On Maximal S-Free Sets and the Helly Number for the Family of S-Convex Sets

We study two combinatorial parameters, which we denote by f(S) and h(S), associated with an arbitrary set S ⊆ Rd, where d ∈ N. In the nondegenerate situation, f(S) is the largest possible number of facets of a d-dimensional polyhedron L such that the interior of L is disjoint with S and L is inclusion-maximal with respect to this property. The parameter h(S) is the Helly number of the family of all sets that can be given as the intersection of S with a convex subset of Rd. We obtain the inequality f(S) ≤ h(S) for an arbitrary S, and the equality f(S) = h(S) for every discrete S. Furthermore, motivated by research in integer and mixed-integer optimization, we show that 2d is the sharp upper bound on f(S) in the case S = (Zd × Rn) ∩ C, where n ≥ 0 and C ⊆ Rd+n is convex. The presented material generalizes and unifies results of various authors, including the result h(Zd) = 2d of Doignon, the related result f(Zd) = 2d of Lovász, and the inequality f(Zd ∩C) ≤ 2d, which has recently been proved for every convex set C ⊆ Rd by Morán and Dey.