Intersection array

In graph theory, given a distance-regular graph G of diameter d, by definition there are integers b_i and c_i such that given any two vertices x and y in G at distance i, y has b_i neighbours at distace i+1 from x and c_i neighbours at distance i-1 from x. The sequence {b₀,...,b_d-1;c₁,...,c_d} is called the intersection array of G.