function pdf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		return 1.0 / ( gpi * (1.0 + pow( (x-x0)/gamma, 2.0 )) );
	}
Evaluates the probability density function (PDF) for a Cauchy distribution.

@private
@param {number} x - input value
@returns {number} evaluated PDF

@example
var y = pdf( 2.3 );
// returns <number>