Leonardo of Pisa, known as Fibonacci (circa 1170–after 1240), was an Italian mathematician who played a crucial role in introducing the Hindu-Arabic numeral system to Europe

. He learned this system during his youth in Bugia (modern-day Bejaia, Algeria), where his father managed a trading post and where Fibonacci encountered Arab mathematics

. Fibonacci traveled extensively around the Mediterranean, studying various mathematical methods used by merchants and scholars, and recognizing the superiority of the Hindu-Arabic place-value system over Roman numerals for calculations

. In 1202, he published his landmark work Liber Abaci ("The Book of Calculation"), which popularized this numeral system in Europe and demonstrated practical arithmetic methods for commerce, such as profit calculation, currency conversion, and interest

. Besides this, Fibonacci is famous for the Fibonacci sequence, a series of numbers where each number is the sum of the two preceding ones (starting with 0 and 1). He introduced this sequence in Liber Abaci as part of a problem about rabbit population growth. The sequence has since been found to have numerous applications in mathematics, nature, art, and computer science

. Fibonacci also made significant contributions to number theory and algebra through other works such as Practica geometriae (1220), Liber quadratorum (1225), and Flos (circa 1225). These works dealt with geometry, square numbers, indeterminate equations, and algebraic solutions, laying important groundwork for later mathematical developments

. In summary, Fibonacci's main achievements include:

• Introducing and popularizing the Hindu-Arabic numeral system in Europe, which revolutionized arithmetic and commerce

• Describing the Fibonacci sequence, which has wide-ranging mathematical and natural applications

• Advancing number theory and algebra through his scholarly works

His influence was recognized in his lifetime, including a pension granted by the Republic of Pisa for his services, and he is considered one of the most important mathematicians of the Middle Ages