Jurjen N.E. Bos <jneb@users.sourceforge.net> added the comment: I stand corrected; more on that later. "backward error" is the mathematical term used for the accuracy of a function. (Forward error is in the result proper; backward error means that you calculate the correct result for a number that is very close to the input.) Since pi is not a machine representable number, it is pretty hard to implement the trig functions with a zero backward error, since you need to divide by 2*pi in any reasonable implementation. For some reason, I was in the impression that the backward error of the sine was zero. I wrote a program to demonstrate the matter, only to find out that I was wrong :P Maybe in the 32 bit version, but not in the 64 bits? Anyway, it is more implementation dependent than I though. Althougth the backward error of the builtin sine function isn't zero, it is still a cool 21 digits, as the program shows. - Jurjen ---------- resolution: -> rejected status: pending -> open Added file: https://bugs.python.org/file48199/sindemo.py _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue35880> _______________________________________