Broom Bridge, Dublin, Ireland

gkat_025.pdf53° 22 22.8 N, 6° 18 0 W


Mathematical Equations As Urban Grafitti

Mathematicians have a habit of thinking of things in the oddest of places: the Irish mathematician Sir William Rowan Hamilton came up with the theory of quaternions while out for a walk with his wife in 1843. Crossing the Broom Bridge in Dublin, Hamilton scratched the quaternion multiplication equation Equation 25-1) into the bridge’s stonework using a knife.


Equation 25-1. The equation of quaternion multiplication

Hamilton’s mathematical vandalism is no longer visible on the bridge, but the bridge itself is still standing and a plaque (see Figure 25-1) was erected by the Irish premier Eamon de Valera in honor of Hamilton. The bridge crosses the Royal Canal in a dubious area of Dublin; the best way to visit it is probably not to try to recreate Hamilton’s stroll, but instead to take a number 120 bus or a local Western Commuter line train to the Broombridge stop.


Figure 25-1. The plaque at Broom Bridge; courtesy of Robert Burke (

Once you have seen Broom Bridge, ...

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