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pi, in mathematics, the ratio of the circumference of a circle to its diameter. The symbol for pi is  . The ratio is the same for all circles and is approximately 3.1416. It is of great importance in mathematics not only in the measurement of the circle but also in more advanced mathematics in connection with such topics as continued fractions, logarithms of imaginary numbers, and periodic functions. Throughout the ages progressively more accurate values have been found for ; an early value was the Greek approximation 3 1/7 , found by considering the circle as the limit of a series of regular polygons with an increasing number of sides inscribed in the circle. About the mid-19th cent. its value was figured to 707 decimal places and by the mid-20th cent. an electronic computer had calculated it to 100,000 digits. It would have taken a person working without error eight hours a day on a desk calculator 30,000 years to make this calculation; it took the computer eight hours. Although it has now been calculated to more than 200,000,000,000 digits, the exact value of cannot be computed. It was shown by the German mathematician Johann Lambert in 1770 that is irrational and by Ferdinand Lindemann in 1882 that is transcendental; i.e., cannot be the root of any algebraic equation with rational coefficients. The important connection between and e, the base of natural logarithms, was found by Leonhard Euler in the famous formula ei=-1, where i=-1.
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