The math module contains log1p but is missing expm1 (the inverse of
log1p). These functions are necessary to avoid loss of precision in
floating point calculations, and are part of the C99 standard math library.

Mike,

Can you propose an implementation, for those platforms that haven't yet 
caught up with C99?  Something comparable to the implementation of log1p 
in Python/pymath.c would be appropriate.  Ideally, such an 
implementation would:

- be accurate to within a few ulps across the whole domain,
- be not too long, and not too slow
- have a decent chance of working with strange floating-point formats 
(Python doesn't assume IEEE 754)
- handle IEEE 754 values 'correctly' (i.e., as recommended by Annex F to 
the C99 standard)

It's too late to get this into Python 2.6/3.0, but patches aimed at 2.7 
or 3.1 would be welcome.

A cheap trick would be to use the identity

  expm1(x) = 2*exp(x/2)*sinh(x/2)

This could also be used in Python as a
workaround, for now.  But I agree that
expm1 should go into the math library.

I'm absorbing this issue into issue 3366.