From NPR's Math Guy, the story of Leonardo of Pisa, the medieval mathematician who introduced Arabic numbers to the West and helped launch the modern era. In 1202, a young Italian published one of the most influential books of all time, introducing modern arithmetic to Western Europe. Leonardo of Pisa (better known today as Fibonacci) had learned the Hindu-Arabic number system when as a teenager he traveled with his father, a customs official for Pisa, to North Africa, then one of the principal mercantile centers of Europe. Devised in India in the seventh and eighth centuries and brought to North Africa by Muslim traders, the Hindu-Arabic system (featuring the numerals 0 through 9) offered a much simpler method of calculation than the then-popular finger reckoning and cumbersome Roman numerals. Though written in scholarly Latin, Fibonacci's book Liber Abbaci (The Book of Calculation) was the first to recognize the power of the 10 numerals, and to aim them at the world of commerce. It spawned generations of popular math texts in colloquial Italian and other languages that made it possible for ordinary people to buy and sell goods, convert currencies, and keep accurate records more readily than ever before—helping transform the West into the dominant force in science, technology, and large-scale international commerce. Liber Abbaci and Fibonacci's other books made him the greatest mathematician of the Middle Ages. Yet despite the ubiquity of his discoveries, Leonardo of Pisa has largely slipped from the pages of history. He is best known today for discovering the "Fibonacci sequence" of numbers that appears with great regularity in biological structures throughout nature, and is used by some to predict the rise and fall of financial markets. Keith Devlin re-creates the life and enduring legacy of an overlooked genius, and in the process makes clear how central numbers and mathematics are to our daily lives. - Publisher.