Leonardo of Pisa or Leonardo Pisano (c. 1175 - 1250), also known as Fibonacci, was an Italian mathematician and is best known for the invention of the Fibonacci numbers and his role in the introduction to Europe of the modern positional decimal system for writing and manipulating numbers (algorism).
Leonardo's father Guilielmo (William) was nicknamed Bonacci ('good natured' or 'simple'). Leonardo was posthumously given the nickname Fibonacci (for filius Bonacci, son of Bonacci). William directed a trading post (by some accounts he was the consul for Pisa) in Bugia, North Africa (now Bejaia, Algeria), and as a young boy Leonardo traveled there to help him. There he learnt from the Arabic numeral system.
Perceiving the superiority of these Arabic numerals, Fibonacci travelled throughout the Mediterranean world to study under the leading Arab mathematicians of the time, returning around 1200. In 1202, at age 27, he published what he had learned in Liber Abaci, or Book of Calculation. This book showed the practical importance of the new number system by applying it to commercial bookkeeping, conversion of weights and measures, the calculation of interests, money-changing, and numerous other applications. The book was well received throughout educated Europe and had a profound impact on European thought, although the use of decimal numerals did not become widespread until the invention of printing almost three centuries later. (See, for example, the 1482 Ptolemaeus map of the world (http://bell.lib.umn.edu/map/PTO/TOUR/1482u.html) printed by Lienhart Holle in Ulm.)
Leonardo became a guest of the Emperor Frederick II, who enjoyed mathematics and science. In 1240 the Republic of Pisa honoured Leonardo, under his alternative name of Leonardo Bigollo, by granting him a salary.
Liber Abaci (1202), a book on calculations.
Practica Geometriae (1220), a compendium on geometry and trigonometry.
Flos (1225), solutions to problems posed by Johannes of Palermo
Liber quadratorum, (The Book of Squares) on Diophantine problems, that is, problems involving Diophantine equations.
Di minor guisa (on commercial arithmetic; lost)
Commentary on Book X of Euclid's Elements (lost)