public static double logPdf(double x,double shape,double scale,double factor){
  if (x <= 0)   return Double.NEGATIVE_INFINITY;
  return factor + shape * Math.log(scale) - (shape + 1) * Math.log(x) - (scale / x) - GammaFunction.lnGamma(shape);
}

public static double logPdf ( double x , double shape , double scale , double factor ) { if ( x <= 0 ) return Double . NEGATIVE_INFINITY ; return factor + shape * Math . log ( scale ) - ( shape + 1 ) * Math . log ( x ) - ( scale / x ) - GammaFunction . lnGamma ( shape ) ; }
the natural log of the probability density function of the distribution
