For the 12.1*10^12 calculation the refernced web site says: Hudnovsky algorithm [1] for the calculation, Bellard BBP formula for verification. Implementation in the programm y-cruncher "A Multi-Threaded Pi-Programy-chruncher" [3] For the math see [4] also.

[1] https://en.wikipedia.org/wiki/Chudnovsky_algorithm

[2] https://en.wikipedia.org/wiki/Bellard%27s_formula

[3] http://www.numberworld.org/y-cruncher/

[4] http://www.numberworld.org/misc_runs/pi-5t/details.html#form...