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# erfinv

> [Inverse error function][inverse-error-function].

<section class="intro">

The [inverse error function][inverse-error-function] is defined in terms of the [Maclaurin series][maclaurin-series]

<!-- <equation class="equation" label="eq:inverse_error_function" align="center" raw="\operatorname{erf}^{-1}(z)=\sum_{k=0}^\infty\frac{c_k}{2k+1}\left (\frac{\sqrt{\pi}}{2}z\right )^{2k+1}" alt="Inverse error function."> -->

<div class="equation" align="center" data-raw-text="\operatorname{erf}^{-1}(z)=\sum_{k=0}^\infty\frac{c_k}{2k+1}\left (\frac{\sqrt{\pi}}{2}z\right )^{2k+1}" data-equation="eq:inverse_error_function">
    <img src="https://cdn.rawgit.com/stdlib-js/stdlib/7e0a95722efd9c771b129597380c63dc6715508b/lib/node_modules/@stdlib/math/base/special/erfinv/docs/img/equation_inverse_error_function.svg" alt="Inverse error function.">
    <br>
</div>

<!-- </equation> -->

where `c_0 = 1` and 

<!-- <equation class="equation" label="eq:inverse_error_function_series_coefficients" align="center" raw="c_k=\sum_{m=0}^{k-1}\frac{c_m c_{k-1-m}}{(m+1)(2m+1)} = \left\{1,1,\frac{7}{6},\frac{127}{90},\frac{4369}{2520},\frac{34807}{16200},\ldots\right\}" alt="Series coefficients."> -->

<div class="equation" align="center" data-raw-text="c_k=\sum_{m=0}^{k-1}\frac{c_m c_{k-1-m}}{(m+1)(2m+1)} = \left\{1,1,\frac{7}{6},\frac{127}{90},\frac{4369}{2520},\frac{34807}{16200},\ldots\right\}" data-equation="eq:inverse_error_function_series_coefficients">
    <img src="https://cdn.rawgit.com/stdlib-js/stdlib/7e0a95722efd9c771b129597380c63dc6715508b/lib/node_modules/@stdlib/math/base/special/erfinv/docs/img/equation_inverse_error_function_series_coefficients.svg" alt="Series coefficients.">
    <br>
</div>

<!-- </equation> -->

</section>

<!-- /.intro -->

<section class="usage">

## Usage

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

#### erfinv( x )

Evaluates the [inverse error function][inverse-error-function].

```javascript
var y = erfinv( 0.5 );
// returns ~0.4769

y = erfinv( 0.8 );
// returns ~0.9062

y = erfinv( -1.0 );
// returns -Infinity

y = erfinv( 1.0 );
// returns Infinity
```

The domain of `x` is restricted to `[-1,1]`. If `|x| > 1`, the function returns `NaN`.

```javascript
var y = erfinv( -3.14 );
// returns NaN
```

If provided `NaN`, the function returns `NaN`.

```javascript
var y = erfinv( NaN );
// returns NaN
```

The [inverse error function][inverse-error-function] is an [odd function][odd-function]; i.e., `erfinv(-x) = -erfinv(x)`. Thus, in accordance with the [IEEE 754][ieee754] standard, if provided `-0`, the function returns `-0`.

```javascript
var y = erfinv( -0.0 );
// returns -0.0
```

</section>

<!-- /.usage -->

<section class="examples">

## Examples

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

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

var x = linspace( -1.0, 1.0, 100 );
var y;
var i;

for ( i = 0; i < x.length; i++ ) {
    y = erfinv( x[ i ] );
    console.log( 'x: %d, erfinv(x): %d', x[ i ], y );
}
```

</section>

<!-- /.examples -->

<section class="links">

[inverse-error-function]: https://en.wikipedia.org/wiki/Error_function#Inverse_functions

[maclaurin-series]: http://mathworld.wolfram.com/MaclaurinSeries.html

[odd-function]: https://en.wikipedia.org/wiki/Even_and_odd_functions

[ieee754]: https://en.wikipedia.org/wiki/IEEE_754-1985

</section>

<!-- /.links -->
