# Fibonacci Day: Why are people celebrating November 23, who is Fibonacci & and what are Fibonacci numbers?

Mathematicians and fans of unusual natural occurrences can celebrate 11/23 as Fibonacci Day, based on the first four numbers of his historic sequence

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November 23 might not sound like a day to celebrate, but for mathematicians around the world today marks a day of celebration for Italian mathematician Fibonacci. Known for creating the Fibonacci Sequence, 11/23 just so happens to be the first four numbers in the famous equation - an equation that scientists and mathematicians rank up with Pi as an important value in both fields.

Also known as Leonardo Bonacci or Leonardo Bigollo Pisano, the Italian has been heralded as “the most talented Western mathematician of the Middle Ages” due to both popularising the use of the Indo–Arabic numeral system, which saw the use of the number zero and the decimal point across Western civilization, the Fibonacci Sequence. The sequence itself has gone on to have a mythos behind it over the centuries due to its approximation to the golden spiral.

Fibonacci created Fibonacci numbers in his 1202 book, Liber Abaci (The Book of Calculations), to calculate the problem of the then ever growing rabbit population. His hypothesis was that for every one rabbit born, they would surely find another rabbit. Those two rabbits would then give birth creating three rabbits, which would then lead to the birth of five rabbits and so forth.

The equation became 1+1=2, therefore 1+2=3, 2+3=5 and so forth; by adding the answers to the two previous equations, the Fibonacci sequence was born. Or, to use rabbits once again:

• At the end of the first month, they mate, but there is still only 1 pair.
• At the end of the second month they produce a new pair, so there are 2 pairs in the field.
• At the end of the third month, the original pair produces a second pair, but the second pair only mate to gestate for a month, so there are 3 pairs in all.
• At the end of the fourth month, the original pair has produced yet another new pair, and the pair born two months ago also produces their first pair, making 5 pairs.

Its importance in maths comes from its unexpected appearances in the subject, especially in the field of research in space and nature. Fibonacci numbers have been used in computer algorithms and more peculiarly creep up in biological settings: branching in trees, the arrangement of leaves on a stem, the fruit sprouts of a pineapple and the flowering of an artichoke are some of the natural occurrences that demonstrate the sequence.

In popular culture, Fibonacci numbers have become as mythicized as the prediction of Nostradamus in terms of its cult appeal. In Dan Brown’s popular novel The Da Vinci Code (2006), the numbers are used to unlock a safe while progressive metal band Tool used the sequence in both the syllables used during the verses and the time signature changes in their 2002 single Lateralus.

In architecture, the Fibonacci numbers appear on buildings such as the Mole Antonelliana in Turin, the Lindenbrauerei in Unna and the Zürich Hauptbahnhof.

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