Leonardo Fibonacci. Portrait of the Italian mathematician Leonardo Fibonacci (1170?-1250?). Fibonacci introduced the Arabian" numeral system to Europe through his Book of the Abacus (1202). He also discovered the Fibonacci series, a sequence of numbers in which each number is the sum of the previous two: 1, 1, 2, 3, 5, 8, 13 and so on. This curious series is often found in nature, for example in the pattern of leaf growth."