Excerpts from Books and Wikipedia
"Like Michael and his Lord Frederick, Leonardo was the product of an increasingly worldly Europe. His father was a Pisan merchant in the city-state's North African enclave, in modern-day Algeria, and he sent his young son to learn the latest arithmetic and accounting methods including the basis for the Italian double entry bookkeeping, from the local Muslim traders. Leonardo later traveled to Sicily, Egypt, southern France, and Constantinople, before returning to his native Italy. There, in 1202, he completed The Book of Calculation--the first comprehensive work in Christian Europe on algebra and geometry. It also provided the most detailed Latin account to date of working with the Arabic number system first spelled out by al-Khwarizmi: 'Here Begins the First Chapter,' writes Leonardo, 'The nine Indian figures are: 987654321. With these nine figures, and with the sign zero which the Arabs call zephyr (al-sifr), any number whatsoever is written.'" [Lyons: House of Wisdom, p. 170]