NAME
log1p() — natural logarithm function
SYNOPSIS
#include <math.h>
double log1p(double x);
DESCRIPTION
The
log1p()
function computes logarithmic functions.
The
log1p()
function is equivalent to
log(1 + x),
but may be more accurate for very small values of
x.
The
expm1()
and
log1p()
functions are useful to guarantee
that financial calculations of (((1+x)**n)-1)/x, namely:
are accurate when
x
is very small (for example, when calculating small
daily interest rates). These functions also simplify writing
accurate inverse hyperbolic functions.
The ISO/ANSI C committee has approved the
log1p()
function for inclusion in the C9X draft standard.
To use this function, compile either with the default
-Ae
option or with the
-Aa
and
-D_HPUX_SOURCE
options. Make sure your program includes
<math.h>.
Link in the math library by specifying
-lm
on the compiler or linker command line.
RETURN VALUE
log1p()
returns the natural logarithm of (1 +
x).
If
x
is +INFINITY,
log1p()
returns +INFINITY.
If
x
= -1.0,
log1p()
returns
-HUGE_VAL.
If
x
is NaN,
log1p()
returns NaN.
If
x
< -1.0,
log1p()
returns NaN and sets
errno
to [EDOM].
ERRORS
If
log1p()
fails,
errno
is set to the following value.
- [EDOM]
x
is less than -1.0.
STANDARDS CONFORMANCE
log1p(): XPG4.2
