Pick an odd n = 2r+1. The vertices are the r-subsets and (r+1)-subsets of
{1,…,n}; two are joined when one contains the other. Hover or step through vertices to light up a neighbourhood.r = 2
|V| = 20|E| = 303-regular
n = 5: 20 vertices and 30 edges, every vertex of degree 3. Generate to draw it.
Choose n and press Generate the graph.