question: public static double logpdf ( double x , double mu , double shape ) { if ( ! ( x > 0 ) || x == Double . POSITIVE_INFINITY ) { return Double . NEGATIVE_INFINITY ; } final double v = ( x - mu ) / mu ; return v < Double . MAX_VALUE ? 0.5 * Math . log ( shape / ( MathUtil . TWOPI * x * x * x ) ) - shape * v * v / ( 2. * x ) : Double . NEGATIVE_INFINITY ; }
target:  Probability density function of the Wald distribution.