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;
}

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 ; }
returns the probability distribution function .
