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### Rising Factorial

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

    namespace boost{ namespace math{

    template <class T>
    calculated-result-type rising_factorial(T x, int i);

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

    }} // namespaces

Returns the rising factorial of x and i:

rising_factorial(x, i) = Γ(x + i) / Γ(x);

or

rising_factorial(x, i) = x(x+1)(x+2)(x+3)...(x+i-1)

Note that both x and i can be negative as well as positive.

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.

May return the result of overflow_error if the result is too large to represent in type T.

The return type of these functions is computed using the result type calculation rules: the type of the result is `double` if T is an integer type, otherwise the type of the result is T.

##### Accuracy

The accuracy will be the same as the tgamma_delta_ratio function.

##### Testing

The spot tests for the rising factorials use data generated by functions.wolfram.com.

##### Implementation

Rising and falling factorials are implemented as ratios of gamma functions using tgamma_delta_ratio. Optimisations for small integer arguments are handled internally by that function.