The work of two parallel devices when we take into account the time of their replacement

This problem was the result of a generalization of the problem from the book "The course of the theory of probability" of B.V. Gnedenko. He examined work of the system consisting of two interchangeable devices. These devices work in the next order: the first device works first, then it falls out repaired, he is replaced by the second device, then and the second falls out and repaired also. If the time operation of the first device is less time to repair the second device, the operation system includes a first device. If no, then we consider that the system finished work and time of work of the system equal to time of work of the first and second device. If the time operation of the second device is less time to repair the first device, the operation system includes a first device et cetera. The characteristic function of time of work of the system is built as a result. Using this characteristic function the expected value of time of work of the system was found and the methods of his increase offer.
In the work we consider a system consisting of two elements operating simultaneously. The time of the work of the system is interrupted, when we carry out repair of both elements. We construct a characteristic function for continuous time of the work of the system. We consider the recommendations for increasing the expected value of time of work of the system.