Chudnovsky algorithm

The Chudnovsky algorithm is a fast method for calculating the digits of π, based on Ramanujan’s π formulae. It was published by the Chudnovsky brothers in 1988[1] and was used in the world record calculations of 2.7 trillion digits of π in December 2009,[2] 10 trillion digits in October 2011,[3][4] 22.4 trillion digits in November 2016,[5] 31.4 trillion digits in September 2018–January 2019,[6] and 50 trillion digits on January 29, 2020.[7]

The algorithm is based on the negated Heegner number ${\displaystyle d=-163}$, the j-function ${\displaystyle \scriptstyle j\left({\frac {1+{\sqrt {-163}}}{2}}\right)=-640320^{3}}$, and on the following rapidly convergent generalized hypergeometric series:[2]

${\displaystyle {\frac {1}{\pi }}=12\sum _{q=0}^{\infty }{\frac {(-1)^{q}(6q)!(545140134q+13591409)}{(3q)!(q!)^{3}\left(640320\right)^{3q+3/2}}}}$

A detailed proof of this formula can be found here:[8]

For a high performance iterative implementation, this can be simplified to

${\displaystyle {\frac {(640320)^{3/2}}{12\pi }}={\frac {426880{\sqrt {10005}}}{\pi }}=\sum _{q=0}^{\infty }{\frac {(6q)!(545140134q+13591409)}{(3q)!(q!)^{3}\left(-262537412640768000\right)^{q}}}}$

There are 3 big integer terms (the multinomial term M_q, the linear term L_q, and the exponential term X_q) that make up the series and π equals the constant C divided by the sum of the series, as below:

${\displaystyle \pi =C\left(\sum _{q=0}^{\infty }{\frac {M_{q}\cdot L_{q}}{X_{q}}}\right)^{-1}}$, where:

${\displaystyle C=426880{\sqrt {10005}}}$,

${\displaystyle M_{q}={\frac {(6q)!}{(3q)!(q!)^{3}}}}$,

${\displaystyle L_{q}=545140134q+13591409}$,

${\displaystyle X_{q}=(-262537412640768000)^{q}}$.

The terms M_q, L_q, and X_q satisfy the following recurrences and can be computed as such:

${\displaystyle {\begin{alignedat}{4}L_{q+1}&=L_{q}+545140134\,\,&&{\textrm {where}}\,\,L_{0}&&=13591409\\[4pt]X_{q+1}&=X_{q}\cdot (-262537412640768000)&&{\textrm {where}}\,\,X_{0}&&=1\\[4pt]M_{q+1}&=M_{q}\cdot \left({\frac {(12q+2)(12q+6)(12q+10)}{(q+1)^{3}}}\right)\,\,&&{\textrm {where}}\,\,M_{0}&&=1\\[4pt]\end{alignedat}}}$

The computation of M_q can be further optimized by introducing an additional term K_q as follows:

${\displaystyle {\begin{alignedat}{4}K_{q+1}&=K_{q}+12\,\,&&{\textrm {where}}\,\,K_{0}&&=6\\[4pt]M_{q+1}&=M_{q}\cdot \left({\frac {K_{q}^{3}-16K_{q}}{(q+1)^{3}}}\right)\,\,&&{\textrm {where}}\,\,M_{0}&&=1\\[12pt]\end{alignedat}}}$

Note that

${\displaystyle e^{\pi {\sqrt {163}}}\approx 640320^{3}+743.99999999999925\dots }$ and

${\displaystyle 640320^{3}=262537412640768000}$

${\displaystyle 545140134=163\cdot 127\cdot 19\cdot 11\cdot 7\cdot 3^{2}\cdot 2}$

${\displaystyle 13591409=13\cdot 1045493}$

This identity is similar to some of Ramanujan's formulas involving π,[2] and is an example of a Ramanujan–Sato series.

The time complexity of the algorithm is ${\displaystyle O(n\log(n)^{3})}$.[9]