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Inverse Cosine and Hyperbolic Cosine Reverse Mode Theory

We use the reverse theory standard math function definition for the functions \(H\) and \(G\). In addition, we use the forward mode notation in acos_forward for

\begin{eqnarray} Q(t) & = & \mp ( X(t) * X(t) - 1 ) \\ B(t) & = & \sqrt{ Q(t) } \end{eqnarray}

We use \(q\) and \(b\) for the p-th order Taylor coefficient row vectors corresponding to these functions and replace \(z^{(j)}\) by

\[( z^{(j)} , b^{(j)} )\]

in the definition for \(G\) and \(H\). The zero order forward mode formulas for the acos function are

\begin{eqnarray} q^{(0)} & = & \mp ( x^{(0)} x^{(0)} - 1) \\ b^{(0)} & = & \sqrt{ q^{(0)} } \\ z^{(0)} & = & F ( x^{(0)} ) \end{eqnarray}

where \(F(x) = \R{acos} (x)\) for \(-\) and \(F(x) = \R{acosh} (x)\) for \(+\). For orders \(j\) greater than zero we have

\begin{eqnarray} q^{(j)} & = & \mp \sum_{k=0}^j x^{(k)} x^{(j-k)} \\ b^{(j)} & = & \frac{1}{j} \frac{1}{ b^{(0)} } \left( \frac{j}{2} q^{(j)} - \sum_{k=1}^{j-1} k b^{(k)} b^{(j-k)} \right) \\ z^{(j)} & = & \frac{1}{j} \frac{1}{ b^{(0)} } \left( \mp j x^{(j)} - \sum_{k=1}^{j-1} k z^{(k)} b^{(j-k)} \right) \end{eqnarray}

If \(j = 0\), we note that \(F^{(1)} ( x^{(0)} ) = \mp 1 / b^{(0)}\) and hence

\begin{eqnarray} \D{H}{ x^{(j)} } & = & \D{G}{ x^{(j)} } + \D{G}{ z^{(j)} } \D{ z^{(j)} }{ x^{(0)} } + \D{G}{ b^{(j)} } \D{ b^{(j)} }{ q^{(0)} } \D{ q^{(0)} }{ x^{(0)} } \\ & = & \D{G}{ x^{(j)} } \mp \D{G}{ z^{(j)} } \frac{1}{ b^{(0)} } \mp \D{G}{ b^{(j)} } \frac{ x^{(0)} }{ b^{(0)} } \end{eqnarray}

If \(j > 0\), then for \(k = 1, \ldots , j-1\)

\begin{eqnarray} \D{H}{ b^{(0)} } & = & \D{G}{ b^{(0)} } + \D{G}{ z^{(j)} } \D{ z^{(j)} }{ b^{(0)} } + \D{G}{ b^{(j)} } \D{ b^{(j)} }{ b^{(0)} } \\ & = & \D{G}{ b^{(0)} } - \D{G}{ z^{(j)} } \frac{ z^{(j)} }{ b^{(0)} } - \D{G}{ b^{(j)} } \frac{ b^{(j)} }{ b^{(0)} } \\ \D{H}{ x^{(0)} } & = & \D{G}{ x^{(0)} } + \D{G}{ b^{(j)} } \D{ b^{(j)} }{ q^{(j)} } \D{ q^{(j)} }{ x^{(0)} } \\ & = & \D{G}{ x^{(0)} } \mp \D{G}{ b^{(j)} } \frac{ x^{(j)} }{ b^{(0)} } \\ \D{H}{ x^{(j)} } & = & \D{G}{ x^{(j)} } + \D{G}{ z^{(j)} } \D{ z^{(j)} }{ x^{(j)} } + \D{G}{ b^{(j)} } \D{ b^{(j)} }{ q^{(j)} } \D{ q^{(j)} }{ x^{(j)} } \\ & = & \D{G}{ x^{(j)} } \mp \D{G}{ z^{(j)} } \frac{1}{ b^{(0)} } \mp \D{G}{ b^{(j)} } \frac{ x^{(0)} }{ b^{(0)} } \\ \D{H}{ b^{(j - k)} } & = & \D{G}{ b^{(j - k)} } + \D{G}{ z^{(j)} } \D{ z^{(j)} }{ b^{(j - k)} } + \D{G}{ b^{(j)} } \D{ b^{(j)} }{ b^{(j - k)} } \\ & = & \D{G}{ b^{(j - k)} } - \D{G}{ z^{(j)} } \frac{k z^{(k)} }{j b^{(0)} } - \D{G}{ b^{(j)} } \frac{ b^{(k)} }{ b^{(0)} } \\ \D{H}{ x^{(k)} } & = & \D{G}{ x^{(k)} } + \D{G}{ z^{(j)} } \D{ z^{(j)} }{ x^{(k)} } + \D{G}{ b^{(j)} } \D{ b^{(j)} }{ q^{(j)} } \D{ q^{(j)} }{ x^{(k)} } \\ & = & \D{G}{ x^{(k)} } \mp \D{G}{ b^{(j)} } \frac{ x^{(j-k)} }{ b^{(0)} } \\ \D{H}{ z^{(k)} } & = & \D{G}{ z^{(k)} } + \D{G}{ z^{(j)} } \D{ z^{(j)} }{ z^{(k)} } + \D{G}{ b^{(j)} } \D{ b^{(j)} }{ z^{(k)} } \\ & = & \D{G}{ z^{(k)} } - \D{G}{ z^{(j)} } \frac{k b^{(j-k)} }{ j b^{(0)} } \end{eqnarray}