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# Kernel Cosine

> Compute the [cosine][cosine] of a number on `[-π/4, π/4]`.

<section class="usage">

## Usage

```javascript
var kernelCos = require( '@stdlib/math/base/special/kernel-cos' );
```

#### kernelCos( x, y )

Computes the [cosine][cosine] of a `number` on `[-π/4, π/4]`. For increased accuracy, the number for which the [cosine][cosine] should be evaluated can be supplied as a [double-double number][double-double-arithmetic] (i.e., a non-evaluated sum of two [double-precision floating-point numbers][ieee754] `x` and `y`).

```javascript
var v = kernelCos( 0.0, 0.0 );
// returns ~1.0

v = kernelCos( 3.141592653589793/6.0, 0.0 );
// returns ~0.866

v = kernelCos( 0.785, -1.144e-17 );
// returns ~0.707

v = kernelCos( NaN, 0.0 );
// returns NaN
```

</section>

<!-- /.usage -->

<section class="notes">

## Notes

-   As components of a [double-double number][double-double-arithmetic], the two [double-precision floating-point numbers][ieee754] `x` and `y` must satisfy 

    <!-- <equation class="equation" label="eq:double_double_inequality" align="center" raw="|y| \leq \frac{1}{2} \operatorname{ulp}(x)" alt="Inequality for the two components of a double-double number."> -->

    <div class="equation" align="center" data-raw-text="|y| \leq \frac{1}{2} \operatorname{ulp}(x)" data-equation="eq:double_double_inequality">
        <img src="https://cdn.rawgit.com/stdlib-js/stdlib/7e0a95722efd9c771b129597380c63dc6715508b/lib/node_modules/@stdlib/math/base/special/kernel-cos/docs/img/equation_double_double_inequality.svg" alt="Inequality for the two components of a double-double number.">
        <br>
    </div>

    <!-- </equation> -->

    where `ulp` stands for [units in the last place][ulp].

</section>

<!-- /.notes -->

<section class="examples">

## Examples

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

```javascript
var linspace = require( '@stdlib/array/linspace' );
var PI = require( '@stdlib/constants/float64/pi' );
var kernelCos = require( '@stdlib/math/base/special/kernel-cos' );

var x = linspace( -PI/4.0, PI/4.0, 100 );
var i;

for ( i = 0; i < x.length; i++ ) {
    console.log( 'kernelCos(%d) = %d', x[ i ], kernelCos( x[ i ], 0.0 ) );
}
```

</section>

<!-- /.examples -->

<section class="links">

[cosine]: https://en.wikipedia.org/wiki/Cosine

[double-double-arithmetic]: https://en.wikipedia.org/wiki/Quadruple-precision_floating-point_format#Double-double_arithmetic

[ieee754]: https://en.wikipedia.org/wiki/IEEE_floating_point

[ulp]: https://en.wikipedia.org/wiki/Unit_in_the_last_place

</section>

<!-- /.links -->
