question: public double pdf ( double x ) { if ( x < 0 ) throw new IllegalArgumentException ( ) ; if ( x == 0 ) { if ( alpha == 1.0 ) return 1.0 / lambda ; else return 0.0 ; } if ( alpha == 1.0 ) return Math . exp ( - x / lambda ) / lambda ; return Math . exp ( ( alpha - 1.0 ) * Math . log ( x / lambda ) - x / lambda - Fun . logGamma ( alpha ) ) / lambda ; }
target:  Returns the probability distribution function.