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### atanh

    #include <boost/math/special_functions/atanh.hpp>

    template<class T>
    calculated-result-type atanh(const T x);

    template<class T, class Policy>
    calculated-result-type atanh(const T x, const Policy&);

Computes the reciprocal of the hyperbolic tangent function, at x.

The final Policy argument is optional and can be used to control the behaviour of the function: how it handles errors, what level of precision to use etc. Refer to the policy documentation for more details.

If x is in the range `]-∞;-1[` or in the range `]+1;+∞[` then returns the result of domain_error.

If x is in the range `[-1;-1+ε[`, then the result of -overflow_error is returned, with ε denoting `std::numeric_limits<T>::epsilon()`.

If x is in the range `]+1-ε;+1]`, then the result of overflow_error is returned, with ε denoting `std::numeric_limits<T>::epsilon()`.

The return type of this function is computed using the result type calculation rules: the return type is `double` when T is an integer type, and T otherwise.

##### Accuracy

Generally accuracy is to within 1 or 2 machine epsilon across all supported platforms.

##### Testing

This function is tested using a combination of random test values designed to give full function coverage computed at high precision using the "naive" formula:

along with a selection of sanity check values computed using functions.wolfram.com to at least 50 decimal digits.

##### Implementation

For sufficiently small x we can use the approximation:

Otherwise the primary definition:

or its equivalent form:

is used.