\(\lambda\)-Designs on \(4p +1\) points

A \(\lambda\)-design on \(v\) points is a family of \(v\) subsets (blocks) of a \(v\)-set such that any two distinct blocks intersect in \(\lambda\) points and not all blocks have the same cardinality.Ryser’s and Woodall’s \(\lambda\)-design conjecture states that each \(\lambda\)-design can be obtained from a symmetric design by complementing with respect to a fixed block. In a recent paper, we proved this conjecture for \(v = p+1, 2p+1, 3p+1\), where \(p\) is prime, and remarked that similar methods might work for \(v = 4p+1\). In the present paper, we prove the conjecture for \(\lambda\)-designs having replication numbers \(r\) and \(r^*\) such that \((r-1, r^*-1) = 4\) and, as a consequence, the \(\lambda\)-design conjecture is proved for \(v = 4p+1\), where \(p\) is prime.