function variance( sigma ) {
	if ( isnan( sigma ) || sigma < 0 ) {
		return NaN;
	}
	return ( 4.0-PI ) * sigma*sigma / 2.0;
}
Returns the variance of a Rayleigh distribution.

@param {NonNegativeNumber} sigma - scale parameter
@returns {NonNegativeNumber} variance

@example
var v = variance( 9.0 );
// returns ~34.765

@example
var v = variance( 2.0 );
// returns ~1.717

@example
var v = variance( -0.2 );
// returns NaN

@example
var v = variance( NaN );
// returns NaN