Erdős-Rényi-Sós 1966: a finite graph in which every two distinct vertices have exactly one common neighbour is a windmill graph F_n (one universal vertex joined to n vertex-disjoint edges). Classical eigenvalue-counting proof; no purely…
Erdős-Rényi-Sós 1966: a finite graph in which every two distinct vertices have exactly one common neighbour is a windmill graph F_n (one universal vertex joined to n vertex-disjoint edges). Classical eigenvalue-counting proof; no purely…