question: public static double pdf ( double x , double mu , double sigma , double skew ) { x = ( x - mu ) / sigma ; if ( Math . abs ( skew ) > 0. ) { x = - Math . log ( 1. - skew * x ) / skew ; } return MathUtil . SQRTHALF * Math . exp ( - .5 * x * x ) / sigma / ( 1 - skew * x ) ; }
target:  Probability density function of the skewed normal distribution.