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-rw-r--r--manual/arith.texi10
1 files changed, 8 insertions, 2 deletions
diff --git a/manual/arith.texi b/manual/arith.texi
index 7f8c205ebb..2e418838db 100644
--- a/manual/arith.texi
+++ b/manual/arith.texi
@@ -414,13 +414,19 @@ This function returns the absolute value of the floating-point number
@deftypefunx float cabsf (complex float @var{z})
@deftypefunx {long double} cabsl (complex long double @var{z})
These functions return the absolute value of the complex number @var{z}.
-The compiler must support complex numbers to use these functions. (See
-also the function @code{hypot} in @ref{Exponents and Logarithms}.) The
+The compiler must support complex numbers to use these functions. The
value is:
@smallexample
sqrt (creal (@var{z}) * creal (@var{z}) + cimag (@var{z}) * cimag (@var{z}))
@end smallexample
+
+This function should always be used instead of the direct formula since
+using the simple straight-forward method can mean to loose accuracy. If
+one of the squared values is neglectable in size compared to the other
+value the result should be the same as the larger value. But squaring
+the value and afterwards using the square root function leads to
+unaccuracy. See @code{hypot} in @xref{Exponents and Logarithms}.
@end deftypefun
@node Normalization Functions