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Frederick II was known among contemporaries as Stupor Mundi, a wonder of the world: a genuinely enlightened ruler, he was said to speak as many as six languages, and his deep interest in studying and observing nature went beyond the borders of the Kingdom of Sicily. The court of Palermo, frequented by educated and distinguished men, was a vibrant intellectual center where he promoted scientific knowledge and the study of disciplines such as philosophy, astrology, mathematics, medicine, and the natural sciences. Leonardo Fibonacci, known as the Christian world's first algebraist and one of the greatest mathematicians and geometry experts of all time, stands out among the prominent figures that Frederick II had the honor of knowing and supporting. He is credited with the famous sequence (known as the "Fibonacci sequence") and the introduction to Europe of the Indo-Arabic numerals and the number zero, which he learned through his travels in the Middle East.
The two met in Pisa, where the emperor had occasion to attend certain mathematical contests. In 1225, Fibonacci dedicated a landmark treatise on algebra, the Liber quadratorum, to the emperor and, according to some scholars, may have contributed to the design of the most majestic of Frederick's fortresses, Castel del Monte, which features intriguing connections between mathematics and architecture in the historical context of medieval Italy.
