powers.qbk

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[section:powers Logs, Powers, Roots and Exponentials]

[section:log1p log1p]

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

   namespace boost{ namespace math{
   
   template <class T>
   ``__sf_result`` log1p(T x);
   
   template <class T, class ``__Policy``>
   ``__sf_result`` log1p(T x, const ``__Policy``&);
   
   }} // namespaces
   
Returns the natural logarithm of `x+1`.

The return type of this function is computed using the __arg_pomotion_rules:
the return is `double` when /x/ is an integer type and T otherwise.

[optional_policy]

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 __te).
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.

The following graph illustrates the behaviour of log1p:

[graph log1p]

[h4 Accuracy]

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

[h4 Testing]

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

[endsect]

[section:expm1 expm1]

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

   namespace boost{ namespace math{
   
   template <class T>
   ``__sf_result`` expm1(T x);
   
   template <class T, class ``__Policy``>
   ``__sf_result`` expm1(T x, const ``__Policy``&);
   
   }} // namespaces
   
Returns e[super x] - 1.

The return type of this function is computed using the __arg_pomotion_rules:
the return is `double` when /x/ is an integer type and T otherwise.

[optional_policy]

For small x, then __ex is very close to 1, as a result calculating __exm1 results
in catastrophic cancellation errors when x is small.  `expm1` calculates __exm1 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 __te).

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:

[graph expm1]
   
[h4 Accuracy]

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

[h4 Testing]

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

[endsect]

[section:cbrt cbrt]

``
#include <boost/math/special_functions/cbrt.hpp>
``

   namespace boost{ namespace math{
   
   template <class T>
   ``__sf_result`` cbrt(T x);
   
   template <class T, class ``__Policy``>
   ``__sf_result`` cbrt(T x, const ``__Policy``&);
   
   }} // namespaces
   
Returns the cubed root of x: x[super 1/3].

The return type of this function is computed using the __arg_pomotion_rules:
the return is `double` when /x/ is an integer type and T otherwise.

[optional_policy]

Implemented using Halley iteration.

The following graph illustrates the behaviour of cbrt:

[graph cbrt]
   
[h4 Accuracy]

For built in floating-point types `cbrt`
should have approximately 2 epsilon accuracy.

[h4 Testing]

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

[endsect]

[section:sqrt1pm1 sqrt1pm1]

``
#include <boost/math/special_functions/sqrt1pm1.hpp>
``

   namespace boost{ namespace math{
   
   template <class T>
   ``__sf_result`` sqrt1pm1(T x);
   
   template <class T, class ``__Policy``>
   ``__sf_result`` sqrt1pm1(T x, const ``__Policy``&);
   
   }} // namespaces
   
Returns `sqrt(1+x) - 1`.

The return type of this function is computed using the __arg_pomotion_rules:
the return is `double` when /x/ is an integer type and T otherwise.

[optional_policy]

This function is useful when you need the difference between sqrt(x) and 1, when
x is itself close to 1.

Implemented in terms of `log1p` and `expm1`.

The following graph illustrates the behaviour of sqrt1pm1:

[graph sqrt1pm1]

[h4 Accuracy]

For built in floating-point types `sqrt1pm1`
should have approximately 3 epsilon accuracy.

[h4 Testing]

A selection of random high precision test values
calculated using NTL::RR at 1000-bit precision.

[endsect]

[section:powm1 powm1]

``
#include <boost/math/special_functions/powm1.hpp>
``

   namespace boost{ namespace math{
   
   template <class T1, class T2>
   ``__sf_result`` powm1(T1 x, T2 y);
   
   template <class T1, class T2, class ``__Policy``>
   ``__sf_result`` powm1(T1 x, T2 y, const ``__Policy``&);
   
   }} // namespaces
   
Returns x[super y ] - 1.

The return type of this function is computed using the __arg_pomotion_rules
when T1 and T2 are dufferent types.

[optional_policy]

There are two domains where this is useful: when y is very small, or when
x is close to 1.

Implemented in terms of `expm1`.

The following graph illustrates the behaviour of powm1:

[graph powm1]

[h4 Accuracy]

Should have approximately 2-3 epsilon accuracy.

[h4 Testing]

A selection of random high precision test values
calculated using NTL::RR at 1000-bit precision.

[endsect]

[section:hypot hypot]

   template <class T1, class T2>
   ``__sf_result`` hypot(T1 x, T2 y);
   
   template <class T1, class T2, class ``__Policy``>
   ``__sf_result`` hypot(T1 x, T2 y, const ``__Policy``&);
   
__effects computes [equation hypot]
in such a way as to avoid undue underflow and overflow.

The return type of this function is computed using the __arg_pomotion_rules
when T1 and T2 are of different types.

[optional_policy]

When calculating [equation hypot] it's quite easy for the intermediate terms to either
overflow or underflow, even though the result is in fact perfectly representable.

[h4 Implementation]

The function is even and symmetric in x and y, so first take assume
['x,y > 0] and ['x > y] (we can permute the arguments if this is not the case).

Then if ['x * [epsilon][space] >= y] we can simply return /x/.

Otherwise the result is given by:

[equation hypot2]

[endsect]

[include pow.qbk]


[endsect][/section:powers Logs, Powers, Roots and Exponentials]

[/ 
  Copyright 2006 John Maddock and Paul A. Bristow.
  Distributed under the Boost Software License, Version 1.0.
  (See accompanying file LICENSE_1_0.txt or copy at
  http://www.boost.org/LICENSE_1_0.txt).
]