A positive integer valued random variable N is a stopping time with respect to the discrete time stochastic process {X1, X2, · · · } if for every n, the event {N = n} is determined by the values of {X1, X2, · · · , Xn}.
(a) Suppose {X1, X2, · · · } is a Bernoulli process. Show that N = the time at which the second success occurs is a stopping time.
(b) Show that if N is a stopping time, then so is Nk = min{N, k}.