https://en.wikipedia.org/wiki/Pi
code
https://github.com/HHS-IntroProgramming/Calculate-Pi
https://github.com/morphx666/CalcPi
https://github.com/Drethic/eve-pi-calc
https://github.com/SaltyPaws/calculator_pi
https://github.com/nohal/Calc_pi_muparser
https://github.com/coq-contribs/pi-calc