lib/number.js
function pdf( x, mu, sigma ) {
	if ( sigma === 0 ) {
		return x === mu ? Number.POSITIVE_INFINITY : 0;
	}
	var s2 = pow( sigma, 2 ),
		A = 1 / ( sqrt( 2 * s2 * PI ) ),
		B = -1 / ( 2 * s2 );
	return A * exp( B * pow( x - mu, 2 ) );
}
function pdf( x, mu, sigma ) {
	if ( sigma === 0 ) {
		return x === mu ? Number.POSITIVE_INFINITY : 0;
	}
	var s2 = pow( sigma, 2 ),
		A = 1 / ( sqrt( 2 * s2 * PI ) ),
		B = -1 / ( 2 * s2 );
	return A * exp( B * pow( x - mu, 2 ) );
}
PDF // 
FUNCTION: pdf( x, mu, sigma )
Evaluates the probability density function (PDF) for a Normal distribution with mean  `mu` and standard deviation `sigma` at a value `x`.

@param {Number} x - input value
@param {Number} mu - mean
@param {Number} sigma - standard deviation
@returns {Number} evaluated PDF