On the minimum number of facets of a 2-neighborly polytope

Abstract

Let $\mu_{\text{2n}}(d,v)$ (respectively, $\mu^{\text{s}}_{\text{2n}}(d,v)$) be the minimal number of facets of a (simplicial) 2-neighborly $d$-polytope with $v$ vertices, $v > d \ge 4$. It is known that $\mu_{\text{2n}}(4,v) = v (v-3)/2$, $\mu_{\text{2n}}(d, d+2) = d+5$, $\mu_{\text{2n}}(d,d+3) = d+7$ for $d \ge 5$, and $\mu_{\text{2n}}(d,d+4) \in [d+5, d+8]$ for $d \ge 6$. We show that $\mu_{\text{2n}}(5, v) = \Omega(v^{4/3})$, $\mu_{\text{2n}}(6, v) \ge v$, and the equality $\mu_{\text{2n}}(6, v) = v$ holds only for a simplex and for a dual 2-neighborly 6-polytope (if it exists) with $v \ge 27$. By using $g$-theorem, we get $\mu^{\text{s}}_{\text{2n}}(d, v) = \Delta (\Delta(d-3) + 3d - 5)/2 + d + 1$, where $\Delta = v - d - 1$. Also we show that $\mu_{\text{2n}}(d, v) \ge d+7$ for $v \ge d+4$.