## Erd ̋os-Szekeres is NP-Hard in 3 Dimensions and What Now?

**Authors:** Thinh Nguyen

The Erd ̋os-Szekeres theorem states that, for every $k$, there is a number $n_k$ such that every set of $n_k$ points in general position in the plane contains a subset of $k$ points in convex position. If we ask the same question for subsets whose convex hull does not contain any other point from the set, this is not true: as shown by Horton, there are sets of arbitrary size that do not contain an empty 7-gon.
These questions have also been studied extensively
from a computational point of view, and polynomial time algorithms for finding the largest (empty) convex set have been given for the planar case. In higher dimension, it is not known how to compute such a set efficiently. In this paper, we show that already in 3 dimensions no polynomial time algorithm exists for determining the largest (empty) convex set (unless $P$=$NP$), by proving that the corresponding decision problem is $NP$-hard. This answers a question by Dobkin, Edelsbrunner and Overmars from 1990.
As a corollary, we derive a similar result for the closely related problem of testing weak $ε$-nets in $R^3$ . Answering a question by Chazelle et al. from 1995, our reduction shows that the problem is $co-NP$-hard.
Finally, we make several suggestions for further research on the subject.

**Comments:** 4 Pages.

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### Submission history

[v1] 2018-07-03 06:11:19

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