The simple Poisson process is characterized by a constant rate at which events occur per unit time. A generalization of this is to suppose that the probability of exactly one event occurring in the interval [ t , t + t ] is ( t ) t + o ( t ) . It can then be shown that the number of events occurring during an interval [ t 1 , t 2 ] has a Poisson distribution with parameter = t 2 t 1 ( t ) d t The occurrence of events over time in this situation is called a nonhomogeneous Poisson process. The article "Inference Based on Retrospective Ascertainment," J. Amer. Stat. Assoc., 1989: 360-372, considers the intensity function ( t ) = exp ( + t ) as appropriate for events involving transmission of HIV (the AIDS virus) via blood transfusion. Suppose that a = 2 and b = .6 (close to values suggested in the paper), with time in years. 2.a What is the expected number of events in the interval [ 0 , 4 ] ? In [ 2 , 6 ] ? 2.b What is the probability that at most 15 events occur in the interval [ 0 , .9907 ] ?.