Timothy M. Chan's Publications: Miscellaneous geometry

Conflict-free coloring of points with respect to rectangles and approximation algorithms for discrete independent set

In the conflict-free coloring problem, for a given range space, we want to bound the minimum value F(n) such that every set P of n points can be colored with F(n) colors with the property that every nonempty range contains a unique color. We prove a new upper bound O(n^{0.368}) with respect to orthogonal ranges in two dimensions (i.e., axis-parallel rectangles), which is the first improvement over the previous bound O(n^{0.382}) by Ajwani, Elbassioni, Govindarajan, and Ray [SPAA'07]. This result leads to an O(n^{1-0.632/2^{d-2}}) upper bound with respect to orthogonal ranges (boxes) in dimension d, and also an O(n^{1-0.632/(2^{d-3}-0.368)}) upper bound with respect to dominance ranges (orthants) in dimension d >= 4.

We also observe that combinatorial results on conflict-free coloring can be applied to the analysis of approximation algorithms for discrete versions of geometric independent set problems. Here, given a set P of (weighted) points and a set S of ranges, we want to select the largest(-weight) subset Q of P with the property that every range of S contains at most one point of Q. We obtain, for example, a randomized O(n^{0.368})-approximation algorithm for this problem with respect to orthogonal ranges in the plane.

Curves of width one and the river shore problem

(with Alexander Golynski,
Alejandro López-Ortiz, and Claude-Guy Quimper)

We consider the problem of finding the shortest curve in the plane that has unit width. This problem was first posed as the "river shore" puzzle by Ogilvy (1972) and is related to the area of on-line searching. Adhikari and Pitman (1989) proved that the optimal solution has length 2.2782... We present a simpler proof, which exploits the fact that the width of a polygon does not decrease under a certain convexification operation.

Tighter bounds on the genus of nonorthogonal polyhedra built from rectangles

Therese Biedl, Erik D. Demaine, Martin Demaine, Paul Nijjar, Ryuhei Uehara, and Ming-Wei Wang)

We prove that there is a polyhedron with genus 6 whose faces are orthogonal polygons (equivalently, rectangles) and yet the angles between some faces are not multiples of 90 degrees, so the polyhedron itself is not orthogonal. On the other hand, we prove that any such polyhedron must have genus at least 3. These results improve the bounds of Donoso and O'Rourke (2001) that there are nonorthogonal polyhedra with orthogonal faces and genus 7 or larger, and any such polyhedron must have genus at least 2. We also demonstrate nonoverlapping one-piece edge-unfoldings (nets) for the genus-7 and genus-6 polyhedra.

The complexity of a single face of a Minkowski sum

Sariel Har-Peled, Boris Aronov, Dan Halperin, and Jack Snoeyink)

This note considers the complexity of a free region in the configuration space of a polygonal robot translating amidst polygonal obstacles in the plane. Specifically, given polygonal sets P and Q with k and n vertices, respectively (k < n), the number of edges and vertices bounding a single face of the complement of the Minkowski sum P + Q is Theta(nk alpha(k)) in the worst case. The lower bound comes from a construction based on lower envelopes of line segments; the upper bound comes from a combinatorial bound on Davenport-Schinzel sequences that satisfy two alternation conditions.

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Timothy Chan (Last updated Aug 2023)