From a $(p,2)$-Theorem to a Tight $(p,q)$-Theorem

Abstract: A family $F$ of sets is said to satisfy the $(p,q)$-property if among any $p$ sets of $F$ some $q$ intersect. The celebrated $(p,q)$-theorem of Alon and Kleitman asserts that any family of compact convex sets in $\mathbb{R}^d$ that satisfies the $(p,q)$-property for some $q \geq d+1$, can be pierced by a fixed number $f_d(p,q)$ of points. The minimum such piercing number is denoted by $HD_d(p,q)$. Already in 1957, Hadwiger and Debrunner showed that whenever $q>\frac{d-1}{d}p+1$ the piercing number is $HD_d(p,q)=p-q+1$; no exact values of $HD_d(p,q)$ were found ever since. While for an arbitrary family of compact convex sets in $\mathbb{R}^d$, $d \geq 2$, a $(p,2)$-property does not imply a bounded piercing number, such bounds were proved for numerous specific families. The best-studied among them is axis-parallel rectangles in the plane. Wegner and (independently) Dol'nikov used a $(p,2)$-theorem for axis-parallel rectangles to show that $HD_{\mathrm{rect}}(p,q)=p-q+1$ holds for all $q>\sqrt{2p}$. These are the only values of $q$ for which $HD_{\mathrm{rect}}(p,q)$ is known exactly. In this paper we present a general method which allows using a $(p,2)$-theorem as a bootstrapping to obtain a tight $(p,q)$-theorem, for families with Helly number 2, even without assuming that the sets in the family are convex or compact. To demonstrate the strength of this method, we obtain a significant improvement of an over 50 year old result by Wegner and Dol'nikov. Namely, we show that $HD_{\mathrm{d-box}}(p,q)=p-q+1$ holds for all $q > c' \log^{d-1} p$, and in particular, $HD_{\mathrm{rect}}(p,q)=p-q+1$ holds for all $q \geq 7 \log_2 p$ (compared to $q \geq \sqrt{2p}$ of Wegner and Dol'nikov). In addition, for several classes of families, we present improved $(p,2)$-theorems, some of which can be used as a bootstrapping to obtain tight $(p,q)$-theorems.