function cdf( x, sigma ) {
	if ( x < 0 ) {
		return 0;
	}
	var s2 = pow( sigma, 2 );
	return -expm1( - pow( x, 2) / ( 2 * s2 ) );
}
function cdf( x, sigma ) {
	if ( x < 0 ) {
		return 0;
	}
	var s2 = pow( sigma, 2 );
	return -expm1( - pow( x, 2) / ( 2 * s2 ) );
}
function cdf ( x , sigma ) { if ( x < 0 ) { return 0 ; } var s2 = pow ( sigma , 2 ) ; return - expm1 ( - pow ( x , 2 ) / ( 2 * s2 ) ) ; }
CDF // 
FUNCTION: cdf( x, sigma )
Evaluates the cumulative distribution function (CDF) for a Rayleigh distribution with scale parameter `sigma` at a value `x`.

@param {Number} x - input value
@param {Number} sigma - scale parameter
@returns {Number} evaluated CDF
CDF // FUNCTION : cdf ( x sigma ) Evaluates the cumulative distribution function ( CDF ) for a Rayleigh distribution with scale parameter sigma at a value x .