# Fibonacci, Leonardo[fibo**nah**chee], also known as **Leonardo Pisano**(c.1170--c.1250) Mathematician, born in Pisa, Italy. Arguably the most outstanding mathematician of the Middle Ages, he popularized the modern decimal system of numerals, and in his greatest work, the *Liber quadratorum* (1225, The Book of Square Numbers), made an advanced contribution to number theory. He also discovered the *Fibonacci sequence* of integers in which each number is equal to the sum of the preceding two (0,1,1,2,3,5,8...). |