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### cos_pi

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

    namespace boost{ namespace math{

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

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

    }} // namespaces

Returns the cosine of π  x.

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.

This function performs exact all-integer arithmetic argument reduction before computing the cosine of π·x.

Table 8.80. Error rates for cos_pi

| GNU C++ version 7.1.0 linux double | GNU C++ version 7.1.0 linux long double | Sun compiler version 0x5150 Sun Solaris long double | Microsoft Visual C++ version 14.1 Win32 double
--- | --- | --- | --- | ---
sin_pi and cos_pi | Max = 0ε (Mean = 0ε) | Max = 0.991ε (Mean = 0.302ε) | Max = 0.991ε (Mean = 0.302ε) | Max = 0.996ε (Mean = 0.284ε)
sin_pi and cos_pi near integers and half integers | Max = 0ε (Mean = 0ε) | Max = 0.976ε (Mean = 0.28ε) | Max = 0.976ε (Mean = 0.28ε) | Max = 0.996ε (Mean = 0.298ε)