Past exam of the integer programming course of the University of Oxford / 2011 / 1 / a

Use vertices $1,2,3,4,5$. Reading each symmetric off-diagonal pair once gives the edge setHere $ij$ abbreviates the undirected edge $\{i,j\}$. The diagram displays these seven edges and no loops. The displayed square matrix is an adjacency matrix in the usual terminology: its two indices both label vertices. A vertex-edge incidence matrix would instead have one index for vertices and one for edges. This explains how the source's node-node terminology is being read.