[/
Copyright (c) 2012 John Maddock
Use, modification and distribution are subject to 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)
]

[section:jacobi Jacobi Elliptic Functions]

[section:jac_over Overvew of the Jacobi Elliptic Functions]

There are twelve Jacobi Elliptic functions, of which the three copolar functions ['sn], ['cn] and ['dn] are the most important
as the other nine can be computed from these three
[footnote [@http://en.wikipedia.org/wiki/Jacobi_elliptic_functions Wikipedia: Jacobi elliptic functions]]
[footnote [@http://mathworld.wolfram.com/JacobiEllipticFunctions.html Weisstein, Eric W. "Jacobi Elliptic Functions." From MathWorld - A Wolfram Web Resource.]]
[footnote [@http://dlmf.nist.gov/22 Digital Library of Mathematical Functions: Jacobian Elliptic Functions]].

These functions each take two arguments: a parameter, and a variable as described below.

Like all elliptic functions these can be parameterised in a number of ways:

* In terms of a parameter ['m].
* In terms of the elliptic modulus ['k] where ['m = k[super 2]].
* In terms of the modular angle [alpha], where ['m = sin[super 2][alpha]].

In our implementation, these functions all take the elliptic modulus /k/ as the parameter.

In addition the variable /u/ is sometimes expressed as an amplitude [phi], in our implementation we always use /u/.

Finally note that our functions all take the elliptic modulus as the first argument - this is for alignment with
the Elliptic Integrals.

There are twenve functions for computing the twelve individual Jacobi elliptic functions: __jacobi_cd, __jacobi_cn, __jacobi_cs,
__jacobi_dc, __jacobi_dn, __jacobi_ds, __jacobi_nc, __jacobi_nd, __jacobi_ns, __jacobi_sc, __jacobi_sd and __jacobi_sn.

They are all called as for example:

   jacobi_cs(k, u);

Note however that these individual functions are all really thin wrappers around the function __jacobi_elliptic which calculates
the three copolar functions ['sn], ['cn] and ['dn] in a single function call.  Thus if you need more than one of these functions
for a given set of arguments, it's most efficient to use __jacobi_elliptic.

[endsect]

[section:jacobi_elliptic Jacobi Elliptic SN, CN and DN]

[heading Synopsis]

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

  namespace boost { namespace math {

   template <class T, class U, class V>
   ``__sf_result`` jacobi_elliptic(T k, U u, V* pcn, V* pdn);

   template <class T, class U, class V, class Policy>
   ``__sf_result`` jacobi_elliptic(T k, U u, V* pcn, V* pdn, const Policy&);

  }} // namespaces
  
[heading Description]

The function __jacobi_elliptic calculates the three copolar Jacobi elliptic functions
['sn(u, k)], ['cn(u, k)] and ['dn(u, k)].  The returned value is 
['sn(u, k)], and if provided, ['*pcn] is
set to ['cn(u, k)], and ['*pdn] is set to ['dn(u, k)].

The functions are defined as follows, given:

[equation jacobi1]

The the angle [phi] is called the ['amplitude] and:

[equation jacobi2]

[note ['[phi]] is called the amplitude.

['k] is called the modulus. 

]
      
[caution Rather like other elliptic functions, the Jacobi functions
are expressed in a variety of different ways.  In particular,
the parameter /k/ (the modulus) may also be expressed using a modular
angle [alpha], or a parameter /m/.  These are related by:

k = sin[alpha]

m = k[super 2] = sin[super 2][alpha]

So that the function ['sn] (for example) may be expressed as
either:

sn(u, k)

sn(u \\ [alpha])

sn(u| m)

To further complicate matters, some texts refer to the ['complement
of the parameter m], or 1 - m, where:

1 - m = 1 - k[super 2] = cos[super 2][alpha]

This implementation uses /k/ throughout, and makes this the first argument
to the functions: this is for alignment with the elliptic integrals which match the requirements
of the [@http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1836.pdf
Technical Report on C++ Library Extensions].  However, you should
be extra careful when using these functions!]

