Almkvist Berndt 1988 begins with an investigation of why the agm is such an efficient algorithm, and proves that it converges quadratically. This is an efficient method to calculate 



π


{\displaystyle \pi }

.

With the same notations used in Arithmetic-geometric mean, we can summarize the paper by writing:





π
=



4


a
g
m

(
1
,
1

/



2



)

2




1
−

∑

n
=
1


∞



2

n
+
1


(

a

n


2


−

g

n


2


)





{\displaystyle \pi ={\frac {4\;\mathrm {agm} (1,1/{\sqrt {2}})^{2}}{1-\sum \limits _{n=1}^{\infty }2^{n+1}(a_{n}^{2}-g_{n}^{2})}}}



This allows you to make the approximation, for any large   N:





π
≈



4


a

N


2




1
−

∑

k
=
1


N



2

k
+
1


(

a

k


2


−

g

k


2


)





{\displaystyle \pi \approx {\frac {4\;a_{N}^{2}}{1-\sum \limits _{k=1}^{N}2^{k+1}(a_{k}^{2}-g_{k}^{2})}}}



The purpose of this task is to demonstrate how to use this approximation in order to compute a large number of decimals of 



π


{\displaystyle \pi }

.

import "/big" for BigRat
 
var digits = 500
var an = BigRat.one
var bn = BigRat.half.sqrt(digits)
var tn = BigRat.half.square
var pn = BigRat.one
while (pn <= digits) {
    var prevAn = an
    an = (bn + an) * BigRat.half
    bn = (bn * prevAn).sqrt(digits)
    prevAn = prevAn - an
    tn = tn - (prevAn.square * pn)
    pn = pn + pn
}
var pi = (an + bn).square / (tn * 4)
System.print(pi.toDecimal(digits, false))