On 16 October 1843 the Irishman William Rowan Hamilton was walking with his wife along the Royal Canal in Dublin when he suddenly solved a problem that had tormented him for fifteen years. He took a penknife from his pocket and cut the formula into a stone of Brougham Bridge: $$i^2=j^2=k^2=ijk=-1.$$The carving wore away long ago, but there is now a plaque on the spot, and every year mathematicians walk to the bridge on 16 October. Rarely has a notation had such a well-documented birth.