Three-colouring arrangements of great circles

Prove that the crossing points of every simple arrangement of great circles can be 3-coloured, or find one that needs four colours.

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1	# Three-colouring arrangements of great circles
2
3	Draw some great circles on a sphere, no three through a point, and colour the crossing points so that neighbours along a circle get different colours. Four colours always suffice; the conjecture is that three do, and it has been checked by computer for up to 11 circles.
4
5	## Problem
6
7	Let $C_1,\dots,C_n$ be great circles on the unit sphere $S^2$ with no three through a common point (a simple arrangement). Any two great circles cross in two antipodal points. The arrangement graph $G$ has these $n(n-1)$ crossing points as vertices, and an edge for each arc of a circle that joins two consecutive crossing points. $G$ is planar and 4-regular; for $n=3$ it is the octahedron.
8
9	Conjecture (Felsner, Hurtado, Noy and Streinu). For every simple arrangement of great circles, the arrangement graph has chromatic number at most 3.
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11	An equivalent form, due to Stan Wagon: the faces of every zonohedron, viewed as a planar map, can be coloured with three colours so that faces sharing an edge differ.
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13	## Background
14
15	Felsner, Hurtado, Noy and Streinu stated the conjecture in 2000 while studying connectivity, Hamiltonicity and colourings of arrangement graphs [1]; Wagon raised the zonohedron form at the CCCG 2002 open-problem session [2], building on his work with Sibley on 3-colouring rhombic Penrose tilings [4]. By the four colour theorem, or Brooks' theorem, every arrangement graph is 4-colourable, so the question is whether the great-circle condition rules out the fourth colour.
16
17	## What is known
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19	- Four colours can be needed nearby. Non-simple great-circle arrangements can require four colours, and so can simple arrangements of ordinary circles; Koester (1985) gave one with 7 circles [3].
20	- Small cases. Aichholzer, Aurenhammer and Krasser verified the conjecture for all arrangements of up to 11 great circles. Chiu, Felsner, Scheucher, Schröder, Steiner and Vogtenhuber re-generated these data and extended the check to all arrangements of up to 11 great-pseudocircles (combinatorial versions that need not be realisable by actual circles) [3].
21	- A special class. The same authors proved 3-colourability for $\triangle$-saturated pseudocircle arrangements, in which every edge lies on exactly one triangular face, and showed that arrangements of $n$ pairwise intersecting pseudocircles have fractional chromatic number at most $3+O(1/n)$ [3].
22	- Stronger conjectures. Based on exhaustive experiments, they proposed strengthenings, including that every simple arrangement of $n\ge 7$ great-pseudocircles has an antipodal 3-colouring, in which antipodal points share a colour [3].
23
24	## What would count as progress
25
26	- A counterexample: great circles given by exact rational normal vectors whose arrangement graph is not 3-colourable. A SAT solver confirms non-3-colourability of a graph of this size quickly.
27	- An exhaustive check for 12 or more great (pseudo)circles, with the enumeration code and the colourings published.
28	- A proof for a new infinite class of arrangements, or for all of them.
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30	## References
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32	1. S. Felsner, F. Hurtado, M. Noy and I. Streinu, Hamiltonicity and colorings of arrangement graphs, Discrete Appl. Math. 154 (2006), 2470–2483; extended abstract in SODA 2000. [doi:10.1016/j.dam.2006.04.006](https://doi.org/10.1016/j.dam.2006.04.006)
33	2. S. Wagon, A machine resolution of a four-color hoax, Proc. 14th Canadian Conf. Comput. Geom. (2002), 174–185. [cccg.ca](https://cccg.ca/proceedings/2002/stan.ps)
34	3. M.-K. Chiu, S. Felsner, M. Scheucher, F. Schröder, R. Steiner and B. Vogtenhuber, Coloring circle arrangements: new 4-chromatic planar graphs, European J. Combin. 121 (2024), 103839. [doi:10.1016/j.ejc.2023.103839](https://doi.org/10.1016/j.ejc.2023.103839), [arXiv:2205.08181](https://arxiv.org/abs/2205.08181)
35	4. T. Sibley and S. Wagon, Rhombic Penrose tilings can be 3-colored, Amer. Math. Monthly 107 (2000), 251–253. [doi:10.1080/00029890.2000.12005187](https://doi.org/10.1080/00029890.2000.12005187)
36
37	## Source
38
39	Imported from [The Open Problems Project, Problem 44](https://topp.openproblem.net/p44), curated by Erik D. Demaine, Joseph S. B. Mitchell and Joseph O'Rourke. Open according to the source as of 2026-10-20. The statement above is written in our own words; see the source for the original wording, history and discussion.