Improved Bounds for Covering Paths and Trees in the Plane

A covering path for a planar point set is a path drawn in the plane with straight-line edges such that every point lies at a vertex or on an edge of the path. A covering tree is defined analogously. Let $\pi(n)$ be the minimum number such that every set of $n$ points in the plane can be covered by a noncrossing path with at most $\pi(n)$ edges. Let $\tau(n)$ be the analogous number for noncrossing covering trees. Dumitrescu, Gerbner, Keszegh, and T\'oth (Discrete & Computational Geometry, 2014) established the following inequalities: $$\frac{5n}{9} - O(1) < \pi(n) < \left(1-\frac{1}{601080391}\right)n, \quad\text{and} \quad\frac{9n}{17} - O(1) < \tau(n)\leqslant \left\lfloor\frac{5n}{6}\right\rfloor.$$ We report the following improved upper bounds: $$\pi(n)\leqslant \left(1-\frac{1}{22}\right)n, \quad\text{and}\quad \tau(n)\leqslant \left\lceil\frac{4n}{5}\right\rceil.$$ In the same context we study rainbow polygons. For a set of colored points in the plane, a perfect rainbow polygon is a simple polygon that contains exactly one point of each color in its interior or on its boundary. Let $\rho(k)$ be the minimum number such that every $k$-colored point set in the plane admits a perfect rainbow polygon of size $\rho(k)$. Flores-Pe\~naloza, Kano, Mart\'inez-Sandoval, Orden, Tejel, T\'oth, Urrutia, and Vogtenhuber (Discrete Mathematics, 2021) proved that $20k/19 - O(1) <\rho(k) < 10k/7 + O(1).$ We report the improved upper bound $\rho(k)< 7k/5 + O(1)$. To obtain the improved bounds we present simple $O(n\log n)$-time algorithms that achieve paths, trees, and polygons with our desired number of edges.