function factory( mu, sigma ) {
	var s2;
	var A;
	var B;
	if (
		isnan( mu ) ||
		isnan( sigma ) ||
		sigma < 0.0
	) {
		return constantFunction( NaN );
	}
	if ( sigma === 0.0 ) {
		return degenerate( mu );
	}
	s2 = pow( sigma, 2.0 );
	A = (-0.5) * ( ( 2.0*ln( sigma ) ) + LN_TWO_PI );
	B = -1.0 / ( 2.0*s2 );
	return logpdf;

	/**
	* Evaluates the natural logarithm of the probability density function (PDF) for a normal distribution.
	*
	* @private
	* @param {number} x - input value
	* @returns {number} evaluated logPDF
	*
	* @example
	* var y = logpdf( -3.14 );
	* // returns <number>
	*/
	function logpdf( x ) {
		return A + ( B * pow( x-mu, 2.0 ) );
	}
}
Returns a function for evaluating the natural logarithm of the probability density function (PDF) for a normal distribution.

@param {number} mu - mean
@param {NonNegativeNumber} sigma - standard deviation
@returns {Function} logPDF

@example
var logpdf = factory( 10.0, 2.0 );
var y = logpdf( 10.0 );
// returns ~-1.612

y = logpdf( 5.0 );
// returns ~-4.737