# gammainc

Evaluates the regularized lower incomplete gamma function:

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

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

and the upper unregularized incomplete gamma function is

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

## Usage

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


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

By default, evaluates the regularized lower 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.

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.

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

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


If provided a negative x, the function returns NaN.

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


If provided a nonpositive s, the function returns NaN.

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

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


## Examples

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 ) );
}