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arXiv · 2003.04450

The number of triangles is more when they have no common vertex

Chuanqi Xiao·Gyula O. H. Katona
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Abstract

By the theorem of Mantel $[5]$ it is known that a graph with $n$ vertices and $\lfloor \frac{n^{2}}{4} \rfloor+1$ edges must contain a triangle. A theorem of Erdős gives a strengthening: there are not only one, but at least $\lfloor\frac{n}{2}\rfloor$ triangles. We give a further improvement: if there is no vertex contained by all triangles then there are at least $n-2$ of them. There are some natural generalizations when $(a)$ complete graphs are considered (rather than triangles), $(b)$ the graph has $t$ extra edges (not only one) or $(c)$ it is supposed that there are no $s$ vertices such that every triangle contains one of them. We were not able to prove these generalizations, they are posed as conjectures.

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Chuanqi Xiao, Gyula O. H. Katona. 2020-03-09. The number of triangles is more when they have no common vertex. https://arxiv.org/abs/2003.04450

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