function pdf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		return exp( logpdf( x ) );
	}
Evaluates the probability density function (PDF) for a beta prime distribution.

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

@example
var y = pdf( 0.3 );
// returns <number>