Directed Graphs and Cannibals

Abstract

In graph theory a graph is defined as any set of points joined by lines, and a simple graph is defined as one that has no loops (lines that join a point to itself) and no parallel lines (two or more lines joining the same pair of points). If an arrowhead is added to each line of a graph, giving each line a direction that orders its end points, the graph becomes a directed graph, or digraph for short. Directed lines are called arcs. Digraphs are the subject here, and the old joke quoted above is appropriate because on some digraphs it is actually impossible to get from one specified point to another.

Stranger in car: “How do I get to the corner of Graham Street and Harary Avenue?”

Native on sidewalk: “You can’t get there from here.”