Looks like it's from here https://github.com/ekmett/approximate/blob/master/cbits/fast.c#L103-L109 https://github.com/ekmett/approximate/blob/master/cbits/fast.

Looks like it's from here https://github.com/ekmett/approximate/blob/master/cbits/fast.c#L103-L109 https://github.com/ekmett/approximate/blob/master/cbits/fast..., there's an explanation here https://nic.schraudolph.org/pubs/Schraudolph99.pdf https://nic.schraudolph.org/pubs/Schraudolph99.pdf