$Id: tan_reverse.omh 3495 2014-12-24 01:16:15Z bradbell $
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CppAD: C++ Algorithmic Differentiation: Copyright (C) 2003-14 Bradley M. Bell

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$begin tan_reverse$$
$spell
	Taylor
$$

$index tan, reverse theory$$
$index theory, tan reverse$$
$index reverse, tan theory$$

$section Tangent and Hyperbolic Tangent Reverse Mode Theory$$

$head Notation$$
We use the reverse theory
$cref/standard math function/ReverseTheory/Standard Math Functions/$$
definition for the functions $latex H$$ and $latex G$$.
In addition, we use the forward mode notation in $cref tan_forward$$ for
$latex X(t)$$, $latex Y(t)$$ and $latex Z(t)$$.

$head Eliminating Y(t)$$
For $latex j > 0$$, the forward mode coefficients are given by
$latex \[
y^{(j-1)} = \sum_{k=0}^{j-1} z^{(k)} z^{(j-k-1)}
\] $$
Fix $latex j > 0$$ and suppose that $latex H$$ is the same as $latex G$$ 
except that $latex y^{(j-1)}$$ is replaced as a function of the Taylor 
coefficients for $latex Z(t)$$.
To be specific, for $latex k = 0 , \ldots , j-1$$,
$latex \[
\begin{array}{rcl}
\D{H}{ z^{(k)} }  
& = & 
\D{G}{ z^{(k)} } + \D{G}{ y^{(j-1)} } \D{ y^{(j-1)} }{ z^{(k)} }
\\
& = & 
\D{G}{ z^{(k)} } + \D{G}{ y^{(j-1)} } 2 z^{(j-k-1)}
\end{array}
\] $$


$head Positive Orders Z(t)$$
For order $latex j > 0$$,
suppose that $latex H$$ is the same as $latex G$$ except that
$latex z^{(j)}$$ is expressed as a function of 
the coefficients for $latex X(t)$$, and the
lower order Taylor coefficients for $latex Y(t)$$, $latex Z(t)$$.
$latex \[
z^{(j)}
=
x^{(j)} \pm \frac{1}{j}  \sum_{k=1}^j k x^{(k)} y^{(j-k)}
\] $$
For $latex k = 1 , \ldots , j$$,
the partial of $latex H$$ with respect to $latex x^{(k)}$$ is given by
$latex \[
\begin{array}{rcl}
\D{H}{ x^{(k)} } & = & 
\D{G}{ x^{(k)} } + \D{G}{ z^{(j)} } \D{ z^{(j)} }{ x^{(k)} }
\\
& = &
\D{G}{ x^{(k)} } + 
\D{G}{ z^{(j)} }
\left[ \delta ( j - k ) \pm \frac{k}{j} y^{(j-k)} \right]
\end{array}
\] $$ 
where $latex \delta ( j - k )$$ is one if $latex j = k$$ and zero
otherwise.
For $latex k = 1 , \ldots ,  j$$ 
The partial of $latex H$$ with respect to $latex y^{j-k}$$,
is given by
$latex \[
\begin{array}{rcl}
\D{H}{ y^{(j-k)} } & = & 
\D{G}{ y^{(j-k)} } + \D{G}{ z^{(j)} } \D{ z^{(j)} }{ y^{(j-k)} }
\\
& = &
\D{G}{ y^{(j-k)} } \pm \D{G}{ z^{(j)} }\frac{k}{j} x^{k}
\end{array}
\] $$ 


$head Order Zero Z(t)$$
The order zero coefficients for the tangent and hyperbolic tangent are
$latex \[
\begin{array}{rcl}
z^{(0)} & = & \left\{
	\begin{array}{c} \tan ( x^{(0)} ) \\ \tanh (  x^{(0)} ) \end{array}
\right.
\end{array}
\] $$
Suppose that $latex H$$ is the same as $latex G$$ except that
$latex z^{(0)}$$ is expressed as a function of the Taylor coefficients
for $latex X(t)$$.
In this case,
$latex \[
\begin{array}{rcl}
\D{H}{ x^{(0)} }  
& = & 
\D{G}{ x^{(0)} }
+ \D{G}{ z^{(0)} } \D{ z^{(0)} }{ x^{(0)} }
\\
& = &
\D{G}{ x^{(0)} } + \D{G}{ z^{(0)} } ( 1 \pm y^{(0)} )
\end{array}
\] $$

$end