 1$ and $LaTeX: M_{rk} = 0$ for $LaTeX: r \neq i,j$. In the directed case, the entry $LaTeX: -1$ indicates the tail: if the arc is directed from $LaTeX: i$ to $LaTeX: j$, $LaTeX: M_{ik} =-1$ and $LaTeX: M_{jk} = 1$.