log1p

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#### log1p

    #include <boost/math/special_functions/log1p.hpp>

    namespace boost{ namespace math{

    template <class T>
    calculated-result-type log1p(T x);

    template <class T, class Policy>
    calculated-result-type log1p(T x, const Policy&);

    }} // namespaces

Returns the natural logarithm of `x+1`.

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.

There are many situations where it is desirable to compute `log(x+1)`. However, for small `x` then `x+1` suffers from catastrophic cancellation errors so that `x+1 == 1` and `log(x+1) == 0`, when in fact for very small x, the best approximation to `log(x+1)` would be `x`. `log1p` calculates the best approximation to `log(1+x)` using a Taylor series expansion for accuracy (less than 2ɛ). Alternatively note that there are faster methods available, for example using the equivalence:

    log(1+x) == (log(1+x) * x) / ((1-x) - 1)

However, experience has shown that these methods tend to fail quite spectacularly once the compiler's optimizations are turned on, consequently they are used only when known not to break with a particular compiler. In contrast, the series expansion method seems to be reasonably immune to optimizer-induced errors.

Finally when BOOST_HAS_LOG1P is defined then the `float/double/long double` specializations of this template simply forward to the platform's native (POSIX) implementation of this function.

##### Accuracy

For built in floating point types `log1p` should have approximately 1 epsilon accuracy.

##### Testing

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