[optional_policy]

The following graphs illustrate how these functions change as /k/ changes: for small /k/
these are sine waves, while as /k/ tends to 1 they become hyperbolic functions:

[graph jacobi_sn]

[graph jacobi_cn]

[graph jacobi_dn]

[heading Accuracy]

These functions are computed using only basic arithmetic operations and trigomometric functions, so
there isn't much variation in accuracy over differing platforms.
Typically errors are trivially small for small angles, and as is typical for cyclic
functions, grow as the angle increases.
Note that only results for the widest floating point type on the 
system are given as narrower types have __zero_error.  All values
are relative errors in units of epsilon.

[table_jacobi_cn]

[table_jacobi_dn]

[table_jacobi_sn]

[heading Testing]

The tests use a mixture of spot test values calculated using the online
calculator at [@http://functions.wolfram.com/ functions.wolfram.com],
and random test data generated using
MPFR at 1000-bit precision and this implementation.

[heading Implementation]

For ['k > 1] we apply the relations:

[equation jacobi3]

Then filter off the special cases:

['sn(0, k) = 0] and ['cn(0, k) = dn(0, k) = 1].

['sn(u, 0) = sin(u), cn(u, 0) = cos(u) and dn(u, 0) = 1].

['sn(u, 1) = tanh(u), cn(u, 1) = dn(u, 1) = 1 / cosh(u)].

And for ['k[super 4] < [epsilon]] we have:

[equation jacobi4]

Otherwise the values are calculated using the method of [@http://dlmf.nist.gov/22.20#SS2 arithmetic geometric means].

[endsect]

[section:jacobi_cd Jacobi Elliptic Function cd]

[heading Synopsis]

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

  namespace boost { namespace math {

   template <class T, class U>
   ``__sf_result`` jacobi_cd(T k, U u);

   template <class T, class U, class Policy>
   ``__sf_result`` jacobi_cd(T k, U u, const Policy& pol);

  }} // namespaces
  
[heading Description]

This function returns the Jacobi elliptic function ['cd].

[optional_policy]

This function is a trivial wrapper around __jacobi_elliptic, with:

['cd(u, k) = cn(u, k) / dn(u, k)]

[graph jacobi_cd]

[endsect]

[section:jacobi_cn Jacobi Elliptic Function cn]

[heading Synopsis]

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

  namespace boost { namespace math {

   template <class T, class U>
   ``__sf_result`` jacobi_cn(T k, U u);

   template <class T, class U, class Policy>
   ``__sf_result`` jacobi_cn(T k, U u, const Policy& pol);

  }} // namespaces
  
[heading Description]

This function returns the Jacobi elliptic function ['cn].

[optional_policy]

This function is a trivial wrapper around __jacobi_elliptic.

[graph jacobi_cn]

[endsect]

[section:jacobi_cs Jacobi Elliptic Function cs]

[heading Synopsis]

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

  namespace boost { namespace math {

   template <class T, class U>
   ``__sf_result`` jacobi_cs(T k, U u);

   template <class T, class U, class Policy>
   ``__sf_result`` jacobi_cs(T k, U u, const Policy& pol);

  }} // namespaces
  
[heading Description]

This function returns the Jacobi elliptic function ['cs].

[optional_policy]

This function is a trivial wrapper around __jacobi_elliptic, with:

['cs(u, k) = cn(u, k) / sn(u, k)]

[graph jacobi_cs]

[endsect]

[section:jacobi_dc Jacobi Elliptic Function dc]

[heading Synopsis]

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

  namespace boost { namespace math {

   template <class T, class U>
   ``__sf_result`` jacobi_dc(T k, U u);

   template <class T, class U, class Policy>
   ``__sf_result`` jacobi_dc(T k, U u, const Policy& pol);

  }} // namespaces
  
[heading Description]

This function returns the Jacobi elliptic function ['dc].

[optional_policy]

This function is a trivial wrapper around __jacobi_elliptic, with:

['dc(u, k) = dn(u, k) / cn(u, k)]

[graph jacobi_dc]

[endsect]

[section:jacobi_dn Jacobi Elliptic Function dn]

[heading Synopsis]

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

  namespace boost { namespace math {

   template <class T, class U>
   ``__sf_result`` jacobi_dn(T k, U u);

   template <class T, class U, class Policy>
   ``__sf_result`` jacobi_dn(T k, U u, const Policy& pol);

  }} // namespaces
  
[heading Description]

This function returns the Jacobi elliptic function ['dn].

[optional_policy]

This function is a trivial wrapper around __jacobi_elliptic.

