Pi(number)
The ratio of the length of a circle to its diameter; it is an infinite non-periodic decimal number
![]() |
One frequently arrives at the number as the limit of certain arithmetic sequences involving simple laws. An example is Leibniz' series
![]() |
which, however, converges very slowly. There are more rapidly-converging series suitable for calculating .
The possibility of a pure analytic definition of is of essential significance for geometry. For example,
also participates in certain formulas in non-Euclidean geometry, but not as the ratio of the length of a circle to its diameter (this ratio is not constant in non-Euclidean geometry). The arithmetic nature of
was finally elucidated in analysis, with a decisive part played by Euler's formula:
![]() |
At the end of the 18th century, J. Lambert and A. Legendre established that is an irrational number, while in the 19th century, F. Lindemann showed that
is a transcendental number.
Comments
A nice account of Lindemann's proof can be found in [a3], Chapt. 6.
The number of known digits of has increased exponentially in recent times. At the moment (1990), the record seems to be half a billion digits (D.V. Chudnovsky and G.V. Chudnovsky). For an account of such computations see [a1]. Up to the 1960's the standard way to calculate
was to use Machin's formula
and the power series of
. Nowadays, some powerful formulas of Ramanujan are used. It is still not known how randomly the digits of
are distributed; in particular, whether
is a normal number.
References
[a1] | J.M. Borwein, P.B. Borwein, "Pi and the AGM" , Interscience (1987) |
[a2] | P. Beckmann, "A history of pi" , The Golem Press , Boulder (Co.) (1971) |
[a3] | I. Stewart, "Galois theory" , Chapman & Hall (1979) |
Pi(number). Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Pi(number)&oldid=31518