# Difference between revisions of "Hadwiger-Nelson problem"

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The Chromatic Number of the Plane (CNP) is the chromatic number of the graph whose vertices are elements of the plane, and two points are connected by an edge if they are a unit distance apart. The Hadwiger-Nelson problem asks to compute CNP. The bounds $4 \leq CNP \leq 7$ are classical; recently [deG2018] it was shown that $CNP \geq 5$. This is achieved by explicitly locating finite unit distance graphs with chromatic number at least 5.

## Notable unit distance graphs

Name Number of vertices Number of edges Structure Colorings
Moser spindle 7 11 Two 60-120-60-120 rhombi with a common vertex, joined at the opposing vertices Not 3-colorable
H 7 12 Vertices and center of a hexagon Has essentially four 4-colorings, two of which contain a monochromatic triangle
J 31 Contains 13 copies of H Has essentially six 4-colorings in which no H has a monochromatic triangle
K 61 Contains 2 copies of J In all 4-colorings without an H with a monochromatic triangle, all linking edges are monochromatic
L 121 Contains two copies of K and 52 copies of H all 4-colorings contain an H with a monochromatic triangle
T 9 15 Contains one Moser spindle and useful symmetry
U 15 Contains three Moser spindles
V 31 Unit vectors at angles consistent with three interlocking Moser spindles
W 301 Cartesian product of V with itself, minus vertices at \sqrt(3) from the centre
M 1345 Cartesian product of W and H, all 4-colorings have a monochromatic triangle in the central copy of H
N 20425 Contains 52 copies of M arranged around the H-copies of L; not 4-colorable

## Bibliography

• [deG2018] A. de Grey, The chromatic number of the plane is at least 5, arXiv:1804.02385
• [H1945] H. Hadwiger, Uberdeckung des euklidischen Raum durch kongruente Mengen, Portugaliae Math. 4 (1945), 238–242.
• [MM1961] L. Moser and M. Moser, Solution to Problem 10, Can. Math. Bull. 4 (1961), 187–189.
• [P1998] D. Pritikin, All unit-distance graphs of order 6197 are 6-colorable, Journal of Combinatorial Theory, Series B 73.2 (1998): 159-163.
• [S2008] A. Soifer, The Mathematical Coloring Book, Springer, 2008, ISBN-13: 978-0387746401.