[graph jacobi_dn]

[endsect]

[section:jacobi_ds Jacobi Elliptic Function ds]

[heading Synopsis]

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

  namespace boost { namespace math {

   template <class T, class U>
   ``__sf_result`` jacobi_ds(T k, U u);

   template <class T, class U, class Policy>
   ``__sf_result`` jacobi_ds(T k, U u, const Policy& pol);

  }} // namespaces
  
[heading Description]

This function returns the Jacobi elliptic function ['ds].

[optional_policy]

This function is a trivial wrapper around __jacobi_elliptic, with:

['ds(u, k) = dn(u, k) / sn(u, k)]

[graph jacobi_ds]

[endsect]

[section:jacobi_nc Jacobi Elliptic Function nc]

[heading Synopsis]

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

  namespace boost { namespace math {

   template <class T, class U>
   ``__sf_result`` jacobi_nc(T k, U u);

   template <class T, class U, class Policy>
   ``__sf_result`` jacobi_nc(T k, U u, const Policy& pol);

  }} // namespaces
  
[heading Description]

This function returns the Jacobi elliptic function ['nc].

[optional_policy]

This function is a trivial wrapper around __jacobi_elliptic, with:

['nc(u, k) = 1 / cn(u, k)]

[graph jacobi_nc]

[endsect]

[section:jacobi_nd Jacobi Elliptic Function nd]

[heading Synopsis]

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

  namespace boost { namespace math {

   template <class T, class U>
   ``__sf_result`` jacobi_nd(T k, U u);

   template <class T, class U, class Policy>
   ``__sf_result`` jacobi_nd(T k, U u, const Policy& pol);

  }} // namespaces
  
[heading Description]

This function returns the Jacobi elliptic function ['nd].

[optional_policy]

This function is a trivial wrapper around __jacobi_elliptic, with:

['nd(u, k) = 1 / dn(u, k)]

[graph jacobi_nd]

[endsect]

[section:jacobi_ns Jacobi Elliptic Function ns]

[heading Synopsis]

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

  namespace boost { namespace math {

   template <class T, class U>
   ``__sf_result`` jacobi_ns(T k, U u);

   template <class T, class U, class Policy>
   ``__sf_result`` jacobi_ns(T k, U u, const Policy& pol);

  }} // namespaces
  
[heading Description]

This function returns the Jacobi elliptic function ['ns].

[optional_policy]

This function is a trivial wrapper around __jacobi_elliptic, with:

['ns(u, k) = 1 / sn(u, k)]

[graph jacobi_ns]

[endsect]

[section:jacobi_sc Jacobi Elliptic Function sc]

[heading Synopsis]

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

  namespace boost { namespace math {

   template <class T, class U>
   ``__sf_result`` jacobi_sc(T k, U u);

   template <class T, class U, class Policy>
   ``__sf_result`` jacobi_sc(T k, U u, const Policy& pol);

  }} // namespaces
  
[heading Description]

This function returns the Jacobi elliptic function ['sc].

[optional_policy]

This function is a trivial wrapper around __jacobi_elliptic, with:

['sc(u, k) = sn(u, k) / cn(u, k)]

[graph jacobi_sc]

[endsect]

[section:jacobi_sd Jacobi Elliptic Function sd]

[heading Synopsis]

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

  namespace boost { namespace math {

   template <class T, class U>
   ``__sf_result`` jacobi_sd(T k, U u);

   template <class T, class U, class Policy>
   ``__sf_result`` jacobi_sd(T k, U u, const Policy& pol);

  }} // namespaces
  
[heading Description]

This function returns the Jacobi elliptic function ['sd].

[optional_policy]

This function is a trivial wrapper around __jacobi_elliptic, with:

['sd(u, k) = sn(u, k) / dn(u, k)]

[graph jacobi_sd]

[endsect]

[section:jacobi_sn Jacobi Elliptic Function sn]

[heading Synopsis]

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

  namespace boost { namespace math {

   template <class T, class U>
   ``__sf_result`` jacobi_sn(T k, U u);

   template <class T, class U, class Policy>
   ``__sf_result`` jacobi_sn(T k, U u, const Policy& pol);

  }} // namespaces
  
[heading Description]

This function returns the Jacobi elliptic function ['sn].

[optional_policy]

This function is a trivial wrapper around __jacobi_elliptic.

[graph jacobi_sn]

[endsect]


[endsect]

