On the 16th October 1843 vaery clever mathamatician was walking with his wife allong the Royal Canal around Cabra. In a flash.an idea came to him.
Not having a note pad he scratched in on to tbe brickwork of the bridge
i2 = j2=k2=ijk=-1
It has to do with vectors and the working out of how high you would have to aim a rocket and at what speed to hit an exact spot a distance away.
It has many many other applications including computer animation
Here is what they say about him in Wickipedia
Today on the 16 a walk takes place to the spot where he scratched his formula