Optimal random replacement policies for repairable systems

For a stationary renewal process with preventive repair optimal policies of preventive repair are established within the class of age-dependent replacement policies with random repair time, provided the expected number of renewals for a fixed time period is given. Under certain conditions the distribution of the random preventive repair time is the degenerate one. Nevertheless, there exist conditions that yield optimal replacement policies with nondegenerate repair time distributions, i.e. random replacement times