# Definition:Inverse Hyperbolic Cosine

## Complex Plane

### Definition 1

The inverse hyperbolic cosine is a multifunction defined as:

$\forall z \in \C: \cosh^{-1} \left({z}\right) := \left\{{w \in \C: z = \cosh \left({w}\right)}\right\}$

where $\cosh \left({w}\right)$ is the hyperbolic cosine function.

### Definition 2

The inverse hyperbolic cosine is a multifunction defined as:

$\forall z \in \C: \cosh^{-1} \left({z}\right) := \left\{{\ln \left({z + \sqrt{\left|{z^2 - 1}\right|} e^{\left({i / 2}\right) \arg \left({z^2 - 1}\right)} }\right) + 2 k \pi i: k \in \Z}\right\}$

where:

$\sqrt{\left|{z^2 - 1}\right|}$ denotes the positive square root of the complex modulus of $z^2 - 1$
$\arg \left({z^2 - 1}\right)$ denotes the argument of $z^2 - 1$
$\ln$ denotes the complex natural logarithm considered as a multifunction.

### Hyperbolic Arccosine

The principal branch of the complex inverse hyperbolic cosine function is defined as:

$\forall z \in \C: \map {\Cosh^{-1} } z := \map \Ln {z + \sqrt {z^2 - 1} }$

where:

$\Ln$ denotes the principal branch of the complex natural logarithm
$\sqrt {z^2 - 1}$ denotes the principal square root of $z^2 - 1$.

## Real Numbers

### Definition 1

The inverse hyperbolic cosine $\cosh^{-1}: S \to \R$ is a real function defined on $S$ as:

$\forall x \in S: \cosh^{-1} \left({x}\right) := y \in \R_{\ge 0}: x = \cosh \left({y}\right)$

where $\cosh \left({y}\right)$ denotes the hyperbolic cosine function.

### Definition 2

The inverse hyperbolic cosine $\cosh^{-1}: S \to \R$ is a real function defined on $S$ as:

$\forall x \in S: \map {\cosh^{-1} } x := \map \ln {x + \sqrt {x^2 - 1} }$

where:

$\sqrt {x^2 - 1}$ denotes the positive square root of $x^2 - 1$
$\ln$ denotes the natural logarithm of a (strictly positive) real number.

## Also see

• Results about the inverse hyperbolic cosine can be found here.