Gauss–Legendre algorithm
The Gauss–Legendre algorithm is an algorithm to compute the digits of Pi|. It is notable for being rapidly convergent, with only 25 iterations producing 45 million correct digits of . However, the drawback is that it is computer memory-intensive and therefore sometimes Machin-like formulas are used instead.
The method is based on the individual work of Carl Friedrich Gauss and Adrien-Marie Legendre combined with modern algorithms for multiplication and square roots. It repeatedly replaces two numbers by their arithmetic and geometric mean, in order to approximate their arithmetic-geometric mean.
The version presented below is also known as the Gauss–Euler, Brent–Salamin algorithm; it was independently discovered in 1975 by Richard Brent and Eugene Salamin. It was used to compute the first 206,158,430,000 decimal digits of on September 18 to 20, 1999, and the results were checked with Borwein's algorithm.Algorithm
1. Initial value setting:
2. Repeat the following instructions until the difference of and is within the desired accuracy:
3. is then approximated as:
The first three iterations give :
The algorithm has quadratic convergence, which essentially means that the number of correct digits doubles with each iteration of the algorithm.Mathematical background
The arithmetic–geometric mean of two numbers, a0 and b0, is found by calculating the limit of the sequences
which both converge to the same limit.
If and then the limit is where is the complete elliptic integral of the first kind
If,, then
where is the complete elliptic integral of the second kind:
Gauss knew of both of these results.Legendre’s identity
For and such that Legendre proved the identity:The Gauss-Legendre algorithm can be proven without elliptic modular functions. This is done here and here using only integral calculus.
