public static double probOf(int k,double lambda){
  return Math.exp((k * Math.log(lambda)) - lambda - Functions.logFactorial(k));
}

public static double probOf ( int k , double lambda ) { return Math . exp ( ( k * Math . log ( lambda ) ) - lambda - Functions . logFactorial ( k ) ) ; }
see the numerical formulation : https : / / en . wikipedia . org / wiki / poisson_distribution .
