function factory( x0, gamma ) {
	if (
		isnan( gamma ) ||
		isnan( x0 ) ||
		gamma <= 0.0
	) {
		return constantFunction( NaN );
	}
	return logpdf;

	/**
	* Evaluates the natural logarithm of the probability density function (logPDF) for a Cauchy distribution.
	*
	* @private
	* @param {number} x - input value
	* @returns {number} evaluated logPDF
	*
	* @example
	* var y = logpdf( 2.3 );
	* // returns <number>
	*/
	function logpdf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		return -( LNPI + ln( gamma ) + log1p( pow( (x-x0)/gamma, 2.0 ) ) );
	}
}
Returns a function for evaluating the natural logarithm of the probability density function (logPDF) for a Cauchy distribution with location parameter `x0` and scale parameter `gamma`.

@param {number} x0 - location parameter
@param {PositiveNumber} gamma - scale parameter
@returns {Function} logPDF

@example
var logpdf = factory( 4.0, 2.0 );

var y = logpdf( 10.0 );
// returns ~-4.14

y = logpdf( 3.0 );
// returns ~-2.061