|How many colors are needed to color the plane so that no two points at unit distance are the same color?|
In geometric graph theory, the Hadwiger–Nelson problem, named after Hugo Hadwiger and Edward Nelson, asks for the minimum number of colors required to color the plane such that no two points at distance 1 from each other have the same color. The answer is unknown, but has been narrowed down to one of the numbers 4, 5, 6 or 7. The correct value may actually depend on the choice of axioms for set theory.
The question can be phrased in graph theoretic terms as follows. Let G be the unit distance graph of the plane: an infinite graph with all points of the plane as vertices and with an edge between two vertices if and only if there is unit distance between the two points. Then the Hadwiger–Nelson problem is to find the chromatic number of G. As a consequence, the problem is often called "finding the chromatic number of the plane". By the de Bruijn–Erdős theorem, a result of de Bruijn & Erdős (1951), the problem is equivalent (under the assumption of the axiom of choice) to that of finding the largest possible chromatic number of a finite unit distance graph.
According to Jensen & Toft (1995), the problem was first formulated by E. Nelson in 1950, and first published by Gardner (1960). Hadwiger (1945) published a related result, showing that any cover of the plane by five congruent closed sets contains a unit distance in one of the sets, and he also mentioned the problem in a later paper (Hadwiger 1961). Soifer (2008) discusses the problem and its history extensively.
Lower and upper bounds
The fact that the chromatic number of the plane must be at least four follows from the existence of a seven-vertex unit distance graph, named the Moser spindle after its discovery in 1961 by the brothers William and Leo Moser, with chromatic number four. This graph consists of two unit equilateral triangles joined at a common vertex–x. Each of these triangles is joined along another edge to another equilateral triangle; the vertices y and z of these joined triangles are at unit distance from each other. If the plane could be three-colored, the coloring within the triangles would force y and z to both have the same color as x, but then, since y and z are at unit distance from each other, we would not have a proper coloring of the unit distance graph of the plane. Therefore, at least four colors are needed to color this graph and the plane containing it. An alternative lower bound in the form of a ten-vertex four-chromatic unit distance graph was discovered at around the same time by Solomon W. Golomb.
The upper bound of seven on the chromatic number follows from the existence of a tessellation of the plane by regular hexagons, with diameter slightly less than one, that can be assigned seven colors in a repeating pattern to form a 7-coloring of the plane; according to Soifer (2008), this upper bound was first observed by John R. Isbell.
Variations of the problem
The problem can easily be extended to higher dimensions. In particular, finding the chromatic number of space usually refers to the 3-dimensional version. As with the version on the plane, the answer is not known, but has been shown to be at least 6 and at most 15.
One can also consider colorings of the plane in which the sets of points of each color are restricted to sets of some particular type. Such restrictions may cause the required number of colors to increase, as they prevent certain colorings from being considered acceptable. For instance, if all color classes are required to be Lebesgue measurable, it is known that at least five colors are required. In the Solovay model of set theory, all point sets are measurable, so this result implies that in this model the chromatic number of the plane is at least five. If a coloring of plane consists of regions bounded by Jordan curves, then at least six colors are required.
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