function pdf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x < 0.0 || x === PINF ) {
			return 0.0;
		}
		return s2i * x * exp( -pow( x, 2.0 ) / ( 2.0 * s2 ) );
	}
Evaluates the probability density function (PDF) for a Rayleigh distribution.

@private
@param {number} x - input value
@returns {number} evaluated PDF

@example
var y = pdf( 2.3 );
// returns <number>