# Cube-Simplex conjecture

 Importance: High ✭✭✭
 Author(s): Kalai, Gil
 Subject: Geometry » Polytopes
 Keywords: cube facet polytope simplex
 Recomm. for undergrads: no
 Posted by: mdevos on: May 9th, 2008
Conjecture   For every positive integer , there exists an integer so that every polytope of dimension has a -dimensional face which is either a simplex or is combinatorially isomorphic to a -dimensional cube.

It is an easy consequence of Euler's formula that every 3-polytope has a face which is either a triangle, a quadrilateral, or a pentagon. The 120-cell is a 4-polytope in which every 2-face is a pentagon (in fact every 3-face is a regular dodecahedron). Perles and Shephard asked whether there exist higher dimensional polytopes in which all 2-faces have at least 5 vertices. This question was answered in the negative by Kalai [K] who showed that every 5-polytope has a 2-face with at most 4 vertices. So, if we define to be the smallest integer satisfying the above conjecture for , or if none exists, then .

This conjecture is still open for simple polytopes. However, it is known that for every positive integer , there exists an integer so that every simple polytope of dimension either has a 2-dimensional face which is a triangle, or a -dimensional face which is combinatorially isomorphic to a cube. This was proved by Kalai [K] using some earlier results of Nikulin and of Blind and Blind. Actually, something much stronger holds here: simple polytopes of sufficiently high dimension without 2-faces which are triangles must have most -dimensional faces combinatorially isomorphic to the -cube.

The following is an interesting weakening of the above conjecture.

Conjecture   For every positive integer , there exists an integer and a finite list of -dimensional polytopes, so that every polytope of dimension has a -dimensional face which appears in .

Defining to be the smallest integer satisfying this conjecture for , or if none exists, we find that (by the consequence of Euler's formula in the first paragraph). Meisinger, Kleinschmidt, and Kalai [MKK] proved that with the help of FLAGTOOL, a computer program which can compute linear relations for -vectors. This weaker conjecture is known to be true for simple polytopes.

## Bibliography

[MKK] G. Meisinger, P. Kleinschmidt, and G. Kalai, Three theorems, with computer-aided proofs, on three-dimensional faces and quotients of polytopes. The Branko Grünbaum birthday issue. Discrete Comput. Geom. 24 (2000), no. 2-3, 413--420. MathSciNet

*[K] G. Kalai, On low-dimensional faces that high-dimensional polytopes must have. Combinatorica 10 (1990), no. 3, 271--280. MathSciNet

* indicates original appearance(s) of problem.