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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diff --git a/Doc/library/math.rst b/Doc/library/math.rst
index e903c5f..39aea29 100644
--- a/Doc/library/math.rst
+++ b/Doc/library/math.rst
@@ -148,6 +148,20 @@
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*).