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Helly-type theorems for hollow axis-aligned boxes
Authors:Konrad J Swanepoel
Institution:Department of Mathematics and Applied Mathematics, University of Pretoria, Pretoria 0002, South Africa
Abstract:A hollow axis-aligned box is the boundary of the cartesian product of $d$ compact intervals in $\mathbb{R}^d$. We show that for $d\geq 3$, if any $2^d$ of a collection of hollow axis-aligned boxes have non-empty intersection, then the whole collection has non-empty intersection; and if any $5$ of a collection of hollow axis-aligned rectangles in $\mathbb{R}^2$ have non-empty intersection, then the whole collection has non-empty intersection. The values $2^d$ for $d\geq 3$ and $5$ for $d=2$ are the best possible in general. We also characterize the collections of hollow boxes which would be counterexamples if $2^d$ were lowered to $2^d-1$, and $5$ to $4$, respectively.

Keywords:Helly-type theorem  box  cube  hypercube
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