function logpdf( x ) {
		return A + ( B * pow( x-mu, 2.0 ) );
	}
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>