Digraph minors

We propose a similarity classification for directed graphs. Two graphs are similar if the number of nodes with a given in-edges and out-edges are the same.

The heading “2:in=0+out=1, 1:in=2+out=0” means that there are two nodes with zero in-edges and one out-edge, and one node with two in-edges and zero out-edges. What follows is the image of all such graphs.

We discard graphs which contain nodes that do not participate in any edge, since those are equivalent to graphs with fewer nodes.