function logpdf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		return -( LNPI + ln( gamma ) + log1p( pow( (x-x0)/gamma, 2.0 ) ) );
	}
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>