For every point x that is not one of the possible values of X, it is evident that f(x)=0. Also, if the sequencex_1,x_2,… includes all the possible values of X, than∑_(i=1)^∞▒〖f(x_i )=1.〗 Atypical p.f. is sketched in Fig. 3.3, in which each vertical segment