# acosh
> Compute the [hyperbolic arccosine][hyperbolic-arccosine] of a number.
## Usage
```javascript
var acosh = require( '@stdlib/math/base/special/acosh' );
```
#### acosh( x )
Computes the [hyperbolic arccosine][hyperbolic-arccosine] of a `number` (in radians).
```javascript
var v = acosh( 1.0 );
// returns 0.0
v = acosh( 2.0 );
// returns ~1.317
v = acosh( 0.5 );
// returns NaN
```
The domain of `x` is restricted to `[1,+infinity)`. If `x < 1`, the function will return `NaN`.
```javascript
var v = acosh( 0.0 );
// returns NaN
```
## Examples
```javascript
var linspace = require( '@stdlib/array/linspace' );
var acosh = require( '@stdlib/math/base/special/acosh' );
var x = linspace( 1.0, 5.0, 100 );
var i;
for ( i = 0; i < x.length; i++ ) {
console.log( acosh( x[ i ] ) );
}
```
[hyperbolic-arccosine]: https://en.wikipedia.org/wiki/Inverse_hyperbolic_function