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# Arcvercosine

> Compute the [inverse versed cosine][inverse-versed-cosine].

<section class="intro">

The [inverse versed cosine][inverse-versed-cosine] is defined as

<!-- <equation class="equation" label="eq:arcvercosine" align="center" raw="\operatorname{avercos}(\theta) = \arccos(1+\theta)" alt="Inverse versed cosine."> -->

<div class="equation" align="center" data-raw-text="\operatorname{avercos}(\theta) = \arccos(1+\theta)" data-equation="eq:arcvercosine">
    <img src="https://cdn.jsdelivr.net/gh/stdlib-js/stdlib@bb29798906e119fcb2af99e94b60407a270c9b32/lib/node_modules/@stdlib/math/base/special/avercos/docs/img/equation_arcvercosine.svg" alt="Inverse versed cosine.">
    <br>
</div>

<!-- </equation> -->

</section>

<!-- /.intro -->

<section class="usage">

## Usage

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

#### avercos( x )

Computes the [inverse versed cosine][inverse-versed-cosine].

```javascript
var v = avercos( 0.0 );
// returns 0.0

v = avercos( -3.141592653589793/2.0 );
// returns ~2.1783

v = avercos( -3.141592653589793/6.0 );
// returns ~1.0742
```

If `x < -2`, `x > 0`, or `x` is `NaN`, the function returns `NaN`.

```javascript
var v = avercos( 1.0 );
// returns NaN

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

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

</section>

<!-- /.usage -->

<section class="examples">

## Examples

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

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

var x = linspace( -2.0, 0.0, 100 );
var i;

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

</section>

<!-- /.examples -->

<section class="links">

[inverse-versed-cosine]: https://en.wikipedia.org/wiki/Versine

</section>

<!-- /.links -->