Merged revisions 76861 via svnmerge from

svn+ssh://pythondev@svn.python.org/python/trunk

........
  r76861 | mark.dickinson | 2009-12-16 20:13:40 +0000 (Wed, 16 Dec 2009) | 3 lines

  Issue #3366: Add expm1 function to math module.  Thanks Eric Smith for
  testing on Windows.
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1 files changed, 14 insertions, 0 deletions

diff --git a/Doc/library/math.rst b/Doc/library/math.rst
index e903c5f902..39aea2903e 100644
--- a/Doc/library/math.rst
+++ b/Doc/library/math.rst
@@ -148,6 +148,20 @@ Power and logarithmic functions
    Return ``e**x``.
 
 
+.. function:: expm1(x)
+
+   Return ``e**x - 1``.  For small floats *x*, the subtraction in
+   ``exp(x) - 1`` can result in a significant loss of precision; the
+   :func:`expm1` function provides a way to compute this quantity to
+   full precision::
+
+      >>> from math import exp, expm1
+      >>> exp(1e-5) - 1  # gives result accurate to 11 places
+      1.0000050000069649e-05
+      >>> expm1(1e-5)    # result accurate to full precision
+      1.0000050000166668e-05
+
+
 .. function:: log(x[, base])
 
    With one argument, return the natural logarithm of *x* (to base *e*).