function factory( a, b ) {
	if (
		isnan( a ) ||
		isnan( b ) ||
		a <= 0.0 ||
		b <= 0.0
	) {
		return constantFunction( NaN );
	}
	return logpdf;

	/**
	* 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>
	*/
	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;
	}
}
Returns a function for evaluating the natural logarithm of the probability density function (PDF) for a Kumaraswamy's double bounded distribution with first shape parameter `a` and second shape parameter `b`.

@param {PositiveNumber} a - first shape parameter
@param {PositiveNumber} b - second shape parameter
@returns {Function} logPDF

@example
var logpdf = factory( 0.5, 0.5 );

var y = logpdf( 0.8 );
// returns ~-0.151

y = logpdf( 0.3 );
// returns ~-0.388