On Fibonacci Day, mathematicians and scientists honour one of their own: Leonardo Bonacci. More commonly known as Fibonacci, the medieval mathematician is notable for popularising the Hindu-Arabic numeral system, with its use of zero, decimal points and 10 numbers instead of the ponderous Roman numeral system. He"s most famous, however, for his sequence of numbers: 0, 1, 1, 2, 3, 5, 8... By adding the first two numbers, you arrive at the third, and adding the previous two numbers, you arrive at the next number, and so on. Disregarding the zero, the first four numbers correspond to today"s date, 11/23.