question: public static double pdf ( double x , double mu , double shape ) { if ( ! ( x > 0 ) || x == Double . POSITIVE_INFINITY ) { return 0 ; } final double v = ( x - mu ) / mu ; double t1 = Math . sqrt ( shape / ( MathUtil . TWOPI * x * x * x ) ) ; return t1 > 0 ? t1 * Math . exp ( - shape * v * v * .5 / x ) : 0 ; }
target:  Probability density function of the Wald distribution.