Let $P$ be the polytope obtained as the convex hull of $n$ points in $\mathbb R^d$. This is the $V$-representation of $P$. Note that $P$ can also be represented as an intersection of closed half-spaces in $\mathbb R^d$. This is the $H$-representation of $P$.

Question. In terms of $n$ and $d$, what is a good upper-bound on the number of half-spaces in the smallest $H$-representation of $P$ ?


1 Answer 1


By the Upper Bound Theorem, the maximum number of $(d-1)$-dimensional faces of an $n$-vertex polytope is achieved by the cyclic polytopes. This number can be explicitly written via the Dehn-Sommerville equations.

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    $\begingroup$ Thanks. For the record, if $f_k(P)$ is the number of $k$-dimensional faces of $P$, then the referenced Dehn-Sommerville equations (combined with Upper Bound Theorem) yield $f_{d-1}(P) \le 2{n-(d+1)/2 \choose n-d}$. $\endgroup$
    – dohmatob
    Commented Dec 27, 2021 at 9:33

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