# Big Line or Big Clique in Planar Point Sets

Let $S$ be a set of points in the plane. Two points $v$ and $w$ in $S$ are \emph{visible} with respect to $S$ if the line segment between $v$ and $w$ contains no other point in $S$.

\begin{conjecture} For all integers $k,\ell\geq2$ there is an integer $n$ such that every set of at least $n$ points in the plane contains at least $\ell$ collinear points or $k$ pairwise visible points. \end{conjecture}

The conjecture is trivial for $\ell \leq 3$.

Kára et al. [KPW] proved the conjecture for $k \leq 4$ and all $\ell$.

Addario-Berry et al. [AFKCW] proved the conjecture for $k=5$ and $\ell=4$.

Abel et al. [ABBCDHKLPW] proved the conjecture for $k=5$ and all $\ell$.

The conjecture is open for $k=6$ or $\ell=4$.

Note that it is easily proved that for all $k,\ell\geq2$, every set of at least $\Omega(\ell k^2)$ points in the plane contains $\ell$ collinear points or $k$ points with no three collinear [Brass].

See [Matousek] for related results and questions.

## Bibliography

[ABBCDHKLPW] Zachary Abel, Brad Ballinger, Prosenjit Bose, Sébastien Collette, Vida Dujmović, Ferran Hurtado, Scott D. Kominers, Stefan Langerman, Attila Pór, David R. Wood. \href[Every Large Point Set contains Many Collinear Points or an Empty Pentagon]{http://dx.doi.org/10.1007/s00373-010-0957-2}, \emph{Graphs and Combinatorics} 27(1): 47-60, 2011.

[AFKCW] Louigi Addario-Berry, Cristina Fernandes, Yoshiharu Kohayakawa, Jos Coelho de Pina, and Yoshiko Wakabayashi. \href[On a geometric Ramsey-style problem]{http://crm.umontreal.ca/cal/en/mois200708.html}, 2007.

[Brass] Peter Brass. On point sets without k collinear points. In \emph{Discrete Geometry}, vol. 253 of Monographs and Textbooks in Pure and Applied Mathematics, pp. 185–192. Dekker, New York, 2003.

*[KPW] Jan Kára, Attila Pór, David R. Wood. \href[On the chromatic number of the visibility graph of a set of points in the plane]{http://dx.doi.org/10.1007/s00454-005-1177-z}, \emph{Discrete and Computational Geometry} 34(3):497-506, 2005.

[Matousek] Jiří Matoušek. \href[Blocking visibility for points in general position]{http://dx.doi.org/10.1007/s00454-009-9185-z}, \emph{Discrete and Computational Geometry} 42(2): 219-223, 2009.

* indicates original appearance(s) of problem.

## Improved bound in the proof for k=5 and arbitrary l

In their proof of the case $k=5$ and arbitrary $\ell$, Abel et al. [ABBCDHKLPW] proved a doubly exponential upper bound on the number $p(\ell)$ of points that guarantees the occurrence of an $\ell$-tuple of collinear points or a $5$-tuple of points with no other point in their convex hull (an \emph{empty pentagon}). The upper bound on $p(\ell)$ was improved by Barát et al. [BDJPSSVW] to $p(\ell) \le 328\ell^2$.

[BDJPSSVW] János Barát, Vida Dujmović, Gwenaël Joret, Michael S. Payne, Ludmila Scharf, Daria Schymura, Pavel Valtr, and David R. Wood. \arxiv[Empty pentagons in point sets with collinearities]{1207.3633}, arxiv:1207.3633, 2012.