Here is his paper on it which is a little 2 pager:
replyhttps://bellard.org/pi/pi_bin.pdf
Though I have to say the last line of the proof "...which gives (1) by reordering the terms" took me much head scratching to understand!
https://bellard.org/pi/pi_bin.pdf
Though I have to say the last line of the proof "...which gives (1) by reordering the terms" took me much head scratching to understand!