question: public double logPdf ( double x ) { recomputeZ ( ) ; if ( x < 0 || x > 1 ) { return Double . NEGATIVE_INFINITY ; } else if ( x == 0 ) { if ( alpha < 1 ) { return Double . NaN ; } return Double . NEGATIVE_INFINITY ; } else if ( x == 1 ) { if ( beta < 1 ) { return Double . NaN ; } return Double . NEGATIVE_INFINITY ; } else { double logX = Math . log ( x ) ; double log1mX = Math . log1p ( - x ) ; return ( alpha - 1 ) * logX + ( beta - 1 ) * log1mX - z ; } }
target:  the natural log of the probability density function of the distribution