Leonardo Fibbonaci, observant mathematician of Pisa
Leonardo Pisano, often called Fibonnaci (son of Bonacci) was born in Pisa in 1150 CE. He travelled through North Africa in childhood with his diplomat father before returning to Pisa to write math books including Practica Geometriae and Liber Quadratorum. Another book, the Liber abaci, introduced what came to be known as the Fibonnaci sequence. Beginning 1,1,2,3,5,8, this sequence predicts the numbers of seeds on a sunflower head as well as numbers of rabbits descending from a pair. It is also foundational work predating Pascal's Triangle.
This sequence in important enough to be the subject of a modern journal called the Fibonnaci Quarterly, which delves into the mathematics of Fibonnaci numbers.