10-23-1852

De Morgan conveyed Four Color Problem to Hamilton, writing,

"A student of mine asked me today to give him a reason for a fact which I did not know was a fact—and do not yet. He says that if a figure be anyhow divided and the compartments differently coloured so that the figures with any portion of common boundary line are differently coloured—four colours may be wanted, but not more… Query cannot a necessity for five or more be invented."

More information about: 

The Four Color Problem
Augustus de Morgan
William Rowan Hamilton

Source: Combinatorics: Ancient and Modern, by Robin Wilson & John J. Watkins (Oxford, 2013)