code: private double getExpectedProbability(int events)
      {
          BigDecimal kFactorial = new BigDecimal(Maths.bigFactorial(events));
          double numerator = Math.exp(-mean) * Math.pow(mean, events);
          return new BigDecimal(numerator).divide(kFactorial, RoundingMode.HALF_UP).doubleValue();
      }
docs: This is the probability mass function
  (http://en.wikipedia.org/wiki/Probability_mass_function) of
  the Poisson distribution represented by this number generator.
  @param events The number of occurrences to determine the
  probability for.
  @return The probability of the specified number of events
  occurring given the current value of lamda.