23rd October

On this day in 1852, Augustus De Morgan conveyed the Four Colour Problem to William 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.