Arthur Cayley FRS and the four-colour map problem

The four-colour map problem (to prove that on any map only four colours are needed to separate countries) is celebrated in mathematics. It resisted the attempts of able mathematicians for over a century and when it was successfully proved in 1976 the ‘computer proof’ was controversial: it did not allow scrutiny in the conventional way. At the height of his influence in 1878, Arthur Cayley had drawn attention to the problem at a meeting of the London Mathematical Society and it was duly ‘announced’ in print. He made a short contribution himself and he encouraged the young A. B. Kempe to publish a paper on the subject. Though ultimately unsuccessful, the work of Cayley and Kempe in the late 1870s brought valuable insights. Using previously unpublished historical sources, of letters and manuscripts, this article attempts to piece together Cayley’s contribution against the backcloth of his other deliberations. Francis Galton is revealed as the ‘go-between’ in suggesting Cayley publish his observations in Proceedings of the Royal Geographical Society. Of particular interest is that Cayley submitted two manuscripts prior to publication. A detailed comparison of these initial and final manuscripts in this article sheds new light on the early history of this great problem.