c - Can not understand this pi calculate algorithm -

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i saw pi calculate algorithm on website , looks that:

#include <stdio.h> int a[52514],b,c=52514,d,e,f=1e4,g,h; main(){  for(;b=c-=14;h=printf("%04d",e+d/f)){     for(e=d%=f;g=--b*2;d/=g){         d=d*b+f*(h?a[b]:f/5);         a[b]=d%--g;}    } }

it said code based on expansion,but not understand relative between code , expansion.

pi= sigma( (i!) ^2*2^(i+1))/(2i+1)! (i=0 infinite)

could me explain it?thanks.

pi+3=sigma( (m!)^2 * 2^m * m / (2*m)! ) (m=1 infinite).

algorithm's s pflouffe use it.


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