# Reciprocal Square Root

Compute the reciprocal of the principal square root of a double-precision floating-point number.

The reciprocal of the principal square root is defined as

## Usage

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


#### rsqrt( x )

Computes the reciprocal (inverse) square root of a double-precision floating-point number.

var v = rsqrt( 1.0 );
// returns 1.0

v = rsqrt( 4.0 );
// returns 0.5

v = rsqrt( 100.0 );
// returns 0.1

v = rsqrt( 0.0 );
// returns Infinity

v = rsqrt( NaN );
// returns NaN

v = rsqrt( Infinity );
// returns 0.0


For negative numbers, the reciprocal square root is not defined.

var v = rsqrt( -4.0 );
// returns NaN


## Examples

var randu = require( '@stdlib/random/base/randu' );
var round = require( '@stdlib/math/base/special/round' );
var rsqrt = require( '@stdlib/math/base/special/rsqrt' );

var x;
var i;

for ( i = 0; i < 100; i++ ) {
x = round( randu() * 100.0 );
console.log( 'rsqrt(%d) = %d', x, rsqrt( x ) );
}


## C APIs

### Usage

#include "stdlib/math/base/special/rsqrt.h"


#### stdlib_base_rsqrt( x )

Computes the reciprocal (inverse) square root of a double-precision floating-point number.

double y = stdlib_base_rsqrt( 4.0 );
// returns 0.5


The function accepts the following arguments:

• x: [in] double input value.
double stdlib_base_rsqrt( const double x );


### Examples

#include "stdlib/math/base/special/rsqrt.h"
#include <stdio.h>

int main() {
double x[] = { 3.14, 9.0, 0.0, 0.0/0.0 };

double y;
int i;
for ( i = 0; i < 4; i++ ) {
y = stdlib_base_rsqrt( x[ i ] );
printf( "rsqrt(%lf) = %lf\n", x[ i ], y );
}
}