Nordhaus-Gaddum-type problems for lines in hypergraphs

Abstract: We study the number of lines in hypergraphs in a more symmetric setting, where both the hypergraph and its complement are considered. In the general case and in some special cases, the lower bounds on the number of lines are much higher than their counterparts in single hypergraph setting or admit more elegant proofs. We show that the minimum value of product of the number of lines in both hypergraphs on $n$ points is easily determined as $\binom{n}{2}$; and the minimum value of their sum is between $\Omega(n)$ and $O(n \log n)$. We also study some restricted classes of hypergraphs; and determine the tight bounds on the minimum sum when the hypergraph is derived from an Euclidean space, a real projective plane, or a tree.