bellard's formula:
PI = 1/2^6 * sum( (-1)^n/2^(10*n) * (-2^5/(4n+1) - 1/(4n+3) + 2^8/(10n+1)-2^6/(10n+3) -2^2/(10n+5)-2^2/(10n+7)+1/(10n+9) ) )

bailey-borwein-plouffe:

PI = sum(1/16^k*(4/(8k+1)-2/(8k+4)-1/(8k+5)-1/(8k+6)))