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# Tribonacci

> Compute the nth [Tribonacci number][tribonacci-number].

<section class="intro">

The [Tribonacci numbers][tribonacci-number] are the integer sequence

<!-- <equation class="equation" label="eq:tribonacci_sequence" align="center" raw="0, 0, 1, 1, 2, 4, 7, 13, 24, 44, 81, 149, 274, 504, 927, 1705, \ldots" alt="Tribonacci sequence"> -->

<div class="equation" align="center" data-raw-text="0, 0, 1, 1, 2, 4, 7, 13, 24, 44, 81, 149, 274, 504, 927, 1705, \ldots" data-equation="eq:tribonacci_sequence">
    <img src="https://cdn.rawgit.com/stdlib-js/stdlib/3249a68fb57cd71148f87ef3b2774be70a04d80a/lib/node_modules/@stdlib/math/base/special/tribonacci/docs/img/equation_tribonacci_sequence.svg" alt="Tribonacci sequence">
    <br>
</div>

<!-- </equation> -->

The sequence is defined by the recurrence relation

<!-- <equation class="equation" label="eq:tribonacci_recurrence_relation" align="center" raw="F_n = F_{n-1} + F_{n-2} + F_{n-3}" alt="Tribonacci sequence recurrence relation"> -->

<div class="equation" align="center" data-raw-text="F_n = F_{n-1} + F_{n-2} + F_{n-3}" data-equation="eq:tribonacci_recurrence_relation">
    <img src="https://cdn.rawgit.com/stdlib-js/stdlib/3249a68fb57cd71148f87ef3b2774be70a04d80a/lib/node_modules/@stdlib/math/base/special/tribonacci/docs/img/equation_tribonacci_recurrence_relation.svg" alt="Tribonacci sequence recurrence relation">
    <br>
</div>

<!-- </equation> -->

with seed values `F_0 = 0`, `F_1 = 0`, and `F_2 = 1`.

</section>

<!-- /.intro -->

<section class="usage">

## Usage

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

#### tribonacci( n )

Computes the nth [Tribonacci number][tribonacci-number].

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

v = tribonacci( 1 );
// returns 0

v = tribonacci( 2 );
// returns 1

v = tribonacci( 3 );
// returns 1

v = tribonacci( 63 );
// returns 8607945812375585
```

If `n > 63`, the function returns `NaN`, as larger [Tribonacci numbers][tribonacci-number] cannot be safely represented in [double-precision floating-point format][ieee754].

```javascript
var v = tribonacci( 64 );
// returns NaN
```

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

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

v = tribonacci( -1 );
// returns NaN
```

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

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

</section>

<!-- /.usage -->

<section class="notes">

</section>

<!-- /.notes -->

<section class="examples">

## Examples

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

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

var v;
var i;

for ( i = 0; i < 64; i++ ) {
    v = tribonacci( i );
    console.log( v );
}
```

</section>

<!-- /.examples -->

<section class="links">

[tribonacci-number]: https://en.wikipedia.org/wiki/Generalizations_of_Fibonacci_numbers#Tribonacci_numbers

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

</section>

<!-- /.links -->
