On colorful Helly numbers and the growth of Tverberg numbers

Abstract

We establish new bounds on colorful Helly and Tverberg numbers in abstract convexity spaces with Radon number $r$. We show that the colorful Helly number is at most $(r-1)2^r$, improving Holmsen--Lee's bound $r^{r^{O(\log r)}}$ and matching the known lower bound $2^{r-1}-1$ up to a factor $O(r)$. Furthermore, we prove that the Tverberg number is $r_k=O(r^{\lceil {\log }_{2}r\rceil }k)$, reducing Pálvölgyi's coefficient $r^{r^{r^{O(\log r)}}}$ to a quasipolynomial function of $r$. For $S_4$-separable spaces, we prove $r_k=O(r^{2}k)$. In the other direction, we construct a sequence of $S_4$-separable spaces with Tverberg number $r_k=\Omega (r^{2}k)$. This shows that the weak Eckhoff conjecture, asserting that $r_k=O(rk)$ in any abstract convexity space, fails already in separable spaces. The same construction refutes the analogue of Reay's conjecture in separable spaces.

arXiv comment30 pages, 3 figures. New over v2: Tight upper bound for Tverberg numbers in S_4 spaces