# Reciprocal Square Root > Compute the reciprocal of the principal [square root][square-root] of a double-precision floating-point number.
The reciprocal of the principal [square root][square-root] is defined as
Reciprocal square root
## Usage ```javascript var rsqrt = require( '@stdlib/math/base/special/rsqrt' ); ``` #### rsqrt( x ) Computes the reciprocal (inverse) square root of a double-precision floating-point number. ```javascript 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. ```javascript var v = rsqrt( -4.0 ); // returns NaN ```
## Examples ```javascript 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 ```c #include "stdlib/math/base/special/rsqrt.h" ``` #### stdlib_base_rsqrt( x ) Computes the reciprocal (inverse) [square root][square-root] of a double-precision floating-point number. ```c double y = stdlib_base_rsqrt( 4.0 ); // returns 0.5 ``` The function accepts the following arguments: - **x**: `[in] double` input value. ```c double stdlib_base_rsqrt( const double x ); ```
### Examples ```c #include "stdlib/math/base/special/rsqrt.h" #include 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 ); } } ```