Odd-order additive gap

Determine the value of \(\rho_{q+1,q-h+3}\) for every odd prime power \(q\) and integer \(2\le h<q\), and establish whether a non-projective finite linear intersecting \((q+1)\)-uniform hypergraph can attain the upper bound \((q+1)/h\) despite the nonexistence of nontrivial maximal \(h\)-arcs in \(PG(2,q)\), or whether the truncated-plane value \(q/h\) is optimal in some odd-order cases.

Background

For r=q+1r=q+1, the paper defines h=qk+3h=q-k+3 and studies the efficiency parameter ρq+1,k\rho_{q+1,k}, the supremum of the ratio between the number of edges and the minimum size of an hh-fold transversal over finite linear intersecting (q+1)(q+1)-uniform hypergraphs. The universal upper bound is (q+1)/h(q+1)/h.

References

Four natural problems remain. Let q be an odd prime power and 2\le h<q. Determine \rho_{q+1,q-h+3}. In particular, can a non-projective intersecting block attain (q+1)/h even though PG(2,q) has no nontrivial maximal h-arc, or is the truncated-plane value q/h optimal in some odd-order cases?

Blocking Amalgamations, Maximal Arcs, and Generalized Crowns  (2608.16035 - Ramani, 17 Aug 2026) in Section 8, Open problems, Problem 8.1 (Odd-order additive gap)