On the number of antipodal or strictly antipodal pairs of points in finite subsets of $\mathbb{R}^d$, III

Abstract

We improve our earlier upper bound on the numbers of antipodal pairs of points among $n$ points in ${\mathbb{R}}^3$, to $2n^2/5+O(n^c)$, for some $c<2$. We prove that the minimal number of antipodal pairs among $n$ points in convex position in ${\mathbb{R}}^d$, affinely spanning ${\mathbb{R}}^d$, is $n + d(d - 1)/2 - 1$. Let ${\underline{sa}}^s_d(n)$ be the minimum of the number of strictly antipodal pairs of points among any $n$ points in ${\mathbb{R}}^d$, with affine hull ${\mathbb{R}}^d$, and in strictly convex position. The value of ${\underline{sa}}^s_d(n)$ was known for $d \le 3$ and any $n$. Moreover, ${\underline{sa}}^s_d(n) = \lceil n/2\rceil $ was known for $n \ge 2d$ even, and $n \ge 4d+1$ odd. We show ${\underline{sa}}^s_d(n) = 2d$ for $2d+1 \le n \le 4d-1$ odd, we determine ${\underline{sa}}^s_d(n)$ for $d=4$ and any $n$, and prove ${\underline{sa}}^s_d(2d -1) = 3(d - 1)$. The cases $d \ge 5 $ and $d+2 \le n \le 2d - 2$ remain open, but we give a lower and an upper bound on ${\underline{sa}}^s_d(n)$ for them, which are of the same order of magnitude, namely $\Theta \left( (d-k)d \right) $. We present a simple example of a strictly antipodal set in ${\mathbb{R}}^d$, of cardinality const\,$\cdot 1.5874...^d$. We give simple proofs of the following statements: if $n$ segments in ${\mathbb{R}}^3$ are pairwise antipodal, or strictly antipodal, then $n \le 4$, or $n \le 3$, respectively, and these are sharp. We describe also the cases of equality.