July 11, 2020

Fibonacci and His Rabbits

1. A Consequential Countdown

Fibonacci and His Rabbits

Leonardo of Pisa, who lived from about 1170 to about 1240, was perhaps the greatest European mathematician of the Middle Ages. He was also called Fibonacci, which means "son of Bonaccio." Although he was born in Pisa, Italy, Fibonacci received much of his early mathematical training from Muslim tutors in Algeria, where his father was an official in an Italian trading business.

Fibonacci's teachers introduced him to the Hindu-Arabic numbering system we use today. He recognized the system's many advantages over the clumsy Roman numerals used in Italy at the time. (Try multiplying XLVIII by CCXI!)

In a math handbook he wrote for merchants, Fibonacci explained the advantages of the Hindu-Arabic notation. Titled Liber abaci (Book of the Abacus), the book helped introduce the decimal numbering system to Europe.

The number sequence named after Fibonacci comes from an arithmetic problem in this book.


USDA ARS

Suppose a pair of rabbits is placed in an enclosure to breed. After two months they breed one female and one male rabbit and another pair each month afterward. Each new pair of rabbits also breeds a pair of rabbits starting after two months.

So after four months there are three pairs of rabbits and after five months there are five pairs of rabbits. If you keep on counting the total number of rabbit pairs each month, you get the numbers of the Fibonacci sequence.


In Fibonacci's rabbit problem, a pair of newborn rabbits waits two months before producing offspring, so only one pair of rabbits is present for the first and second months (first two columns). It produces offspring in the third month, so now a total of two pairs of rabbits are present (third column). In the fourth month, the original pair produces another pair, but the other pair isn't ready to produce offspring yet, giving a total of three pairs of rabbits (fourth column).


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