# Configurations of Non-crossing Rays and Related Problems

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## Abstract

Let S be a set of n points in the plane and let R be a set of n pairwise non-crossing rays, each with an apex at a different point of S. Two sets of non-crossing rays $$R_1$$ and $$R_2$$ are considered to be different if the cyclic permutations they induce at infinity are different. In this paper, we study the number r(S) of different configurations of non-crossing rays that can be obtained from a given point set S. We define the extremal values

\begin{aligned} \overline{r}(n) = \max _{|S|=n} r(S)\quad \text { and } \quad \underline{r}(n) = \min _{|S|=n} r(S), \end{aligned}

and we prove that $$\underline{r}(n) = \Omega ^* (2^n)$$, $$\underline{r}(n) = O^* (3.516^n)$$ and that $$\overline{r}(n) = \Theta ^* (4^n)$$. We also consider the number of different ways, $$r^\gamma (S)$$, in which a point set S can be connected to a simple curve $$\gamma$$ using a set of non-crossing straight-line segments. We define and study

\begin{aligned} \overline{r}^{\gamma }(n) = \max _{|S|=n} r^{\gamma }(S) \quad \text {and } \quad \underline{r}^{\gamma }(n) = \min _{|S|=n} r^{\gamma }(S), \end{aligned}

and we find these values for the following cases: When $$\gamma$$ is a line and the points of S are in one of the halfplanes defined by $$\gamma$$, then $$\underline{r}^\gamma (n) = \Theta ^* (2^n)$$ and $$\overline{r}^\gamma (n) = \Theta ^* (4^n)$$. When $$\gamma$$ is a convex curve enclosing S, then $$\overline{r}^\gamma (n) = O^* (16^n)$$. If all the points of S belong to a convex closed curve $$\gamma$$, then $$\underline{r}^{\gamma }(n) = \overline{r}^{\gamma }(n) = \Theta ^* (5^n)$$.

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## Acknowledgments

A. García and J. Tejel: Partially supported by Projects Gob. Arag. E58 (ESF), MICINN MTM2009-07242, MINECO MTM2012-30951 and ESF EUROCORES programme EuroGIGA, CRP ComPoSe: MICINN Project EUI-EURC-2011-4306. F. Hurtado: Partially supported by Projects Gen. Cat DGR2009SGR1040, MICINN MTM2009-07242, MINECO MTM2012-30951 and ESF EUROCORES programme EuroGIGA, CRP ComPoSe: MICINN Project EUI-EURC-2011-4306. J. Urrutia: Partially supported by Projects MTM2006-03909 (Spain) and SEP-CONACYT of Mexico, Project 80268.

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Correspondence to Jorge Urrutia.

Editor in Charge: János Pach

A preliminary version of this work was presented at the XII Spanish Meeting on Computational Geometry [13]. This full version improves on many of the results presented there.

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García, A., Hurtado, F., Tejel, J. et al. Configurations of Non-crossing Rays and Related Problems. Discrete Comput Geom 55, 522–549 (2016). https://doi.org/10.1007/s00454-016-9765-7

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• DOI: https://doi.org/10.1007/s00454-016-9765-7