Crossing and intersecting families of geometric graphs on point sets
| Autor: | Álvarez-Rebollar, José Luis, Cravioto-Lagos, Jorge, Marín, Nestaly, Solé-Pi, Oriol, Urrutia, Jorge |
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| Rok vydání: | 2022 |
| Předmět: | |
| Druh dokumentu: | Working Paper |
| Popis: | Let $S$ be a set of $n$ points in the plane in general position. Two line segments connecting pairs of points of $S$ cross if they have an interior point in common. Two vertex disjoint geometric graphs with vertices in $S$ cross if there are two edges, one from each graph, which cross. A set of vertex disjoint geometric graphs with vertices in $S$ is called mutually crossing if any two of them cross. We show that there exists a constant $c$ such that from any family of $n$ mutually crossing triangles, one can always obtain a family of at least $n^c$ mutually crossing $2$-paths (each of which is the result of deleting an edge from one of the triangles) and then provide an example that implies that $c$ cannot be taken to be larger than $2/3$. For every $n$ we determine the maximum number of crossings that a Hamiltonian cycle on a set of $n$ points might have. Next, we construct a point set whose longest perfect matching contains no crossings. We also consider edges consisting of a horizontal and a vertical line segment joining pairs of points of $S$, which we call elbows, and prove that in any point set $S$ there exists a family of $\lfloor n/4 \rfloor$ vertex disjoint mutually crossing elbows. Additionally, we show a point set that admits no more than $n/3$ mutually crossing elbows. Finally we study intersecting families of graphs, which are not necessarily vertex disjoint. A set of edge disjoint graphs with vertices in $S$ is called an intersecting family if for any two graphs in the set we can choose an edge in each of them such that they cross. We prove a conjecture by Lara and Rubio-Montiel, namely, that any set $S$ of $n$ points in general position admits a family of intersecting triangles with a quadratic number of elements. Some other results are obtained throughout this work. Comment: 19 pages, 14 figures |
| Databáze: | arXiv |
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