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### 1.9 `atanh`, `atanhf`—inverse hyperbolic tangent

Synopsis

    #include <math.h>
    double atanh(double x);
    float atanhf(float x);

Description
`atanh` calculates the inverse hyperbolic tangent of x.

`atanhf` is identical, other than taking and returning `float` values.

Returns
`atanh` and `atanhf` return the calculated value.

If

    x|

is greater than 1, the global `errno` is set to `EDOM` and the result is a NaN. A `DOMAIN error` is reported.

If

    x|

is 1, the global `errno` is set to `EDOM`; and the result is infinity with the same sign as `x`. A `SING error` is reported.

Portability
Neither `atanh` nor `atanhf` are ANSI C.