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# Technical Reference: Base Operating System and Extensions, Volume 1

## atanh, atanhf, or atanhl Subroutine

### Purpose

Computes the inverse hyperbolic tangent.

### Syntax

```#include <math.h>

float atanhf (x)
float x;

long double atanhl (x)
long double x;

double atanh (x)
double x;```

### Description

The atanhf, atanhl, and atanh subroutines compute the inverse hyperbolic tangent of the x parameter.

An application wishing to check for error situations should set the errno global variable to zero and call feclearexcept(FE_ALL_EXCEPT) before calling these functions. On return, if errno is nonzero or fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is nonzero, an error has occurred.

### Parameters

 x Specifies the value to be computed.

### Return Values

Upon successful completion, the atanhf, atanhl, and atanh subroutines return the inverse hyperbolic tangent of the given argument.

If x is ±1, a pole error occurs, and atanhf, atanhl , and atanh return the value of the macro HUGE_VALF, HUGE_VALL, and HUGE_VAL respectively, with the same sign as the correct value of the function.

For finite |x|>1, a domain error occurs, and a NaN is returned.

If x is NaN, a NaN is returned.

If x is 0, x is returned.

If x is ±Inf, a domain error shall occur, and a NaN is returned.

If x is subnormal, a range error may occur and x is returned.

### Error Codes

The atanhf, atanhl, and atanh subroutines return NaNQ and set errno to EDOM if the absolute value of x is greater than 1.

### Related Information

exp, expf, or expl Subroutine

math.h in AIX 5L Version 5.2 Files Reference.

Subroutines Overview in AIX 5L Version 5.2 General Programming Concepts: Writing and Debugging Programs.

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