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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.

The following graph illustrates the behaviour of cbrt:

##### Accuracy

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

Table 6.80. Error rates for cbrt

| Microsoft Visual C++ version 12.0 Win32 double | GNU C++ version 5.1.0 linux double | GNU C++ version 5.1.0 linux long double | Sun compiler version 0x5130 Sun Solaris long double
--- | --- | --- | --- | ---
cbrt Function | Max = 1.7ε (Mean = 0.565ε) (<math.h>: Max = 1.7ε (Mean = 0.565ε)) | Max = 0ε (Mean = 0ε) (Cephes: Max = 0ε (Mean = 0ε)) | Max = 1.34ε (Mean = 0.471ε) (<tr1/cmath>: Max = 1.34ε (Mean = 0.471ε)) (<math.h>: Max = 1.34ε (Mean = 0.471ε)) | Max = 1.11ε (Mean = 0.424ε) (<math.h>: Max = 1.11ε (Mean = 0.424ε))

##### Testing

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