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# gammainc

> [Incomplete gamma function][incomplete-gamma-function].

<section class="intro">

Evaluates the regularized lower [incomplete gamma function][incomplete-gamma-function]:

<!-- <equation class="equation" label="eq:lower_incomplete_gamma" align="center" raw="P( x, a ) = \frac{\gamma(a,x)}{\Gamma(a)} = \frac{1}{\Gamma(a)} \int_0^x t^{a-1} e^{-t} \; dt" alt="Regularized lower incomplete gamma function."> -->

<div class="equation" align="center" data-raw-text="P( x, a ) = \frac{\gamma(a,x)}{\Gamma(a)} = \frac{1}{\Gamma(a)} \int_0^x t^{a-1} e^{-t} \; dt" data-equation="eq:lower_incomplete_gamma">
    <img src="https://cdn.rawgit.com/stdlib-js/stdlib/7e0a95722efd9c771b129597380c63dc6715508b/lib/node_modules/@stdlib/math/base/special/gammainc/docs/img/equation_lower_incomplete_gamma.svg" alt="Regularized lower incomplete gamma function.">
    <br>
</div>

<!-- </equation> -->

The function can also be used to evaluate the regularized upper incomplete gamma function, which is defined as follows:

<!-- <equation class="equation" label="eq:upper_incomplete_gamma" align="center" raw="Q( x, a ) = \frac{\Gamma(a,x)}{\Gamma(a)} = \frac{1}{\Gamma(a)} \int_x^\infty t^{a-1} e^{-t} \; dt" alt="Regularized upper incomplete gamma function."> -->

<div class="equation" align="center" data-raw-text="Q( x, a ) = \frac{\Gamma(a,x)}{\Gamma(a)} = \frac{1}{\Gamma(a)} \int_x^\infty t^{a-1} e^{-t} \; dt" data-equation="eq:upper_incomplete_gamma">
    <img src="https://cdn.rawgit.com/stdlib-js/stdlib/7e0a95722efd9c771b129597380c63dc6715508b/lib/node_modules/@stdlib/math/base/special/gammainc/docs/img/equation_upper_incomplete_gamma.svg" alt="Regularized upper incomplete gamma function.">
    <br>
</div>

<!-- </equation> -->

The two functions have the relationship `Q(x,a) = 1 - P(x,a)`.

In addition, this routine can be used to evaluate the _unregularized_ gamma functions. The range of above functions is `[0,1]`, which is not the case for the _unregularized_ versions. The unregularized lower incomplete gamma function is defined as

<!-- <equation class="equation" label="eq:unreg_lower_incomplete_gamma" align="center" raw="\gamma(a,x) = \int_0^x t^{a-1} e^{-t} \; dt" alt="Unregularized lower incomplete gamma function."> -->

<div class="equation" align="center" data-raw-text="\gamma(a,x) = \int_0^x t^{a-1} e^{-t} \; dt" data-equation="eq:unreg_lower_incomplete_gamma">
    <img src="https://cdn.rawgit.com/stdlib-js/stdlib/7e0a95722efd9c771b129597380c63dc6715508b/lib/node_modules/@stdlib/math/base/special/gammainc/docs/img/equation_unreg_lower_incomplete_gamma.svg" alt="Unregularized lower incomplete gamma function.">
    <br>
</div>

<!-- </equation> -->

and the upper unregularized incomplete gamma function is

<!-- <equation class="equation" label="eq:unreg_upper_incomplete_gamma" align="center" raw="\Gamma(a,x)= \int_x^\infty t^{a-1} e^{-t} \; dt" alt="Unregularized upper incomplete gamma function."> -->

<div class="equation" align="center" data-raw-text="\Gamma(a,x)= \int_x^\infty t^{a-1} e^{-t} \; dt" data-equation="eq:unreg_upper_incomplete_gamma">
    <img src="https://cdn.rawgit.com/stdlib-js/stdlib/7e0a95722efd9c771b129597380c63dc6715508b/lib/node_modules/@stdlib/math/base/special/gammainc/docs/img/equation_unreg_upper_incomplete_gamma.svg" alt="Unregularized upper incomplete gamma function.">
    <br>
</div>

<!-- </equation> -->

The relationship between the two functions is `Γ(a,x) = γ(a,x) + Γ(a)`.

</section>

<!-- /.intro -->

<section class="usage">

## Usage

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

#### gammainc( x, s\[, regularized\[, upper ]] )

By default, evaluates the regularized lower [incomplete gamma function][incomplete-gamma-function] for inputs `x` and `s`. The third and fourth parameters of the function can be used to specify whether instead to evaluate the non-regularized and/or upper incomplete gamma functions, respectively.

```javascript
var y = gammainc( 6.0, 2.0 );
// returns ~0.9826

y = gammainc( 1.0, 2.0, true, true );
// returns ~0.7358

y = gammainc( 7.0, 5.0 );
// returns ~0.8270

y = gammainc( 7.0, 5.0, false );
// returns ~19.8482
```

If provided `NaN` as any argument, the function returns `NaN`.

```javascript
var y = gammainc( NaN, 1.0 );
// returns NaN

y = gammainc( 1.0, NaN );
// returns NaN
```

If provided a negative `x`, the function returns `NaN`.

```javascript
var y = gammainc( -2.0, 2.0 );
// returns NaN
```

If provided a nonpositive `s`, the function returns `NaN`.

```javascript
var y = gammainc( 2.0, -1.0 );
// returns NaN

y = gammainc( 2.0, 0.0 );
// returns NaN
```

</section>

<!-- /.usage -->

<section class="examples">

## Examples

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

```javascript
var randu = require( '@stdlib/random/base/randu' );
var gammainc = require( '@stdlib/math/base/special/gammainc' );

var i;
var x;
var s;

for ( i = 0; i < 100; i++ ) {
    x = randu() * 10.0;
    s = randu() * 10.0;
    console.log( 'x: %d, \t s: %d, \t f(x,s): %d', x.toFixed( 4 ), s.toFixed( 4 ), gammainc( x, s ).toFixed( 4 ) );
}
```

</section>

<!-- /.examples -->

<section class="links">

[incomplete-gamma-function]: https://en.wikipedia.org/wiki/Incomplete_gamma_function

</section>

<!-- /.links -->
