function pdf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x <= 0.0 || x >= 1.0 ) {
			return 0.0;
		}
		return ( a*b ) * pow( x, a - 1.0 ) * pow( 1.0 - pow( x, a ), b - 1.0 );
	}
Evaluates the probability density function (PDF) for a Kumaraswamy's double bounded distribution.

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

@example
var y = pdf( 2.0 );
// returns <number>