The number Pi ( /pai/ ) is the ratio of a circle’s circumference to its diameter and is, for millenniums, a fascination for mathematicians but also for other people.

Pi is the most known constant in mathematics. It is used to calculate the area of a circle and the volume of a sphere, but also in many other formulas. It is an irrational number, which means its value cannot be expressed exactly or using a fraction and it has infinitely many decimals.

** A famous number**

No other number managed to make the transition between the academic environment and the pop culture like the number pi did. It inspired books, clubs, and websites, which relate about the “apparition” of the symbol in clouds, but also about the decimals that appear in cornfields. In 1998, the movie Pi had as a principal theme the story of a paranoiac mathematician who became almost crazy trying to find a number that would describe all the patterns in the universe. Pi was also mentioned in the movie-series Matrix, in Star Trek and The Simpsons. The singer Kate Bush has on her album Aerial (2005) a song named Pi (lyrics). She sings more than 100 decimals of the number. But mathematicians verified the song and found out that, after the 50^{th} decimal, she starts to invent them. But the song is really nice and she has a sweet voice.

**Pi, in strange places:**

- Sport: on the 400m running track, the number is used to calculate the different places to start. This way the runners are running the same distance.
- Tradition: the students from the Technologic Institute from Boston use 3.14159 when cheering in their sportive competitions.
- Nature: the geologist Hans-Henrik Stolum, from the University of Chambridge, found that the length of a river from A to B, divided by the straight line from A to B is, often, equal to pi.

**History**

1900-1652 BC – The Egyptians and Babylonians estimated pi to 3.1605, respectively 3.125.

287 BC – The mathematician Arhimede from Saracuza was born. He found the relation between the circle’s circumference and the circle’s area.

429 – The Chinese mathematician Zu Chongzhi was born. He estimated pi to be 355/115, which remains the best approximation for 1000 years.

1579 – Francois Viete calculates the first 9 decimals of pi.

1610 – Ludolph van Ceulen, from Germany, calculates the first 35 decimals. Dies in the same year and has the pi value he calculated on his grave stone.

1706 – The symbol Õ (pi) is used for the first time by the mathematician William Jones to represent the ratio between the circle’s circumference and its diameter.

1761 – The mathematician Johann Heinrich Lambert proves, for the 1^{st} time, the pi is irrational.

1794 – Adrien-Marie Legendre proves that the square of pi is also irrational.

1937 – The ceiling of the 31 room from DecouvertePalace was decorated with the decimals of pi. The 528^{th} was wrong and it was corrected in 1949.

1988 – It is celebrated the first time the Pi Day, on 14 march, in San Francisco.

2005 – The Chinese student Chao Lu recites the first 67,890 decimals of pi from memory, establishing a new world record.

2010 – Shigeru Kondo, system engineer in Japan, and Alexander Yee, computer science student in SUA, needed 90 days to write the first 5,000 billion decimals of pi on a computer. ( in the first million decimals of pi, the number 5 appears more often: 100,359 times)

2009 – SUA Congress recognizes officially the National Day of Pi.

2012 – 12 march is the International Day of Pi and the birthday of Albert Einstein (1879).

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I love mathematics and I think your blog is fun 🙂 Annie

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