atanh() [9/11]

var stan::math::atanh ( const var &  x )

inline

The inverse hyperbolic tangent function for variables (C99).

The derivative is defined by

$\frac{d}{dx} \mbox{atanh}(x) = \frac{1}{1 - x^2}$.

$$\mbox{atanh}(x) = \begin{cases} \textrm{NaN} & \mbox{if } x < -1\\ \tanh^{-1}(x) & \mbox{if } -1\leq x \leq 1 \\ \textrm{NaN} & \mbox{if } x > 1\\[6pt] \textrm{NaN} & \mbox{if } x = \textrm{NaN} \end{cases}$$

$$\frac{\partial\, \mbox{atanh}(x)}{\partial x} = \begin{cases} \textrm{NaN} & \mbox{if } x < -1\\ \frac{\partial\, \tanh^{-1}(x)}{\partial x} & \mbox{if } -1\leq x\leq 1 \\ \textrm{NaN} & \mbox{if } x > 1\\[6pt] \textrm{NaN} & \mbox{if } x = \textrm{NaN} \end{cases}$$

$$\tanh^{-1}(x)=\frac{1}{2}\ln\left(\frac{1+x}{1-x}\right)$$

$$\frac{\partial \, \tanh^{-1}(x)}{\partial x} = \frac{1}{1-x^2}$$

Parameters

x	The variable.

Returns

Inverse hyperbolic tangent of the variable.

Exceptions

std::domain_error

if a < -1 or a > 1

Definition at line 58 of file atanh.hpp.