$Id: sin_cos_reverse.omh 3686 2015-05-11 16:14:25Z bradbell $
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CppAD: C++ Algorithmic Differentiation: Copyright (C) 2003-15 Bradley M. Bell

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$begin sin_cos_reverse$$
$spell
	Taylor
	cos
$$

$section Trigonometric and Hyperbolic Sine and Cosine Reverse Theory$$
$mindex sin, sinh, cos, cosh$$


We use the reverse theory
$cref%standard math function
	%ReverseTheory
	%Standard Math Functions
%$$
definition for the functions $latex H$$ and $latex G$$.
In addition,
we use the following definitions for $latex s$$ and $latex c$$
and the integer $latex \ell$$

$table
Coefficients
	$cnext $cnext $latex s$$
	$cnext $cnext $latex c$$
	$cnext $cnext $latex \ell$$
$rnext
Trigonometric Case
	$cnext $cnext $latex \sin [ X(t) ]$$
	$cnext $cnext $latex \cos [ X(t) ]$$
	$cnext $cnext 1
$rnext
Hyperbolic Case
	$cnext $cnext $latex \sinh [ X(t) ]$$
	$cnext $cnext $latex \cosh [ X(t) ]$$
	$cnext $cnext -1
$tend
We use the value
$latex \[
	z^{(j)} = ( s^{(j)} , c^{(j)} )
\] $$
in the definition for $latex G$$ and $latex H$$.
The forward mode formulas for the
$cref/sine and cosine/sin_cos_forward/$$ functions are
$latex \[
\begin{array}{rcl}
s^{(j)}  & = & \frac{1 + \ell}{2} \sin ( x^{(0)} )
           +   \frac{1 - \ell}{2} \sinh ( x^{(0)} )
\\
c^{(j)}  & = & \frac{1 + \ell}{2} \cos ( x^{(0)} )
           +   \frac{1 - \ell}{2} \cosh ( x^{(0)} )
\end{array}
\] $$

for the case $latex j = 0$$, and for $latex j > 0$$,

$latex \[
\begin{array}{rcl}
s^{(j)} & = & \frac{1}{j}
	\sum_{k=1}^{j} k x^{(k)} c^{(j-k)}  \\
c^{(j)} & = & \ell \frac{1}{j}
	\sum_{k=1}^{j} k x^{(k)} s^{(j-k)}
\end{array}
\] $$

If $latex j = 0$$, we have the relation

$latex \[
\begin{array}{rcl}
\D{H}{ x^{(j)} } & = &
\D{G}{ x^{(j)} }
+ \D{G}{ s^{(j)} } c^{(0)}
+ \ell \D{G}{ c^{(j)} } s^{(0)}
\end{array}
\] $$

If $latex j > 0$$, then for $latex k = 1, \ldots , j-1$$

$latex \[
\begin{array}{rcl}
\D{H}{ x^{(k)} } & = &
\D{G}{ x^{(k)} }
+ \D{G}{ s^{(j)} } \frac{1}{j} k c^{(j-k)}
+ \ell \D{G}{ c^{(j)} } \frac{1}{j} k s^{(j-k)}
\\
\D{H}{ s^{(j-k)} } & = &
\D{G}{ s^{(j-k)} } + \ell \D{G}{ c^{(j)} } k x^{(k)}
\\
\D{H}{ c^{(j-k)} } & = &
\D{G}{ c^{(j-k)} } + \D{G}{ s^{(j)} } k x^{(k)}
\end{array}
\] $$

$end