Leibniz formula for π
|Part of a series of articles on the|
|mathematical constant π|
- See Leibniz (disambiguation) for other formulas known under the same name.
Using summation notation:
The infinite series above is called the Leibniz series. It is also called the Gregory–Leibniz series, recognizing the work of James Gregory. The formula was originally discovered by Madhava of Sangamagrama in the 14th century, but was not widely known in the West. Since Leibniz, however, was the first person to rediscover the series in continental Europe, it is named after him. In retrospect, the naming is sometimes modified to Madhava–Leibniz series in order to recognize Madhava's contribution.
Considering only the integral in the last line, we have:
Therefore, as n → ∞ we are left with the Leibniz series:
Leibniz's formula converges slowly. Calculating π to 10 correct decimal places using direct summation of the series requires about 5,000,000,000 terms because for .
However, the Leibniz formula can be used to calculate π to high precision (hundreds of digits or more) using various convergence acceleration techniques. For example, the Shanks transformation, Euler transform or Van Wijngaarden transformation, which are general methods for alternating series, can be applied effectively to the partial sums of the Leibniz series. Further, combining terms pairwise gives the non-alternating series
which can be evaluated to high precision from a small number of terms using Richardson extrapolation or the Euler–Maclaurin formula. This series can also be transformed into an integral by means of the Abel–Plana formula and evaluated using techniques for numerical integration.
Unusual behavior 
If the series is truncated at the right time, the decimal expansion of the approximation will agree with that of π for many more digits, except for isolated digits or digit groups. For example, taking 5,000,000 terms yields
where N is an integer divisible by 4. If N is chosen to be a power of ten, each term in the right sum becomes a finite decimal fraction. The formula is a special case of the Boole summation formula for alternating series, providing yet another example of a convergence acceleration technique that can be applied to the Leibniz series. In 1992, Jonathan Borwein and Mark Limber used the first thousand Euler numbers to calculate π to 5,263 decimal places with Leibniz' formula.
See also 
- Jonathan Borwein, David Bailey & Roland Girgensohn, Experimentation in Mathematics - Computational Paths to Discovery, A K Peters 2003, ISBN 1-56881-136-5, pages 28–30.