Boost C++ Libraries of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

This is the documentation for an old version of Boost. Click here to view this page for the latest version.

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)


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.


The accuracy will be the same as the tgamma_delta_ratio function.


The spot tests for the rising factorials use data generated by


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