cbrt

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#### cbrt

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

    namespace boost{ namespace math{

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

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

    }} // namespaces

Returns the cubed root of x: x1/3.

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

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.

Implemented using Halley iteration.

##### Accuracy

For built in floating-point types `cbrt` should have approximately 2 epsilon accuracy.

##### Testing

A mixture of spot test sanity checks, and random high precision test values calculated using NTL::RR at 1000-bit precision.