23 November: Fibonacci Day

Fibonacci Day. You've probably heard of the Fibonacci number sequence named after the mathematician Fibonacci. Here are 10 things you may not know about the man and his numbers.

1. He was born in around 1170 in Pisa, Italy, and his father was a wealthy and well travelled merchant and possibly customs officer called Guglielmo Bonacci. His given name was Leonardo.
2. Fibonacci was just one of many names he was known by. His father's name, Bonacci, means "good natured one", hence Leonardo became known as Fibonacci, a contraction of filius Bonacci, which means "son of the good-natured one." He was also referred to as Leonardo of Pisa, Leonardo Pisano, Leonardo Bigollo (traveller), Leonardo Bonacci, and Leonardo Fibonacci.
3. As a boy, Fibonacci travelled with his father. One port of call they made was Bugia, in present day Algeria, where Fibonacci first learned about the Hindu–Arabic numeral system (ie the numbers we use today as opposed to the Roman numeral system in use in Italy at the time).
4. He wrote books about mathematics using Hindu–Arabic numerals, and helped popularise them for use in trade and accounting. His most famous book is Liber Abaci (Book of Abacus or Book of Calculation). The first edition in 1202 showed the practical use of the new Arabic numeral system in commercial bookkeeping, converting weights and measures, calculation of interest, money-changing, and other applications. There are no known copies of this book in existence.
5. He wrote a second edition in around 1228 which introduced the Arabic numeral system and compared it with other systems, such as Roman numerals, and showed how to convert other numeral systems into Arabic numerals. He also wrote The Book of Squares, (Liber Quadratorum), a book on algebra. There are only copies of about four of his books in existence, even though it's thought he wrote many more. They didn't have printing in those days so books had to be written and copied out by hand, so even in Fibonacci's time there wouldn't have been many copies.
6. He didn't discover the number system named after him - the sequence had been noted by Indian mathematicians as early as the sixth century. Fibonacci merely wrote about it and brought it to the attention of people in Europe. The Fibonacci sequence only became really famous during the late nineteenth century when the mathematician Edouard Lucas gave it its present name and wrote about its properties in his book Theorie des nombres.
7. The first illustration of the number sequence in Fibonacci's book involved the growth of a population of rabbits.
8. The sequence has so many properties that there is even a mathematical journal dedicated to it - The Fibonacci Quarterly.
9. Very little is known about Fibonacci's personal life - whether he had a wife, or children, or how he died in the end. We do know that he provided accounting advice and taught mathematics in Pisa, because the Republic of Pisa issued a decree granting him a salary.
10. He also wrote about prime numbers and observed that, in order to test whether a number is a prime, all you need to do is divide it by the prime numbers up to the square root of the number you're interested in. This is the first known test for a prime number.