The random variable x is said to have the yule simon distribution if

The Yule-Simon distribution is a discrete probability distribution. It is named after Udny Yule and Herbert A. Simon. 

The Yule–Simon distribution was originally created by Yule as a limiting distribution model for a particular stochastic process, called the "Yule process" or the "preferential attachment process," in his study of the distribution of biological taxa and subtaxa.

The random variable X is said to have the Yule-Simon distribution if 

P (X=k) =              4                    where   k = 1,2,...
                     k (k+1)(k+2)