To introduce the problem in the title, we begin with a quotation from the Foreword, written by Branko Grünbaum, to the book by Alexander Soifer (2009): *The Mathematical Coloring Book: Mathematics of Coloring and the Colorful Life of its Creators*:

*If each point of the plane is to be given a color, how many colors do we need if every two points at unit distance are to receive distinct colors?*

About 70 years ago it was shown that the least number of colours needed for such a colouring is one of 4, 5, 6 and 7. But which of these is the correct number? Despite efforts by many very clever people, some of whom had solved problems that appeared to be much harder, no advance has been made to narrow the gap

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