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# Archavercosine

> Compute the [inverse half-value versed cosine][archavercosine].

<section class="intro">

The [inverse half-value versed cosine][archavercosine] is defined as

<!-- <equation class="equation" label="eq:archavercosine" align="center" raw="\operatorname{ahavercos}(\theta) = 2 \cdot \arccos(\sqrt{\theta})" alt="Inverse half-value versed cosine."> -->

<div class="equation" align="center" data-raw-text="\operatorname{ahavercos}(\theta) = 2 \cdot \arccos(\sqrt{\theta})" data-equation="eq:archavercosine">
    <img src="https://cdn.rawgit.com/stdlib-js/stdlib/7e0a95722efd9c771b129597380c63dc6715508b/lib/node_modules/@stdlib/math/base/special/ahavercos/docs/img/equation_archavercosine.svg" alt="Inverse half-value versed cosine.">
    <br>
</div>

<!-- </equation> -->

</section>

<!-- /.intro -->

<section class="usage">

## Usage

```javascript
var ahavercos = require( '@stdlib/math/base/special/ahavercos' );
```

#### ahavercos( x )

Computes the [inverse half-value versed cosine][archavercosine].

```javascript
var v = ahavercos( 0.0 );
// returns ~3.1416

v = ahavercos( 1.0 );
// returns 0.0

v = ahavercos( 0.5 );
// returns ~1.5708
```

If `x < 0`, `x > 1`, or `x` is `NaN`, the function returns `NaN`.

```javascript
var v = ahavercos( 1.5 );
// returns NaN

v = ahavercos( -3.14 );
// returns NaN

v = ahavercos( NaN );
// returns NaN
```

</section>

<!-- /.usage -->

<section class="examples">

## Examples

<!-- eslint no-undef: "error" -->

```javascript
var linspace = require( '@stdlib/array/linspace' );
var ahavercos = require( '@stdlib/math/base/special/ahavercos' );

var x = linspace( 0.0, 1.0, 100 );
var i;

for ( i = 0; i < x.length; i++ ) {
    console.log( ahavercos( x[ i ] ) );
}
```

</section>

<!-- /.examples -->

<section class="links">

[archavercosine]: https://en.wikipedia.org/wiki/Versine

</section>

<!-- /.links -->
