/// <summary>
/// Gets the cumulative distribution function (cdf) for
/// the this distribution evaluated at point <c>x</c>.
/// </summary>
///
/// <param name="x">
/// A single point in the distribution range.</param>
///
/// <remarks>
/// The Cumulative Distribution Function (CDF) describes the cumulative
/// probability that a given value or any value smaller than it will occur.
/// </remarks>
///
public override double DistributionFunction(int x)
{
return(Special.IGamma(x + 1, lambda) / Special.Factorial(x));
}