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# negaFibonacci

> Compute the nth [negaFibonacci number][fibonacci-number].

<section class="intro">

The [negaFibonacci numbers][fibonacci-number] are the integer sequence

<!-- <equation class="equation" label="eq:negafibonacci_sequence" align="center" raw="0, 1, -1, 2, -3, 5, -8, 13, -21, 34, -55, 89, -144, \ldots" alt="NegaFibonacci sequence"> -->

<div class="equation" align="center" data-raw-text="0, 1, -1, 2, -3, 5, -8, 13, -21, 34, -55, 89, -144, \ldots" data-equation="eq:negafibonacci_sequence">
    <img src="https://cdn.rawgit.com/stdlib-js/stdlib/7e0a95722efd9c771b129597380c63dc6715508b/lib/node_modules/@stdlib/math/base/special/negafibonacci/docs/img/equation_negafibonacci_sequence.svg" alt="NegaFibonacci sequence">
    <br>
</div>

<!-- </equation> -->

The sequence is defined by the recurrence relation

<!-- <equation class="equation" label="eq:negafibonacci_recurrence_relation" align="center" raw="F_{n-2} = F_{n} - F_{n-1}" alt="NegaFibonacci sequence recurrence relation"> -->

<div class="equation" align="center" data-raw-text="F_{n-2} = F_{n} - F_{n-1}" data-equation="eq:negafibonacci_recurrence_relation">
    <img src="https://cdn.rawgit.com/stdlib-js/stdlib/7e0a95722efd9c771b129597380c63dc6715508b/lib/node_modules/@stdlib/math/base/special/negafibonacci/docs/img/equation_negafibonacci_recurrence_relation.svg" alt="NegaFibonacci sequence recurrence relation">
    <br>
</div>

<!-- </equation> -->

which yields

<!-- <equation class="equation" label="eq:negafibonacci_fibonacci" align="center" raw="F_{-n} = (-1)^{n+1} F_n" alt="NegaFibonacci relationship to Fibonacci numbers"> -->

<div class="equation" align="center" data-raw-text="F_{-n} = (-1)^{n+1} F_n" data-equation="eq:negafibonacci_fibonacci">
    <img src="https://cdn.rawgit.com/stdlib-js/stdlib/7e0a95722efd9c771b129597380c63dc6715508b/lib/node_modules/@stdlib/math/base/special/negafibonacci/docs/img/equation_negafibonacci_fibonacci.svg" alt="NegaFibonacci relationship to Fibonacci numbers">
    <br>
</div>

<!-- </equation> -->

with seed values `F_0 = 0` and `F_{-1} = 1`.

</section>

<!-- /.intro -->

<section class="usage">

## Usage

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

#### negafibonacci( n )

Computes the nth [negaFibonacci number][fibonacci-number].

```javascript
var v = negafibonacci( 0 );
// returns 0

v = negafibonacci( -1 );
// returns 1

v = negafibonacci( -2 );
// returns -1

v = negafibonacci( -3 );
// returns 2

v = negafibonacci( -78 );
// returns -8944394323791464
```

If `n < -78`, the function returns `NaN`, as larger [negaFibonacci numbers][fibonacci-number] cannot be safely represented in [double-precision floating-point format][ieee754].

```javascript
var v = negafibonacci( -79 );
// returns NaN
```

If not provided a nonpositive integer value, the function returns `NaN`.

```javascript
var v = negafibonacci( -3.14 );
// returns NaN

v = negafibonacci( 1 );
// returns NaN
```

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

```javascript
var v = negafibonacci( NaN );
// returns NaN
```

</section>

<!-- /.usage -->

<section class="notes">

</section>

<!-- /.notes -->

<section class="examples">

## Examples

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

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

var v;
var i;

for ( i = 0; i > -79; i-- ) {
    v = negafibonacci( i );
    console.log( v );
}
```

</section>

<!-- /.examples -->

<section class="links">

[fibonacci-number]: https://en.wikipedia.org/wiki/Fibonacci_number

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

</section>

<!-- /.links -->
