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### expm1

    #include <boost/math/special_functions/expm1.hpp>

    namespace boost{ namespace math{

    template <class T>
    calculated-result-type expm1(T x);

    template <class T, class Policy>
    calculated-result-type expm1(T x, const Policy&);

    }} // namespaces

Returns ex - 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.

For small x, then `ex` is very close to 1, as a result calculating `ex - 1` results in catastrophic cancellation errors when x is small. `expm1` calculates `ex - 1` using rational approximations (for up to 128-bit long doubles), otherwise via a series expansion when x is small (giving an accuracy of less than 2ɛ).

Finally when BOOST_HAS_EXPM1 is defined then the `float/double/long double` specializations of this template simply forward to the platform's native (POSIX) implementation of this function.

The following graph illustrates the behaviour of expm1:

##### Accuracy

For built in floating point types `expm1` should have approximately 1 epsilon accuracy.

Table 8.82. Error rates for expm1

| GNU C++ version 7.1.0 linux long double | GNU C++ version 7.1.0 linux double | Sun compiler version 0x5150 Sun Solaris long double | Microsoft Visual C++ version 14.1 Win32 double
--- | --- | --- | --- | ---
Random test data | Max = 0.992ε (Mean = 0.402ε) (<cmath>: Max = 0.992ε (Mean = 0.402ε)) (<math.h>: Max = 0.992ε (Mean = 0.402ε)) | Max = 0.793ε (Mean = 0.126ε) (Rmath 3.2.3: Max = 0.793ε (Mean = 0.126ε)) | Max = 1.31ε (Mean = 0.428ε) (<math.h>: Max = 0.996ε (Mean = 0.426ε)) | Max = 1.31ε (Mean = 0.496ε) (<math.h>: Max = 1.31ε (Mean = 0.496ε))

##### Testing

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