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Let $P$ be a (convex, bounded) polytope with the following property: for every vertex $v$, there are exactly two facets which do not contain $v$. Does it follow that $P$ is (combinatorially) a Cartesian product of two simplices?

Some remarks

  • by "facet" I mean "face of codimension $1$"
  • a Cartesian product of two simplices has the desired property
  • this is trivially true for $\dim P = 2$
  • this is true for $\dim P = 3$: by playing with the Euler formula and the list of polyhedra with small $f$-vectors, one checks that only the triangular prism satisfies the condition
  • it is a classical fact that a $n$-dimensional simple polytope with $n+2$ facets is a Cartesian product of two simplices. So the answer is positive if $P$ is assumed to be simple.
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  • $\begingroup$ The triangular prism (=cartesian product of a triangle and an interval) has 5 facets. Any given vertex is contained in 3 facets and therefore in all facets except 2. $\endgroup$ Commented Sep 1, 2021 at 18:14
  • $\begingroup$ Just curious, is it the case that the product of $n$ simplices (of any dimensions) has each vertex not belong to exactly $n$ facets? $\endgroup$ Commented Sep 1, 2021 at 18:25
  • $\begingroup$ Yes this is true. Also note that the converse to your statement is false for $n \geq 3$: for $n=3$ the pentagon (in dimension $2$) and the "pentagonal wedge" depicted in en.wikipedia.org/wiki/Hexahedron#/media/File:Hexahedron4.svg (in dimension 3) are counterexamples. $\endgroup$ Commented Sep 1, 2021 at 18:38
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    $\begingroup$ fyi: A quick check with sage reveals that this is also true for all polytopes of dimension 4 with less than 10 vertices (or less than 10 facets). "playing with the Euler formula" might then also give the result for all polytopes in dimension 4. $\endgroup$ Commented Sep 1, 2021 at 19:49
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    $\begingroup$ Is it equivalent to say: If P is a convex polytope in which every vertex is contained in all but two facets, then P must be simple. $\endgroup$ Commented Sep 1, 2021 at 20:07

1 Answer 1

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There are other polytopes with this property that can be obtained via the free join construction.

Given two polytopes $P_1\subset\Bbb R^{d_1}$ and $P_2\subset\Bbb R^{d_2}$, the free join $P_1\bowtie P_2$ is obtained by embedding $P_1$ and $P_2$ into skew affine subspaces of $\Bbb R^{d_1+d_2+1}$ and taking the convex hull.

The claim is that if $P_1$ and $P_2$ have your property, then so does $P_1\bowtie P_2$. To see this, note that the facets of $P_1\bowtie P_2$ are exactly of the form $P_1\bowtie f_2$ and $f_1\bowtie P_2$, where $f_i$ is a facet of $P_i$. Now, if $v$ is a vertex in, say, $P_1\subset P_1\bowtie P_2$ that is in all facets of $P_1$ except for $f,f'\subset P_1$, then $v$ is in all facets of $P_1\bowtie P_2$ except for $f\bowtie P_2$ and $f'\bowtie P_2$. Equivalently for vertices in $P_2$.

Example. Take the free join of two squares, which is a 5-dimensional polytope with 8 vertices, 8 facets and vertex degree 6. This polytope is not simple and can thus not be the cartesian product of two simplices.


This construction yields counterexamples in dimension $d\ge 5$. Concerning $d=4$, there should not be any counterexamples in this dimension.

Suppose that $P\subset\Bbb R^4$ is a counterexample. According to Moritz Firsching's comment we can assume that it has $n\ge 10$ facets. Each vertex is then contained in exactly $n-2\ge 8$ of them. More generally, each set of $k$ vertices is contained in at least $n-2k$ facets. But let $f\subset P$ be a 2-face of $P$ and $v_1,v_2,v_3\in f$ three affinely independent vertices, then this set of vertices, and thus also $f$, is contained in $n-6\ge 4$ facets. This cannot be, since each 2-face of $P$ is contained in exactly two facets.

I believe this generalizes to show that any polytope with your property must have $\le 2d$ facets.

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    $\begingroup$ Given that this construction is also "canonical" like the one with the Cartesian product, one might ask a modified question whether these are the only possible cases. Starting in the reverse direction: Maybe there are some highly/strongly regular (etc.) graphs which can be casted as polytopes in a suitable number of dimensions without being any kind of "product"? $\endgroup$
    – Wolfgang
    Commented Sep 3, 2021 at 6:30
  • $\begingroup$ In fact, certain eigenspaces of association schemes could be the place to look for. $\endgroup$
    – Wolfgang
    Commented Sep 3, 2021 at 6:51
  • $\begingroup$ The free join of two squares also has $8$ faces, as opposed to $7$ for both $S_1\times S_4$ and $S_2\times S_3$. $\endgroup$ Commented Sep 3, 2021 at 6:56
  • $\begingroup$ Great! That constructions gives counter-examples for dimension 5 and up! In dimension 3, there aren't any, which leaves the question open for dimension 4, I guess... $\endgroup$ Commented Sep 3, 2021 at 11:32
  • $\begingroup$ @MoritzFirsching - my failed attempt: in dimension $4$ (as in all others) cutting a symplex with a generic hyperplane creates creates pairs of polytopes all with the required property (except in case of a half with only one old vertex), but they all turn out to be likely products of $2$ simplexes, in that they have exactly the same number of edges of each dimension as those. $\endgroup$ Commented Sep 3, 2021 at 12:13

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