### NAME

atanh, atanhf, atanhl - inverse hyperbolic tangent function

### SYNOPSIS

**#include <math.h>**

**double atanh(double ***x***);**
**float atanhf(float ***x***);**
**long double atanhl(long double ***x***);**

Link with *-lm*.

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

**atanh**(): _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 500 || _ISOC99_SOURCE; or
*cc -std=c99* **atanhf**(), **atanhl**(): _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 600 || _ISOC99_SOURCE; or
*cc -std=c99*

### DESCRIPTION

The
**atanh**() function calculates the inverse hyperbolic tangent of
*x*; that is the value whose hyperbolic tangent is *x*.

### RETURN VALUE

On success, these functions return the inverse hyperbolic tangent of
*x*.

If
*x* is a NaN, a NaN is returned.

If
*x* is +0 (-0), +0 (-0) is returned.

If
*x* is +1 or -1,
a pole error occurs,
and the functions return
**HUGE_VAL**, **HUGE_VALF**, or
**HUGE_VALL**, respectively, with the mathematically correct sign.

If the absolute value of
*x* is greater than 1,
a domain error occurs,
and a NaN is returned.

### ERRORS

See math_error(7) for information on how to determine whether an error has occurred when calling these functions.

The following errors can occur:

Domain error: x less than -1 or greater than +1
| |

errno is set to
EDOM. An invalid floating-point exception
(FE_INVALID) is raised.
| |

Pole error: x is +1 or -1
| |

errno is set to
ERANGE (but see BUGS).
A divide-by-zero floating-point exception
(FE_DIVBYZERO) is raised.
| |

### CONFORMING TO

C99, POSIX.1-2001.
The variant returning
*double* also conforms to
SVr4, 4.3BSD, C89.

### BUGS

For a pole error,
*errno* is set to
**EDOM**; POSIX.1 says it should be set to
**ERANGE**.

### SEE ALSO

acosh(3), asinh(3), catanh(3), cosh(3), sinh(3), tanh(3)

### COLOPHON

This page is part of release 3.23 of the Linux
*man-pages* project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.