# On the Existence of Ordinary Triangles

- Published in 2017
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Let $P$ be a finite point set in the plane. A $c$-ordinary triangle in $P$ is a subset of $P$ consisting of three non-collinear points such that each of the three lines determined by the three points contains at most $c$ points of $P$. We prove that there exists a constant $c>0$ such that $P$ contains a $c$-ordinary triangle, provided that $P$ is not contained in the union of two lines. Furthermore, the number of $c$-ordinary triangles in $P$ is $\Omega(|P|)$.

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### BibTeX entry

@article{OntheExistenceofOrdinaryTriangles, title = {On the Existence of Ordinary Triangles}, abstract = {Let {\$}P{\$} be a finite point set in the plane. A {\$}c{\$}-ordinary triangle in {\$}P{\$} is a subset of {\$}P{\$} consisting of three non-collinear points such that each of the three lines determined by the three points contains at most {\$}c{\$} points of {\$}P{\$}. We prove that there exists a constant {\$}c>0{\$} such that {\$}P{\$} contains a {\$}c{\$}-ordinary triangle, provided that {\$}P{\$} is not contained in the union of two lines. Furthermore, the number of {\$}c{\$}-ordinary triangles in {\$}P{\$} is {\$}\Omega(|P|){\$}.}, url = {http://arxiv.org/abs/1701.08183v1 http://arxiv.org/pdf/1701.08183v1}, author = {Radoslav Fulek and Hossein Nassajian Mojarrad and M{\'{a}}rton Nasz{\'{o}}di and J{\'{o}}zsef Solymosi and Sebastian U. Stich and May Szedl{\'{a}}k}, comment = {}, urldate = {2017-02-06}, archivePrefix = {arXiv}, eprint = {1701.08183}, primaryClass = {math.CO}, year = 2017, collections = {Geometry,Fun maths facts} }