If u and v are vertices in the graph G then dist G (u, v) (or just dist(u, v)) will denote the distance between them in G. The set of vertices at distance r from u will be denoted by Sr (u). A connected graph G is distance-regular if, for any two vertices u and v in G and any two integers i and j, | S i ( u ) ∩ S j ( v ) | https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315137131/3e789cab-79f2-434a-a841-e2acf5146c2d/content/eq923.tif"/>