libm.info: atanh

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`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.
   You can modify the error handling for these routines using `matherr'.
*Portability*
Neither `atanh' nor `atanhf' are ANSI C.