Liber quadratorum
From Tardis Wiki, the free Doctor Who reference
The Liber quadratorum was a mathematical text written in the 13th century by Fibonacci. It enabled the solving of diophantine equations of the second degree.
The Eighth Doctor stopped the Sulumian Thorgan from killing Fibonacci before the Italian had written the Liber quadratorum. (PROSE: The Gallifrey Chronicles [+]Loading...["The Gallifrey Chronicles (novel)"])