# Difference between revisions of "Hadwiger-Nelson problem"

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== Further questions == | == Further questions == | ||

− | * What are the independence ratios of the above unit distance graphs? | + | * What are the [http://mathworld.wolfram.com/IndependenceRatio.html independence ratios] of the above unit distance graphs? |

− | * What are the fractional chromatic numbers of these graphs? | + | * What are the [https://en.wikipedia.org/wiki/Fractional_coloring fractional chromatic numbers] of these graphs? |

− | * What are the Lovasz numbers of these graphs? | + | * What are the [https://en.wikipedia.org/wiki/Lov%C3%A1sz_number Lovasz numbers] of these graphs? |

== Blog posts and other online forums == | == Blog posts and other online forums == |

## Revision as of 11:23, 14 April 2018

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 [math]4 \leq CNP \leq 7[/math] are classical; recently [deG2018] it was shown that [math]CNP \geq 5[/math]. This is achieved by explicitly locating finite unit distance graphs with chromatic number at least 5.

The **Polymath16** project seeks to simplify the graphs used in [deG2018] to establish this lower bound.

## Contents

## Polymath threads

- Polymath proposal: finding simpler unit distance graphs of chromatic number 5, Aubrey de Grey, Apr 10 2018. (
**Active discussion thread**) - Polymath16, first thread: Simplifying de Grey’s graph, Dustin Mixon, Apr 14, 2018. (
**Active research thread**)

## Notable unit distance graphs

A **unit distance graph** is a graph that can be realised as a collection of vertices in the plane, with two vertices connected by an edge if they are precisely a unit distance apart. The chromatic number of any such graph is a lower bound for [math]CNP[/math]; in particular, if one can find a unit distance graph with no 4-colorings, then [math]CNP \geq 5[/math].

Name | Number of vertices | Number of edges | Structure | Colorings | |
---|---|---|---|---|---|

Moser spindle | 7 | 11 | Two 60-120-60-120 rhombi with a common vertex, with one pair of sharp vertices coincident and the other joined | 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 | 72 | Contains 13 copies of H | Has essentially six 4-colorings in which no H has a monochromatic triangle | |

K | 61 | 150 | Contains 2 copies of J | In all 4-colorings lacking an H with a monochromatic triangle, all pairs of vertices at distance 4 are monochromatic | |

L | 121 | 301 | 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; three vertices form an equilateral triangle | ||

U | 15 | 33 | Three copies of T at 120-degree rotations | ||

V | 31 | 30 | Unit vectors at angles consistent with three interlocking Moser spindles | ||

W | 301 | 1230 | Cartesian product of V with itself, minus vertices at more than [math]\sqrt{3}[/math] from the centre | ||

M | 1345 | 8268 | Cartesian product of W and H | All 4-colorings have a monochromatic triangle in the central copy of H | |

N | 20425 | 151311 | Contains 52 copies of M arranged around the H-copies of L | Not 4-colorable | |

[math]G_0[/math] | 1585 | N "shrunk" by stepwise deletions and replacements of vertices | Not 4-colorable | ||

G | 1581 | 7877 | Deleting 4 vertices from [math]G_0[/math] | Not 4-colorable | |

G' | 1577 | Deleting 8 vertices from [math]G_0[/math] | Not 4-colorable | ||

L' | 97 | Has 40 copies of H | All 4-colorings contain an H with a monochromatic triangle |

## Further questions

- What are the independence ratios of the above unit distance graphs?
- What are the fractional chromatic numbers of these graphs?
- What are the Lovasz numbers of these graphs?

## Blog posts and other online forums

- Has there been a computer search for a 5-chromatic unit distance graph?, Juno, Apr 16, 2016.
- The chromatic number of the plane is at least 5, Jordan Ellenberg, Apr 9 2018.
- Aubrey de Grey: The chromatic number of the plane is at least 5, Gil Kalai, Apr 10 2018.
- The chromatic number of the plane is at least 5, Dustin Mixon, Apr 10, 2018.
- Amazing progress on long-standing problems, Scott Aaronson, Apr 11 2018.
- https://dustingmixon.wordpress.com/2018/04/13/the-chromatic-number-of-the-plane-is-at-least-5-part-ii/, Dustin Mixon, Apr 13 2018.

## Code and data

This dropbox folder will contain most of the data and images for the project.

Some specific files:

- The 1585-vertex graph in DIMACS format
- A naive translation of 4-colorability of this graph into a SAT problem in DIMACS format
- The vertices of this graph in explicit Sage notation

## Wikipedia

## Bibliography

- [deBE1951] N. G. de Bruijn, P. Erdős, A colour problem for infinite graphs and a problem in the theory of relations, Nederl. Akad. Wetensch. Proc. Ser. A, 54 (1951): 371–373, MR 0046630.

- [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.