can you explain?

Definition 3.7 Pascal (k, p) Random Variable X is a Pascal (k,p) random variable if the PMF of X has the form w= EN x 1. In general, the number of Bernoulli trials that take place until one of the two outcomes is observed k times is a Pascal random variable. For a Pascal (k, p) 3.3. FAMILIES OF DISCRETE RANDOM VARIABLES 73 random variable X, Px(x) is nonzero only for = k,k+1,.... Definition 3.7 does not state the values of k for which Px(a2) = 0 because in Definition 3.6 we have (*) = 0 for x {0,1,...,n}. Also note that the Pascal (1, p) random variable is the geometric (p) random variable