Pi History - Euler & Pi
The Illustrated History of Pi
 
Euler & Pi - Pi not only gets a calculation method, but a name
   
 

Leonhard Euler, a brilliant mathematician and number theorist, not only formulated key relationships of π, but popularized the use of the symbol itself.

Giving Pi not just a value, but a name:   Seventeeth century English mathematicians William Oughtred, Isaac Barrow, and David Gregory had all used the Greek letter π as a symbol for the the circumference, or periphery, of a circle. In 1706 William Jones, a Welsh mathematician, gave π its first modern usage - symbolizing the ratio of a circle’s circumference to its diameter. But it was when Euler, with his considerable prestige, adopted this symbol, circa 1736-7, that π came into common usage, as we know it.
   
  Euler’s simple elegant ‘recipe’ for π utilized a harmonic series, where the terms are wavelengths of successive harmonics of a vibrating string. Relationships to Bernoulli numbers and even Riemann’s zeta function can be found in it.
 
   
  Euler’s two angle arctan formula built on the work of John Machin:
 
  Euler’s equation referred to as “Euler’s Identity” not only incorporated π but related it to the chief symbols in mathematical history up to that time:  the principal whole numbers 0 and 1, the chief mathematical relations + and =, the symbol i for the "impossible" square root of minus one, and the logarithm base e.
   
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