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### Binomial Coefficients

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

    namespace boost{ namespace math{

    template <class T>
    T binomial_coefficient(unsigned n, unsigned k);

    template <class T, class Policy>
    T binomial_coefficient(unsigned n, unsigned k, const Policy&);

    }} // namespaces

Returns the binomial coefficient: nCk.

Requires k <= n.

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.

| Important
The functions described above are templates where the template argument `T` can not be deduced from the arguments passed to the function. Therefore if you write something like: `boost::math::binomial_coefficient(10, 2);` You will get a compiler error, usually indicating that there is no such function to be found. Instead you need to specify the return type explicitly and write: `boost::math::binomial_coefficient<double>(10, 2);` So that the return type is known. Further, the template argument must be a real-valued type such as `float` or `double` and not an integer type - that would overflow far too easily!

##### Accuracy

The accuracy will be the same as for the factorials for small arguments (i.e. no more than one or two epsilon), and the beta function for larger arguments.

##### Testing

The spot tests for the binomial coefficients use data generated by Wolfram Alpha.

##### Implementation

Binomial coefficients are calculated using table lookup of factorials where possible using:

> nCk = n! / (k!(n-k)!)

Otherwise it is implemented in terms of the beta function using the relations:

> nCk = 1 / (k * beta(k, n-k+1))

and

> nCk = 1 / ((n-k) * beta(k+1, n-k))