[New-bugs-announce] [issue35880] math.sin has no backward error; this isn't documented

[Jurjen N.E. Bos]

New submission from Jurjen N.E. Bos <jneb at users.sourceforge.net>:

The documentation of math.sin (and related trig functions) doesn't speak about backward error.
In cPython, as far as I can see, there is no backward error at all, which is quite uncommon.
This may vary between implementations; many math libraries of other languages have a backward error, resulting in large errors for large arguments.
e.g. sin(1<<500) is correctly computed as 0.42925739234242827, where a backward error as small as 1e-150 can give a completely wrong result.

Some text could be added (which I am happy to produce) that explains what backward error means, and under which circumstances you can expect an accurate result.

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assignee: docs at python
components: Documentation
messages: 334672
nosy: docs at python, jneb
priority: normal
severity: normal
status: open
title: math.sin has no backward error; this isn't documented
versions: Python 2.7, Python 3.4, Python 3.5, Python 3.6, Python 3.7, Python 3.8

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue35880>
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