function logpdf( x ) {
		var out;

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

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

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