Largest dyadic dual VC-dimension of non-piercing families

The dyadic dual VC-dimension of a set system \( \mathcal{F} \) is the largest integer \( \ell \) such that there exist \( \ell \) sets \( F_1, F_{2}, \dots, F_\ell \in \mathcal{F} \), where every pair \( \{i, j\} \in \binom{[\ell]}{2} \) is witnessed by an element \( a_{i,j} \in F_i \cap F_j \) that does not belong to any other set \( F_k \) with \( k \in [\ell] \setminus \{i, j\} \). In this paper, we determine the largest dyadic dual VC-dimension of a non-piercing family is exactly 4, providing a rare example where the maximum of this parameter can be determined for a natural family arising from geometry. As an application, we give a short and direct proof that the transversal number \( \tau(\mathcal{F}) \) of any non-piercing family is at most \(C\nu(\mathcal{F})^9 \), where \( \nu(\mathcal{F}) \) is the matching number and C is a constant. This improves a recent result of Pálvölgyi and Zólomy.